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1. The Species Concept Image References:
Wikimedia Commons. (2004). Flamingo (Aaron Logan). Retrieved 3-1-07, from http://commons.wikimedia.org/wiki/Image:Lightmatter_flamingo2.jpg.
Wikipedia. (2005). Phalaenopsis hybrid (Andre Karwath). Retrieved 3-1-07, from http://en.wikipedia.org/wiki/Image:Phalaenopsis_%28aka%29.jpg.
Wikimedia Commons. (2005). Orangutan in Aalborg Zoo, Denmark (Malene Thyssen). Retrieved 3-1-07, from http://commons.wikimedia.org/wiki/Image:Pongo_pygmaeus_%28orangutang%29.jpg.
Wikimedia Commons. (2006). Paradise fish (Macropodus opercularis) (André Karwath). Retrieved 3-1-07, from http://commons.wikimedia.org/wiki/Image:Macropodus_opercularis_-_front_%28aka%29.jpg.
Wikimedia Commons. (2007). A young grasshopper on a grass stalk (fir0002). Retrieved 3-1-07, from http://commons.wikimedia.org/wiki/Image:Young_grasshopper_on_grass_stalk03.jpg.
Image References:
Wikimedia Commons. (2004). Flamingo (Aaron Logan). Retrieved 3-1-07, from http://commons.wikimedia.org/wiki/Image:Lightmatter_flamingo2.jpg.
Wikipedia. (2005). Phalaenopsis hybrid (Andre Karwath). Retrieved 3-1-07, from http://en.wikipedia.org/wiki/Image:Phalaenopsis_%28aka%29.jpg.
Wikimedia Commons. (2005). Orangutan in Aalborg Zoo, Denmark (Malene Thyssen). Retrieved 3-1-07, from http://commons.wikimedia.org/wiki/Image:Pongo_pygmaeus_%28orangutang%29.jpg.
Wikimedia Commons. (2006). Paradise fish (Macropodus opercularis) (André Karwath). Retrieved 3-1-07, from http://commons.wikimedia.org/wiki/Image:Macropodus_opercularis_-_front_%28aka%29.jpg.
Wikimedia Commons. (2007). A young grasshopper on a grass stalk (fir0002). Retrieved 3-1-07, from http://commons.wikimedia.org/wiki/Image:Young_grasshopper_on_grass_stalk03.jpg.
2. The Species Problem The Species Problem
Although scientists generally agree that species represent the most basic and fundamental groups into which living and extinct organisms are classified, there is debate over how species should be identified as well as the precise definition of the word species. This controversy, known as the species problem, stems from the quest for a single, universal, and functional conception of species. This essential species concept would have to identify species unambiguously as discreet, natural units in addition to explaining the existence of species and how they arise in nature. While a number of different species concepts have been proposed, none are able to meet these requirements for all species.
In response to this problem, many scientists recommend a pluralistic approach, that is, the recognition that there are multiple ways to think about and define species. Investigators must therefore choose an appropriate species concept based on their needs. For example, a paleontologist who tries to understand prehistoric vertebrates by studying fossils is likely to require and apply a different conceptualization of species than a microbiologist who studies heredity and variation in populations of bacteria.
References:
Campbell, N.E. & Reece, J.B. (2002). Biology (6th ed.). San Francisco: Benjamin Cummings.
Coyne, J.A. & Orr, H.A. (2004). Speciation. Sunderland, MA: Sinauer Associates.
Futuyma, D.J. (2005). Evolution. Sunderland, MA: Sinauer Associates.
Image References:
Wikimedia Commons. (2004). Flamingo (Aaron Logan). Retrieved 3-1-07, from http://commons.wikimedia.org/wiki/Image:Lightmatter_flamingo2.jpg.
Wikipedia. (2005). Phalaenopsis hybrid (Andre Karwath). Retrieved 3-1-07, from http://en.wikipedia.org/wiki/Image:Phalaenopsis_%28aka%29.jpg.
Wikimedia Commons. (2005). Orangutan in Aalborg Zoo, Denmark (Malene Thyssen). Retrieved 3-1-07, from http://commons.wikimedia.org/wiki/Image:Pongo_pygmaeus_%28orangutang%29.jpg.
Wikimedia Commons. (2006). Paradise fish (Macropodus opercularis) (André Karwath). Retrieved 3-1-07, from http://commons.wikimedia.org/wiki/Image:Macropodus_opercularis_-_front_%28aka%29.jpg.
Wikimedia Commons. (2007). A young grasshopper on a grass stalk (fir0002). Retrieved 3-1-07, from http://commons.wikimedia.org/wiki/Image:Young_grasshopper_on_grass_stalk03.jpg.
The Species Problem
Although scientists generally agree that species represent the most basic and fundamental groups into which living and extinct organisms are classified, there is debate over how species should be identified as well as the precise definition of the word species. This controversy, known as the species problem, stems from the quest for a single, universal, and functional conception of species. This essential species concept would have to identify species unambiguously as discreet, natural units in addition to explaining the existence of species and how they arise in nature. While a number of different species concepts have been proposed, none are able to meet these requirements for all species.
In response to this problem, many scientists recommend a pluralistic approach, that is, the recognition that there are multiple ways to think about and define species. Investigators must therefore choose an appropriate species concept based on their needs. For example, a paleontologist who tries to understand prehistoric vertebrates by studying fossils is likely to require and apply a different conceptualization of species than a microbiologist who studies heredity and variation in populations of bacteria.
References:
Campbell, N.E. & Reece, J.B. (2002). Biology (6th ed.). San Francisco: Benjamin Cummings.
Coyne, J.A. & Orr, H.A. (2004). Speciation. Sunderland, MA: Sinauer Associates.
Futuyma, D.J. (2005). Evolution. Sunderland, MA: Sinauer Associates.
Image References:
Wikimedia Commons. (2004). Flamingo (Aaron Logan). Retrieved 3-1-07, from http://commons.wikimedia.org/wiki/Image:Lightmatter_flamingo2.jpg.
Wikipedia. (2005). Phalaenopsis hybrid (Andre Karwath). Retrieved 3-1-07, from http://en.wikipedia.org/wiki/Image:Phalaenopsis_%28aka%29.jpg.
Wikimedia Commons. (2005). Orangutan in Aalborg Zoo, Denmark (Malene Thyssen). Retrieved 3-1-07, from http://commons.wikimedia.org/wiki/Image:Pongo_pygmaeus_%28orangutang%29.jpg.
Wikimedia Commons. (2006). Paradise fish (Macropodus opercularis) (André Karwath). Retrieved 3-1-07, from http://commons.wikimedia.org/wiki/Image:Macropodus_opercularis_-_front_%28aka%29.jpg.
Wikimedia Commons. (2007). A young grasshopper on a grass stalk (fir0002). Retrieved 3-1-07, from http://commons.wikimedia.org/wiki/Image:Young_grasshopper_on_grass_stalk03.jpg.
3. The Biological Species Concept A species is a group of
interbreeding individuals which are
reproductively isolated from other populations.
Reproductive isolation allows species to evolve independently of other species. The Biological Species Concept
Scientists working to understand evolution and speciation (the processes by which species arise), frequently apply the biological species concept. Ernst Mayr, one of the leading evolutionary biologists of the 20th century, provided the most commonly accepted version of the biological species concept, which posits that species are groups of interbreeding natural populations that are reproductively isolated from other such groups.
Mayers work focused on vertebrate animals, a group for which the biological species concept is particularly useful. It also has been applied to plants and other populations. However, most plant biologists also consider evolutionary history and morphology in their species definitions. The biological species concept defines species in terms of interbreeding populations, and is therefore most useful for understanding organisms that reproduce sexually. For these organisms, the formation of a new species involves the accumulation of enough genetic differences within individuals of a population to prevent them from breeding with individuals of other populations a condition referred to as reproductive isolation. When reproductively isolated, species accumulate genetic changes and evolve along independent trajectories. Without reproductive isolation, populations merge in a genetic sense (they share a common gene pool), and thus converge (become similar) in an evolutionary sense.
Many biologists do not interpret the biological species concept rigidly, and characterize species as populations with substantial but not necessarily complete reproductive isolation (Coyne and Orr, 2004).
Reference:
Campbell, N.E. & Reece, J.B. (2002). Biology (6th ed.). San Francisco: Benjamin Cummings.
Coyne, J. and Orr, H. (2004). Speciation. Sunderland, MA: Sinauer Associates, Inc.
Mayr, E. (2002). What Evolution Is. London: Phoenix, p 183.
Image Reference:
Wikipedia. (2006). School of squirrelfish, Sargocentron xantherythrum (James Watt). Retrieved 3-1-07, from http://en.wikipedia.org/wiki/Image:School_of_reef_fish_at_Rapture_Reef%2C_French_Frigate_Shoals.jpg.The Biological Species Concept
Scientists working to understand evolution and speciation (the processes by which species arise), frequently apply the biological species concept. Ernst Mayr, one of the leading evolutionary biologists of the 20th century, provided the most commonly accepted version of the biological species concept, which posits that species are groups of interbreeding natural populations that are reproductively isolated from other such groups.
Mayers work focused on vertebrate animals, a group for which the biological species concept is particularly useful. It also has been applied to plants and other populations. However, most plant biologists also consider evolutionary history and morphology in their species definitions. The biological species concept defines species in terms of interbreeding populations, and is therefore most useful for understanding organisms that reproduce sexually. For these organisms, the formation of a new species involves the accumulation of enough genetic differences within individuals of a population to prevent them from breeding with individuals of other populations a condition referred to as reproductive isolation. When reproductively isolated, species accumulate genetic changes and evolve along independent trajectories. Without reproductive isolation, populations merge in a genetic sense (they share a common gene pool), and thus converge (become similar) in an evolutionary sense.
Many biologists do not interpret the biological species concept rigidly, and characterize species as populations with substantial but not necessarily complete reproductive isolation (Coyne and Orr, 2004).
Reference:
Campbell, N.E. & Reece, J.B. (2002). Biology (6th ed.). San Francisco: Benjamin Cummings.
Coyne, J. and Orr, H. (2004). Speciation. Sunderland, MA: Sinauer Associates, Inc.
Mayr, E. (2002). What Evolution Is. London: Phoenix, p 183.
Image Reference:
Wikipedia. (2006). School of squirrelfish, Sargocentron xantherythrum (James Watt). Retrieved 3-1-07, from http://en.wikipedia.org/wiki/Image:School_of_reef_fish_at_Rapture_Reef%2C_French_Frigate_Shoals.jpg.
4. Are these the same species? The Biological Species Concept
Scientists working to understand evolution and speciation (the processes by which species arise), frequently apply the biological species concept. Ernst Mayr, one of the leading evolutionary biologists of the 20th century, provided the most commonly accepted version of the biological species concept, which posits that species are groups of interbreeding natural populations that are reproductively isolated from other such groups.
Mayers work focused on vertebrate animals, a group for which the biological species concept is particularly useful. It also has been applied to plants and other populations. However, most plant biologists also consider evolutionary history and morphology in their species definitions. The biological species concept defines species in terms of interbreeding populations, and is therefore most useful for understanding organisms that reproduce sexually. For these organisms, the formation of a new species involves the accumulation of enough genetic differences within individuals of a population to prevent them from breeding with individuals of other populations a condition referred to as reproductive isolation. When reproductively isolated, species accumulate genetic changes and evolve along independent trajectories. Without reproductive isolation, populations merge in a genetic sense (they share a common gene pool), and thus converge (become similar) in an evolutionary sense.
Many biologists do not interpret the biological species concept rigidly, and characterize species as populations with substantial but not necessarily complete reproductive isolation (Coyne and Orr, 2004).
Reference:
Campbell, N.E. & Reece, J.B. (2002). Biology (6th ed.). San Francisco: Benjamin Cummings.
Coyne, J. and Orr, H. (2004). Speciation. Sunderland, MA: Sinauer Associates, Inc.
Mayr, E. (2002). What Evolution Is. London: Phoenix, p 183.
Image Reference:
Wikipedia. (2006). School of squirrelfish, Sargocentron xantherythrum (James Watt). Retrieved 3-1-07, from http://en.wikipedia.org/wiki/Image:School_of_reef_fish_at_Rapture_Reef%2C_French_Frigate_Shoals.jpg.The Biological Species Concept
Scientists working to understand evolution and speciation (the processes by which species arise), frequently apply the biological species concept. Ernst Mayr, one of the leading evolutionary biologists of the 20th century, provided the most commonly accepted version of the biological species concept, which posits that species are groups of interbreeding natural populations that are reproductively isolated from other such groups.
Mayers work focused on vertebrate animals, a group for which the biological species concept is particularly useful. It also has been applied to plants and other populations. However, most plant biologists also consider evolutionary history and morphology in their species definitions. The biological species concept defines species in terms of interbreeding populations, and is therefore most useful for understanding organisms that reproduce sexually. For these organisms, the formation of a new species involves the accumulation of enough genetic differences within individuals of a population to prevent them from breeding with individuals of other populations a condition referred to as reproductive isolation. When reproductively isolated, species accumulate genetic changes and evolve along independent trajectories. Without reproductive isolation, populations merge in a genetic sense (they share a common gene pool), and thus converge (become similar) in an evolutionary sense.
Many biologists do not interpret the biological species concept rigidly, and characterize species as populations with substantial but not necessarily complete reproductive isolation (Coyne and Orr, 2004).
Reference:
Campbell, N.E. & Reece, J.B. (2002). Biology (6th ed.). San Francisco: Benjamin Cummings.
Coyne, J. and Orr, H. (2004). Speciation. Sunderland, MA: Sinauer Associates, Inc.
Mayr, E. (2002). What Evolution Is. London: Phoenix, p 183.
Image Reference:
Wikipedia. (2006). School of squirrelfish, Sargocentron xantherythrum (James Watt). Retrieved 3-1-07, from http://en.wikipedia.org/wiki/Image:School_of_reef_fish_at_Rapture_Reef%2C_French_Frigate_Shoals.jpg.
5. And these? The Biological Species Concept
Scientists working to understand evolution and speciation (the processes by which species arise), frequently apply the biological species concept. Ernst Mayr, one of the leading evolutionary biologists of the 20th century, provided the most commonly accepted version of the biological species concept, which posits that species are groups of interbreeding natural populations that are reproductively isolated from other such groups.
Mayers work focused on vertebrate animals, a group for which the biological species concept is particularly useful. It also has been applied to plants and other populations. However, most plant biologists also consider evolutionary history and morphology in their species definitions. The biological species concept defines species in terms of interbreeding populations, and is therefore most useful for understanding organisms that reproduce sexually. For these organisms, the formation of a new species involves the accumulation of enough genetic differences within individuals of a population to prevent them from breeding with individuals of other populations a condition referred to as reproductive isolation. When reproductively isolated, species accumulate genetic changes and evolve along independent trajectories. Without reproductive isolation, populations merge in a genetic sense (they share a common gene pool), and thus converge (become similar) in an evolutionary sense.
Many biologists do not interpret the biological species concept rigidly, and characterize species as populations with substantial but not necessarily complete reproductive isolation (Coyne and Orr, 2004).
Reference:
Campbell, N.E. & Reece, J.B. (2002). Biology (6th ed.). San Francisco: Benjamin Cummings.
Coyne, J. and Orr, H. (2004). Speciation. Sunderland, MA: Sinauer Associates, Inc.
Mayr, E. (2002). What Evolution Is. London: Phoenix, p 183.
Image Reference:
Wikipedia. (2006). School of squirrelfish, Sargocentron xantherythrum (James Watt). Retrieved 3-1-07, from http://en.wikipedia.org/wiki/Image:School_of_reef_fish_at_Rapture_Reef%2C_French_Frigate_Shoals.jpg.The Biological Species Concept
Scientists working to understand evolution and speciation (the processes by which species arise), frequently apply the biological species concept. Ernst Mayr, one of the leading evolutionary biologists of the 20th century, provided the most commonly accepted version of the biological species concept, which posits that species are groups of interbreeding natural populations that are reproductively isolated from other such groups.
Mayers work focused on vertebrate animals, a group for which the biological species concept is particularly useful. It also has been applied to plants and other populations. However, most plant biologists also consider evolutionary history and morphology in their species definitions. The biological species concept defines species in terms of interbreeding populations, and is therefore most useful for understanding organisms that reproduce sexually. For these organisms, the formation of a new species involves the accumulation of enough genetic differences within individuals of a population to prevent them from breeding with individuals of other populations a condition referred to as reproductive isolation. When reproductively isolated, species accumulate genetic changes and evolve along independent trajectories. Without reproductive isolation, populations merge in a genetic sense (they share a common gene pool), and thus converge (become similar) in an evolutionary sense.
Many biologists do not interpret the biological species concept rigidly, and characterize species as populations with substantial but not necessarily complete reproductive isolation (Coyne and Orr, 2004).
Reference:
Campbell, N.E. & Reece, J.B. (2002). Biology (6th ed.). San Francisco: Benjamin Cummings.
Coyne, J. and Orr, H. (2004). Speciation. Sunderland, MA: Sinauer Associates, Inc.
Mayr, E. (2002). What Evolution Is. London: Phoenix, p 183.
Image Reference:
Wikipedia. (2006). School of squirrelfish, Sargocentron xantherythrum (James Watt). Retrieved 3-1-07, from http://en.wikipedia.org/wiki/Image:School_of_reef_fish_at_Rapture_Reef%2C_French_Frigate_Shoals.jpg.
6. Characteristics of species: Reproductive Isolation
Barriers to interbreeding and successful gene flow exist between species.
Categories of reproductive isolation barriers :
Premating/prezygotic isolation (before mating occurs)
Postmating/prezygotic isolation (after mating, but before fertilization occurs)
Postmating /postzygotic isolation (after mating and after fertilization) Reproductive Isolation
The divergence of species from parent or ancestral populations is reinforced by reproductive barriers that limit exchange of genes with members of other populations. These isolating barriers, originally described by Dobzhanky (1937) as isolation mechanisms, are defined as biological properties of individual organisms that impede gene exchange with members of other populations. External factors, such as geographic barriers are not considered to be reproductive isolating barriers.
Reproductive isolation barriers can be categorized in different ways: relative to the act of mating or relative to the production of zygotes. Premating/prezygotic isolation barriers impede mating between members of different species. However, if members of different species do mate, postmating/prezygotic isolation barriers prevent the formation of a zygote. If these isolating barriers fail and a zygote is formed after the members of two species mate, postmating/postzygotic isolation barriers reduce the viability and/or fertility of the zygote.
Reference:
Campbell, N.E. & Reece, J.B. (2002). Biology (6th ed.). San Francisco: Benjamin Cummings.Reproductive Isolation
The divergence of species from parent or ancestral populations is reinforced by reproductive barriers that limit exchange of genes with members of other populations. These isolating barriers, originally described by Dobzhanky (1937) as isolation mechanisms, are defined as biological properties of individual organisms that impede gene exchange with members of other populations. External factors, such as geographic barriers are not considered to be reproductive isolating barriers.
Reproductive isolation barriers can be categorized in different ways: relative to the act of mating or relative to the production of zygotes. Premating/prezygotic isolation barriers impede mating between members of different species. However, if members of different species do mate, postmating/prezygotic isolation barriers prevent the formation of a zygote. If these isolating barriers fail and a zygote is formed after the members of two species mate, postmating/postzygotic isolation barriers reduce the viability and/or fertility of the zygote.
Reference:
Campbell, N.E. & Reece, J.B. (2002). Biology (6th ed.). San Francisco: Benjamin Cummings.
7. Premating / Prezygotic Ecological Isolation
Reproductive isolation arises when species occupy different habitats.
Example: lions and tigers occupy different habitats within the same geographic area, yet do not interbreed. Ecological Isolation
Ecological isolation occurs when different species live in the same geographic area but occupy different habitats within that area. These barriers are byproducts of different adaptations to local environments. Under these circumstances, individuals of different species do not hybridize simply because they rarely encounter one another.
For example, until recently, the natural ranges of lions and tigers in India overlapped. However, these two species have different habitats: lions live and breed in the open grasslands while tigers generally stay in the forest. Thus, even though lions and tigers technically can mate and produce viable offspring, this rarely, if ever occurs in natural settings.
Ecological isolation is a premating and prezygotic isolating barrier.
Reference:
Campbell, N.E. & Reece, J.B. (2002). Biology (6th ed.). San Francisco: Benjamin Cummings.
Raven, P.H., Johnson, G.B., Losos, J.B., Singer, S.R. (2005). Biology (7th ed.). New York: McGraw-Hill.
Image Reference:
Wikipedia. (2006). A lion in Namibia (yaaay). Retrieved 2-2-09, from http://en.wikipedia.org/wiki/File:Lion_waiting_in_Nambia.jpg.
Wikipedia. (2006). Tiger (Mila Zinkova). Retrieved 2-2-09, from http://en.wikipedia.org/wiki/File:Siberian_Tiger_sf.jpg.Ecological Isolation
Ecological isolation occurs when different species live in the same geographic area but occupy different habitats within that area. These barriers are byproducts of different adaptations to local environments. Under these circumstances, individuals of different species do not hybridize simply because they rarely encounter one another.
For example, until recently, the natural ranges of lions and tigers in India overlapped. However, these two species have different habitats: lions live and breed in the open grasslands while tigers generally stay in the forest. Thus, even though lions and tigers technically can mate and produce viable offspring, this rarely, if ever occurs in natural settings.
Ecological isolation is a premating and prezygotic isolating barrier.
Reference:
Campbell, N.E. & Reece, J.B. (2002). Biology (6th ed.). San Francisco: Benjamin Cummings.
Raven, P.H., Johnson, G.B., Losos, J.B., Singer, S.R. (2005). Biology (7th ed.). New York: McGraw-Hill.
Image Reference:
Wikipedia. (2006). A lion in Namibia (yaaay). Retrieved 2-2-09, from http://en.wikipedia.org/wiki/File:Lion_waiting_in_Nambia.jpg.
Wikipedia. (2006). Tiger (Mila Zinkova). Retrieved 2-2-09, from http://en.wikipedia.org/wiki/File:Siberian_Tiger_sf.jpg.
8. Premating / Prezygotic Example: lions and tigers occupy different habitats within the same geographic area, yet do not interbreed (in nature).
But in captivity: Ecological Isolation
Ecological isolation occurs when different species live in the same geographic area but occupy different habitats within that area. These barriers are byproducts of different adaptations to local environments. Under these circumstances, individuals of different species do not hybridize simply because they rarely encounter one another.
For example, until recently, the natural ranges of lions and tigers in India overlapped. However, these two species have different habitats: lions live and breed in the open grasslands while tigers generally stay in the forest. Thus, even though lions and tigers technically can mate and produce viable offspring, this rarely, if ever occurs in natural settings.
Ecological isolation is a premating and prezygotic isolating barrier.
Reference:
Campbell, N.E. & Reece, J.B. (2002). Biology (6th ed.). San Francisco: Benjamin Cummings.
Raven, P.H., Johnson, G.B., Losos, J.B., Singer, S.R. (2005). Biology (7th ed.). New York: McGraw-Hill.
Image Reference:
Wikipedia. (2006). A lion in Namibia (yaaay). Retrieved 2-2-09, from http://en.wikipedia.org/wiki/File:Lion_waiting_in_Nambia.jpg.
Wikipedia. (2006). Tiger (Mila Zinkova). Retrieved 2-2-09, from http://en.wikipedia.org/wiki/File:Siberian_Tiger_sf.jpg.Ecological Isolation
Ecological isolation occurs when different species live in the same geographic area but occupy different habitats within that area. These barriers are byproducts of different adaptations to local environments. Under these circumstances, individuals of different species do not hybridize simply because they rarely encounter one another.
For example, until recently, the natural ranges of lions and tigers in India overlapped. However, these two species have different habitats: lions live and breed in the open grasslands while tigers generally stay in the forest. Thus, even though lions and tigers technically can mate and produce viable offspring, this rarely, if ever occurs in natural settings.
Ecological isolation is a premating and prezygotic isolating barrier.
Reference:
Campbell, N.E. & Reece, J.B. (2002). Biology (6th ed.). San Francisco: Benjamin Cummings.
Raven, P.H., Johnson, G.B., Losos, J.B., Singer, S.R. (2005). Biology (7th ed.). New York: McGraw-Hill.
Image Reference:
Wikipedia. (2006). A lion in Namibia (yaaay). Retrieved 2-2-09, from http://en.wikipedia.org/wiki/File:Lion_waiting_in_Nambia.jpg.
Wikipedia. (2006). Tiger (Mila Zinkova). Retrieved 2-2-09, from http://en.wikipedia.org/wiki/File:Siberian_Tiger_sf.jpg.
9. Premating / Prezygotic Temporal Isolation
Differences in the timing of mating prevent different species from interbreeding.
Example: different frog species live and breed in the same pond, but they reproduce at different times of the year. Temporal Isolation
Temporal isolation occurs when members of two species occupy similar habitats, but breed at different times. Thus, gene flow between populations is impeded even when the species populations occupy the same habitat within the same geographic area.
Different species of frogs, for example, may share the same pond for reproduction, but will not hybridize because they use the pond at different times of the year. Similarly, differences in the flowering times of two closely related species of plants can keep them from being cross-pollinated by one another.
Temporal isolation prevents mating between different species, and therefore also prevents the formation of a zygote, so it is a premating isolating barrier as well as a prezygotic isolating barrier.
Reference:
Campbell, N.E. & Reece, J.B. (2002). Biology (6th ed.). San Francisco: Benjamin Cummings.
Image Reference:
Wikimedia Commons. (2006). Common Water Frog (Rana esculenta) (Holger Groschl). Retrieved 3-1-07, from http://commons.wikimedia.org/wiki/Image:Wasserfrosch.jpg.Temporal Isolation
Temporal isolation occurs when members of two species occupy similar habitats, but breed at different times. Thus, gene flow between populations is impeded even when the species populations occupy the same habitat within the same geographic area.
Different species of frogs, for example, may share the same pond for reproduction, but will not hybridize because they use the pond at different times of the year. Similarly, differences in the flowering times of two closely related species of plants can keep them from being cross-pollinated by one another.
Temporal isolation prevents mating between different species, and therefore also prevents the formation of a zygote, so it is a premating isolating barrier as well as a prezygotic isolating barrier.
Reference:
Campbell, N.E. & Reece, J.B. (2002). Biology (6th ed.). San Francisco: Benjamin Cummings.
Image Reference:
Wikimedia Commons. (2006). Common Water Frog (Rana esculenta) (Holger Groschl). Retrieved 3-1-07, from http://commons.wikimedia.org/wiki/Image:Wasserfrosch.jpg.
10. Premating / Prezygotic Behavioral Isolation
Differences in mating behavior lead to reproductive isolation.
Examples:
courtship displays
pheromones
bird songs Ethological Isolation
Ethological (behavioral) isolation results from differences in courtship or mating behavior that keep members of different species from mating, even when they inhabit the same geographic area. These barriers often consist of special signals or elaborate behaviors that are used by members of a species to attract or recognize and accept mates. Examples include courtship displays (such as with the blue-footed boobies in this photo), pheromones (chemical signals), and the songs of birds.
Ethological isolation specifically prevents mating between different species and therefore also prevents the formation of a zygote. It is a premating isolating barrier as well as a prezygotic isolating barrier.
Reference:
Campbell, N.E. & Reece, J.B. (2002). Biology (6th ed.). San Francisco: Benjamin Cummings.
Image Reference:
Wikimedia Commons. (2008). Two Blue Footed Boobies (Hersfold). Retrieved 2-2-09, from http://commons.wikimedia.org/wiki/File:Blue-footed_Booby_Comparison.jpg.Ethological Isolation
Ethological (behavioral) isolation results from differences in courtship or mating behavior that keep members of different species from mating, even when they inhabit the same geographic area. These barriers often consist of special signals or elaborate behaviors that are used by members of a species to attract or recognize and accept mates. Examples include courtship displays (such as with the blue-footed boobies in this photo), pheromones (chemical signals), and the songs of birds.
Ethological isolation specifically prevents mating between different species and therefore also prevents the formation of a zygote. It is a premating isolating barrier as well as a prezygotic isolating barrier.
Reference:
Campbell, N.E. & Reece, J.B. (2002). Biology (6th ed.). San Francisco: Benjamin Cummings.
Image Reference:
Wikimedia Commons. (2008). Two Blue Footed Boobies (Hersfold). Retrieved 2-2-09, from http://commons.wikimedia.org/wiki/File:Blue-footed_Booby_Comparison.jpg.
11. Premating / Prezygotic Mechanical Isolation
Anatomical differences between species make mating physically impossible.
Examples:
shape of the genitalia
body size Ethological Isolation
Ethological (behavioral) isolation results from differences in courtship or mating behavior that keep members of different species from mating, even when they inhabit the same geographic area. These barriers often consist of special signals or elaborate behaviors that are used by members of a species to attract or recognize and accept mates. Examples include courtship displays (such as with the blue-footed boobies in this photo), pheromones (chemical signals), and the songs of birds.
Ethological isolation specifically prevents mating between different species and therefore also prevents the formation of a zygote. It is a premating isolating barrier as well as a prezygotic isolating barrier.
Reference:
Campbell, N.E. & Reece, J.B. (2002). Biology (6th ed.). San Francisco: Benjamin Cummings.
Image Reference:
Wikimedia Commons. (2008). Two Blue Footed Boobies (Hersfold). Retrieved 2-2-09, from http://commons.wikimedia.org/wiki/File:Blue-footed_Booby_Comparison.jpg.Ethological Isolation
Ethological (behavioral) isolation results from differences in courtship or mating behavior that keep members of different species from mating, even when they inhabit the same geographic area. These barriers often consist of special signals or elaborate behaviors that are used by members of a species to attract or recognize and accept mates. Examples include courtship displays (such as with the blue-footed boobies in this photo), pheromones (chemical signals), and the songs of birds.
Ethological isolation specifically prevents mating between different species and therefore also prevents the formation of a zygote. It is a premating isolating barrier as well as a prezygotic isolating barrier.
Reference:
Campbell, N.E. & Reece, J.B. (2002). Biology (6th ed.). San Francisco: Benjamin Cummings.
Image Reference:
Wikimedia Commons. (2008). Two Blue Footed Boobies (Hersfold). Retrieved 2-2-09, from http://commons.wikimedia.org/wiki/File:Blue-footed_Booby_Comparison.jpg.
12. Postmating / Prezygotic Gametic Isolation
Reproductive isolation arises when the gametes of different species cannot fuse.
Example: different species of sea urchin shed gametes at the same time and place but their gametes do not fuse. Gametic Isolation
When the gametes of one species cannot fuse with the gametes of another species to form a zygote, gametic isolation has occurred. This is a kind of lock-and-key isolating mechanism in which, despite successful mating or pollination, hybridization will not occur because the gametes of one species function poorly with the gametes or reproductive tract of another species.
Sea urchins provide a good example of this type of reproductive isolating barrier. Many sea urchin species live in sympatry (within the same geographic region) and shed their gametes at the same time (no temporal isolation), but remain evolutionarily distinct. In this case, the formation of hybrid zygotes is prevented because the surface proteins of the ovule (the lock) and sperm, or male gametes (the keys) of different species do not fit together.
Gametic isolation is a postmating isolating barrier while still a prezygotic isolating barrier.
Reference:
Campbell, N.E. & Reece, J.B. (2002). Biology (6th ed.). San Francisco: Benjamin Cummings.
Summers, R.G. & Hylander, B.L. (1975). Species-specificity of acrosome reaction and primary gamete binding in echinoids. Experimental Cell Research 96:63-68.
Image Reference:
Wikipedia. (2008). Strongylocentrotus franciscanus (Kirt L. Onthank). Retrieved 2-2-09, from http://en.wikipedia.org/wiki/File:Strongylocentrotus_franciscanus.jpg. Gametic Isolation
When the gametes of one species cannot fuse with the gametes of another species to form a zygote, gametic isolation has occurred. This is a kind of lock-and-key isolating mechanism in which, despite successful mating or pollination, hybridization will not occur because the gametes of one species function poorly with the gametes or reproductive tract of another species.
Sea urchins provide a good example of this type of reproductive isolating barrier. Many sea urchin species live in sympatry (within the same geographic region) and shed their gametes at the same time (no temporal isolation), but remain evolutionarily distinct. In this case, the formation of hybrid zygotes is prevented because the surface proteins of the ovule (the lock) and sperm, or male gametes (the keys) of different species do not fit together.
Gametic isolation is a postmating isolating barrier while still a prezygotic isolating barrier.
Reference:
Campbell, N.E. & Reece, J.B. (2002). Biology (6th ed.). San Francisco: Benjamin Cummings.
Summers, R.G. & Hylander, B.L. (1975). Species-specificity of acrosome reaction and primary gamete binding in echinoids. Experimental Cell Research 96:63-68.
Image Reference:
Wikipedia. (2008). Strongylocentrotus franciscanus (Kirt L. Onthank). Retrieved 2-2-09, from http://en.wikipedia.org/wiki/File:Strongylocentrotus_franciscanus.jpg.
13. Postmating / Postzygotic Hybrids
The offspring of parents from two genetically distinct populations is known as a hybrid.
Postzygotic isolating barriers frequently reduce the viability or fertility of hybrids.
Reproduction of fertile hybrids can lead to formation of new species. Hybrids
The reproductive isolation mechanisms discussed so far are all examples of prezygotic isolation, where gene flow between species is prevented often before mating can occur and before a zygote can form.. In contrast, postzygotic isolation occurs after members of two different species have mated and fertilization has occurred. The offspring of such crosses are called hybrids. In most cases, the viability or fertility is reduced, and can be considered a postzygotic isolating barrier. Sometimes, however, hybrid offspring and their progeny are viable and fertile. When successful hybrid populations arise, the lines between existing natural species or new species (particularly in plants) can become blurred.
For example, as we discussed previously, lions and tigers rarely meet in their natural environments. However, if held together in captivity, they will interbreed. A liger is the offspring of a male lion and a female tiger, and a tigon is a hybrid between a male tiger and a female lion.
Reference:
Campbell, N.E. & Reece, J.B. (2002). Biology (6th ed.). San Francisco: Benjamin Cummings.
Public Broadcasting Service. (2001). Evolution. Retrieved 04-13-07 from http://www.pbs.org/wgbh/evolution/library/05/2/l_052_02.html
Image Reference:
Wikipedia. (2007). Liger couple (Hkandy). Retrieved 2-2-09, from http://en.wikipedia.org/wiki/File:Liger_couple.jpg.Hybrids
The reproductive isolation mechanisms discussed so far are all examples of prezygotic isolation, where gene flow between species is prevented often before mating can occur and before a zygote can form.. In contrast, postzygotic isolation occurs after members of two different species have mated and fertilization has occurred. The offspring of such crosses are called hybrids. In most cases, the viability or fertility is reduced, and can be considered a postzygotic isolating barrier. Sometimes, however, hybrid offspring and their progeny are viable and fertile. When successful hybrid populations arise, the lines between existing natural species or new species (particularly in plants) can become blurred.
For example, as we discussed previously, lions and tigers rarely meet in their natural environments. However, if held together in captivity, they will interbreed. A liger is the offspring of a male lion and a female tiger, and a tigon is a hybrid between a male tiger and a female lion.
Reference:
Campbell, N.E. & Reece, J.B. (2002). Biology (6th ed.). San Francisco: Benjamin Cummings.
Public Broadcasting Service. (2001). Evolution. Retrieved 04-13-07 from http://www.pbs.org/wgbh/evolution/library/05/2/l_052_02.html
Image Reference:
Wikipedia. (2007). Liger couple (Hkandy). Retrieved 2-2-09, from http://en.wikipedia.org/wiki/File:Liger_couple.jpg.
14. Postmating / Postzygotic Hybrid Inviability
Hybrid zygote fails to develop into an adult hybrid.
Example: frogs of the genus Rana can form hybrid tadpoles but the tadpoles die before becoming adults. Hybrid Inviability
When hybrid zygotes are formed, they frequently die at some point during their development due to genetic incompatibility between the two parent species. This isolation mechanism, known as hybrid inviability, prevents the genomes of the two species from mixing. In this way, the gene flow between the two populations is impeded.
One example of hybrid inviability occurs in the frog genus, Rana. Interbreeding between some species of Rana results in the formation of hybrid tadpoles, but the tadpoles die before they become reproductive adults. Thus, the populations corresponding to different species remain genetically isolated.
Hybrid inviability is a postmating isolating barrier and also a postzygotic isolating barrier.
Reference:
Campbell, N.E. & Reece, J.B. (2002). Biology (6th ed.). San Francisco: Benjamin Cummings.
Moore, J.A. (1950). Further studies on Rana pipiens racial hybrids. The American Naturalist 84:247-254.
Image Reference:
Wikimedia Commons. (2006). North American Bullfrog (Rana catesbeiana) (Carl D. Howe). Retrieved 2-2-09, fromhttp://en.wikipedia.org/wiki/File:North-American-bullfrog1.jpg. Hybrid Inviability
When hybrid zygotes are formed, they frequently die at some point during their development due to genetic incompatibility between the two parent species. This isolation mechanism, known as hybrid inviability, prevents the genomes of the two species from mixing. In this way, the gene flow between the two populations is impeded.
One example of hybrid inviability occurs in the frog genus, Rana. Interbreeding between some species of Rana results in the formation of hybrid tadpoles, but the tadpoles die before they become reproductive adults. Thus, the populations corresponding to different species remain genetically isolated.
Hybrid inviability is a postmating isolating barrier and also a postzygotic isolating barrier.
Reference:
Campbell, N.E. & Reece, J.B. (2002). Biology (6th ed.). San Francisco: Benjamin Cummings.
Moore, J.A. (1950). Further studies on Rana pipiens racial hybrids. The American Naturalist 84:247-254.
Image Reference:
Wikimedia Commons. (2006). North American Bullfrog (Rana catesbeiana) (Carl D. Howe). Retrieved 2-2-09, fromhttp://en.wikipedia.org/wiki/File:North-American-bullfrog1.jpg.
15. Postmating / Postzygotic Hybrid Sterility
Hybrids live to adulthood but cannot produce viable gametes.
Examples of sterile hybrids:
Mules are the sterile offspring of a female horse and a male donkey.
In Drosophilas male hybrids are sterile while female hybrids usually are fertile.
Hybrid Sterility
In some cases, hybrid zygotes successfully develop into adults but the adult individuals do not produce viable gametes. This isolating barrier, known as hybrid sterility, restricts the amount of genome mixing that can occur across two species.
A classic example of a sterile hybrid is the mule, the offspring of a female horse and a male donkey. All mules typically are sterile. However, within other taxonomic groups, hybrid sterility is displayed in only one sex. For example, in the fly genus Drosophila, crosses between different species produce sterile male hybrids and female hybrids that usually are fertile.
When hybrid sterility affects one sex preferentially, it is generally the heterogametic sex (the sex with two different sex chromosomes) that is sterile, while the homogametic sex (the sex with a matching pair of sex chromosomes) is fertile. This is known as Haldanes rule. In most mammals and insects, including flies, the males are heterogametic and the females are homogametic. Birds and reptiles are the opposite. That is, the males are homogametic and the females are heterogametic.
Hybrid sterility is both a postmating and a postzygotic isolating barrier.
Reference:
Campbell, N.E. & Reece, J.B. (2002). Biology (6th ed.). San Francisco: Benjamin Cummings.
Orr, H.A. & Presgraves, D.C. (2000). Speciation by postzygotic isolation: forces, genes and molecules. BioEssays, 22:1085-1094.
Ridley, M. (2004). Evolution (3rd ed.). Oxford: Blackwell Publishing Limited.
Image Reference:
Wikimedia Commons. (2004). Mules in south Italy (Einar Faanes). Retrieved 2-2-09, from http://commons.wikimedia.org/wiki/File:Mule.jpg.Hybrid Sterility
In some cases, hybrid zygotes successfully develop into adults but the adult individuals do not produce viable gametes. This isolating barrier, known as hybrid sterility, restricts the amount of genome mixing that can occur across two species.
A classic example of a sterile hybrid is the mule, the offspring of a female horse and a male donkey. All mules typically are sterile. However, within other taxonomic groups, hybrid sterility is displayed in only one sex. For example, in the fly genus Drosophila, crosses between different species produce sterile male hybrids and female hybrids that usually are fertile.
When hybrid sterility affects one sex preferentially, it is generally the heterogametic sex (the sex with two different sex chromosomes) that is sterile, while the homogametic sex (the sex with a matching pair of sex chromosomes) is fertile. This is known as Haldanes rule. In most mammals and insects, including flies, the males are heterogametic and the females are homogametic. Birds and reptiles are the opposite. That is, the males are homogametic and the females are heterogametic.
Hybrid sterility is both a postmating and a postzygotic isolating barrier.
Reference:
Campbell, N.E. & Reece, J.B. (2002). Biology (6th ed.). San Francisco: Benjamin Cummings.
Orr, H.A. & Presgraves, D.C. (2000). Speciation by postzygotic isolation: forces, genes and molecules. BioEssays, 22:1085-1094.
Ridley, M. (2004). Evolution (3rd ed.). Oxford: Blackwell Publishing Limited.
Image Reference:
Wikimedia Commons. (2004). Mules in south Italy (Einar Faanes). Retrieved 2-2-09, from http://commons.wikimedia.org/wiki/File:Mule.jpg.
16. Limitations of the Biological Species Concept Limitations of the Biological Species Concept
While the biological species concept has had a profound influence on the development of current evolutionary theory, it has limitations. For example, because the biological species concept defines species in terms of interbreeding, it cannot be used to determine if similar populations that are geographically separated belong to the same species, nor is it useful for the classification of species in extinct populations. In addition, the biological species concept is most applicable to organisms that reproduce sexually and is less useful for the classification of organisms that reproduce asexually, including single-celled organisms like bacteria, many plant species, and even some vertebrates. The biological species concept also can be problematic when reproductive isolating barriers are incomplete. Biologists would consider this strong evidence of reproductive isolation. For example, some estimates suggest that 10% of bird species hybridize. Hybridization is even more common among flowering plants. These limitations of the biological species concept have led scientists to propose alternative species concepts.
References:
Campbell, N.E. & Reece, J.B. (2002). Biology (6th ed.). San Francisco: Benjamin Cummings.
Coyne, J.A. & Orr, H.A. (2004). Speciation. Sunderland, MA: Sinauer Associates.
Futuyma, D.J. (2005). Evolution. Sunderland, MA: Sinauer Associates.
Image Reference:
Wikimedia Commons. (2005). Scanning electron micrograph of Escherichia coli (Rocky Mountain Laboratories, NIAID, NIH). Retrieved 2-2-09, from http://commons.wikimedia.org/wiki/File:EscherichiaColi_NIAID.jpg. Limitations of the Biological Species Concept
While the biological species concept has had a profound influence on the development of current evolutionary theory, it has limitations. For example, because the biological species concept defines species in terms of interbreeding, it cannot be used to determine if similar populations that are geographically separated belong to the same species, nor is it useful for the classification of species in extinct populations. In addition, the biological species concept is most applicable to organisms that reproduce sexually and is less useful for the classification of organisms that reproduce asexually, including single-celled organisms like bacteria, many plant species, and even some vertebrates. The biological species concept also can be problematic when reproductive isolating barriers are incomplete. Biologists would consider this strong evidence of reproductive isolation. For example, some estimates suggest that 10% of bird species hybridize. Hybridization is even more common among flowering plants. These limitations of the biological species concept have led scientists to propose alternative species concepts.
References:
Campbell, N.E. & Reece, J.B. (2002). Biology (6th ed.). San Francisco: Benjamin Cummings.
Coyne, J.A. & Orr, H.A. (2004). Speciation. Sunderland, MA: Sinauer Associates.
Futuyma, D.J. (2005). Evolution. Sunderland, MA: Sinauer Associates.
Image Reference:
Wikimedia Commons. (2005). Scanning electron micrograph of Escherichia coli (Rocky Mountain Laboratories, NIAID, NIH). Retrieved 2-2-09, from http://commons.wikimedia.org/wiki/File:EscherichiaColi_NIAID.jpg.
17. Other Species Concepts Alternatives to the biological species concept establish different criteria for the definition and identification of species.
Alternative species concepts include:
phylogenetic species concept;
morphological species concept;
ecological species concept; and
recognition species concept.
There is no universally applicable species concept. Other Species Concepts
Alternatives to the biological species concept establish different criteria for the definition and identification of species. One alternative is the phylogenetic species concept, which defines species as discreet, irreducible groups of organisms that are diagnosably different from other groups and share a common ancestor. Subtle variations of this idea have been proposed, such as the genealogical species concept that defines a species as a group whose members are more closely related to one another than to any organism outside the group. Together, these concepts highlight the role of evolutionary history and genetic divergence in the process of speciation. Phylogenetic relationships are often inferred through the use of quantitative methods. For example, current molecular techniques now permit the direct comparison of genetic information to create groupings or to assign individuals to specific species groups. Before the advent and wide use of DNA, and other sequencing information, phylogenetic relationships were inferred from morphology, geographical distribution, and other characteristics related to phenotype.
While the phylogenetic species concepts are concerned with the identification of historically related groups, a number of alternative species concepts emphasize the origins of the discreet groups seen in nature. For example, the ecological species concept defines species as a group of organisms that has adapted to a particular niche in an environment and evolves independently from all groups outside of its range. Thus, species are identified by the use of a common set of environmental resources. In contrast, the recognition species concept defines a species as a group of biparental organisms that share a common fertilization system. According to this species concept, species are identified as a group of organisms that only recognize other members of the group as potential mates.
These, and other species concepts, have been proposed to address some of the limitations of the biological species concept. For example, both the ecological and phenotypic species concepts are used to define species in groups that reproduce asexually, for which the biological species concept is not useful.
References:
Campbell, N.E. & Reece, J.B. (2002). Biology (6th ed.). San Francisco: Benjamin Cummings.
Coyne, J.A. & Orr, H.A. (2004). Speciation. Sunderland, MA: Sinauer Associates.
Futuyma, D.J. (2005). Evolution. Sunderland, MA: Sinauer Associates.
Other Species Concepts
Alternatives to the biological species concept establish different criteria for the definition and identification of species. One alternative is the phylogenetic species concept, which defines species as discreet, irreducible groups of organisms that are diagnosably different from other groups and share a common ancestor. Subtle variations of this idea have been proposed, such as the genealogical species concept that defines a species as a group whose members are more closely related to one another than to any organism outside the group. Together, these concepts highlight the role of evolutionary history and genetic divergence in the process of speciation. Phylogenetic relationships are often inferred through the use of quantitative methods. For example, current molecular techniques now permit the direct comparison of genetic information to create groupings or to assign individuals to specific species groups. Before the advent and wide use of DNA, and other sequencing information, phylogenetic relationships were inferred from morphology, geographical distribution, and other characteristics related to phenotype.
While the phylogenetic species concepts are concerned with the identification of historically related groups, a number of alternative species concepts emphasize the origins of the discreet groups seen in nature. For example, the ecological species concept defines species as a group of organisms that has adapted to a particular niche in an environment and evolves independently from all groups outside of its range. Thus, species are identified by the use of a common set of environmental resources. In contrast, the recognition species concept defines a species as a group of biparental organisms that share a common fertilization system. According to this species concept, species are identified as a group of organisms that only recognize other members of the group as potential mates.
These, and other species concepts, have been proposed to address some of the limitations of the biological species concept. For example, both the ecological and phenotypic species concepts are used to define species in groups that reproduce asexually, for which the biological species concept is not useful.
References:
Campbell, N.E. & Reece, J.B. (2002). Biology (6th ed.). San Francisco: Benjamin Cummings.
Coyne, J.A. & Orr, H.A. (2004). Speciation. Sunderland, MA: Sinauer Associates.
Futuyma, D.J. (2005). Evolution. Sunderland, MA: Sinauer Associates.
18. Modes of Speciation Modes of speciation address the origins of reproductive isolation.
There are four geographic modes of speciation:
allopatric
peripatric
parapatric
sympatric
Modes of Speciation
Many biologists interpret species as the fundamental unit into which populations of organisms can be classified. However, what is considered to constitute a species varies by taxonomic group as well as the purpose for which this classification is being used. The biological species concept, which defines species as interbreeding populations that are reproductively isolated from other populations, is a useful definition, particularly for thinking about how many species of sexually reproducing animals and plants arise.
There are four geographic modes of speciation that are based on the extent to which the incipient species (populations that are in the process of forming distinct species) are geographically isolated from one another. Allopatric speciation occurs in populations that became separated by a geographic barrier. Peripatric speciation, also known as founder effect speciation, is a special type of allopatric speciation. It occurs when a small population becomes isolated from its parent species. Parapatric speciation arises between neighboring populations that share small zones of contact and exhibit modest gene exchange. Sympatric speciation occurs within a single, freely interbreeding population, and is believed to occur only rarely. These modes were first described for animals and also are useful for distinguishing patterns of speciation in plants.
References:
Campbell, N.E. & Reece, J.B. (2002). Biology (6th ed.). San Francisco: Benjamin Cummings.
Coyne, J.A. & Orr, H.A. (2004). Speciation. Sunderland, MA: Sinauer Associates.
Futuyma, D.J. (2005). Evolution. Sunderland, MA: Sinauer Associates.
Mayr, E. (1976). Evolution and the diversity of life. Cambridge, MA: Belknap Press of Harvard University Press.
Simpson, G. (1951). The species concept. Evolution, Vol. 5, No. 4: 285-298.
Image Reference:
Wikimedia Commons. (2006). Speciation modes (Ilmari Karonen). Retrieved on 3-1-07, from http://commons.wikimedia.org/wiki/File:Speciation_modes.svg.Modes of Speciation
Many biologists interpret species as the fundamental unit into which populations of organisms can be classified. However, what is considered to constitute a species varies by taxonomic group as well as the purpose for which this classification is being used. The biological species concept, which defines species as interbreeding populations that are reproductively isolated from other populations, is a useful definition, particularly for thinking about how many species of sexually reproducing animals and plants arise.
There are four geographic modes of speciation that are based on the extent to which the incipient species (populations that are in the process of forming distinct species) are geographically isolated from one another. Allopatric speciation occurs in populations that became separated by a geographic barrier. Peripatric speciation, also known as founder effect speciation, is a special type of allopatric speciation. It occurs when a small population becomes isolated from its parent species. Parapatric speciation arises between neighboring populations that share small zones of contact and exhibit modest gene exchange. Sympatric speciation occurs within a single, freely interbreeding population, and is believed to occur only rarely. These modes were first described for animals and also are useful for distinguishing patterns of speciation in plants.
References:
Campbell, N.E. & Reece, J.B. (2002). Biology (6th ed.). San Francisco: Benjamin Cummings.
Coyne, J.A. & Orr, H.A. (2004). Speciation. Sunderland, MA: Sinauer Associates.
Futuyma, D.J. (2005). Evolution. Sunderland, MA: Sinauer Associates.
Mayr, E. (1976). Evolution and the diversity of life. Cambridge, MA: Belknap Press of Harvard University Press.
Simpson, G. (1951). The species concept. Evolution, Vol. 5, No. 4: 285-298.
Image Reference:
Wikimedia Commons. (2006). Speciation modes (Ilmari Karonen). Retrieved on 3-1-07, from http://commons.wikimedia.org/wiki/File:Speciation_modes.svg.
19. Allopatric Speciation Reproductive isolation evolves in geographically isolated populations.
Genetic divergence arises largely through natural selection, mutation, and genetic drift.
Believed to be the most common mode of speciation.
Allopatric Speciation
Allopatric speciation is the evolution of reproductive isolation in populations that are separated by geographic barriers. What constitutes a geographic barrier is not strictly defined, rather, it can be understood as any environmental factor that prevents or dramatically reduces gene flow between two populations. Thus, while geographic barriers most commonly result from large-scale climatic and geological events (mountain formation, glaciation, continental drift), they can also result from strict habitat preferences that microgeographically isolate populations.
Geographic isolation prevents gene flow among previously interbreeding populations and allows them to evolve independently. This almost inevitably leads to divergence between the two populations over time as distinct evolutionary changes accumulate: different mutations arise in the different populations, genetic drift fixes different genes in the populations, and the populations undergo different adaptive changes in response to natural selection. Over time, the two populations may become reproductively incompatible or isolated, essentially as a by-product of the genetic divergence of other traits. Eventually, the two (initially identical) species will no longer interbreed, even if they are brought back into contact with one another under natural conditions. Allopatric speciation is considered to be the most common of the known modes of speciation.
References:
Campbell, N.E. & Reece, J.B. (2002). Biology (6th ed.). San Francisco: Benjamin Cummings.
Coyne, J.A. & Orr, H.A. (2004). Speciation. Sunderland, MA: Sinauer Associates.
Futuyma, D.J. (2005). Evolution. Sunderland, MA: Sinauer Associates.
Mayr, E. (1976). Evolution and the diversity of life. Cambridge, MA: Harvard University Press.
Image References:
Wikimedia Commons. (2006). Liskamm (Pawel Kuznair). Retrieved 2-27-07, from http://commons.wikimedia.org/wiki/File:Liskamm_01.jpg.
Wikimedia Commons. (2005). The Gambia River winds through the Niokolokoba National Park. (U.S. Geological Service). Retrieved 2-27-07, from http://commons.wikimedia.org/wiki/File:River_gambia_Niokolokoba_National_Park.gif.Allopatric Speciation
Allopatric speciation is the evolution of reproductive isolation in populations that are separated by geographic barriers. What constitutes a geographic barrier is not strictly defined, rather, it can be understood as any environmental factor that prevents or dramatically reduces gene flow between two populations. Thus, while geographic barriers most commonly result from large-scale climatic and geological events (mountain formation, glaciation, continental drift), they can also result from strict habitat preferences that microgeographically isolate populations.
Geographic isolation prevents gene flow among previously interbreeding populations and allows them to evolve independently. This almost inevitably leads to divergence between the two populations over time as distinct evolutionary changes accumulate: different mutations arise in the different populations, genetic drift fixes different genes in the populations, and the populations undergo different adaptive changes in response to natural selection. Over time, the two populations may become reproductively incompatible or isolated, essentially as a by-product of the genetic divergence of other traits. Eventually, the two (initially identical) species will no longer interbreed, even if they are brought back into contact with one another under natural conditions. Allopatric speciation is considered to be the most common of the known modes of speciation.
References:
Campbell, N.E. & Reece, J.B. (2002). Biology (6th ed.). San Francisco: Benjamin Cummings.
Coyne, J.A. & Orr, H.A. (2004). Speciation. Sunderland, MA: Sinauer Associates.
Futuyma, D.J. (2005). Evolution. Sunderland, MA: Sinauer Associates.
Mayr, E. (1976). Evolution and the diversity of life. Cambridge, MA: Harvard University Press.
Image References:
Wikimedia Commons. (2006). Liskamm (Pawel Kuznair). Retrieved 2-27-07, from http://commons.wikimedia.org/wiki/File:Liskamm_01.jpg.
Wikimedia Commons. (2005). The Gambia River winds through the Niokolokoba National Park. (U.S. Geological Service). Retrieved 2-27-07, from http://commons.wikimedia.org/wiki/File:River_gambia_Niokolokoba_National_Park.gif.
20. Sympatric Speciation Reproductive isolation evolves within a single, freely interbreeding population.
Genetic divergence arises largely through diversifying selection and polyploidy.
The most controversial of the geographic modes of speciation.
Sympatric Speciation
In sympatric speciation, reproductive isolation is said to arise within a single, freely, and randomly interbreeding population in the absence of any spatial segregation. Thus, gene flow is initially restricted by biological features of organisms rather than by geography or distance. Because even low levels of gene exchange can swamp out the build up of genetic differentiation that is required for speciation, the occurrence of sympatric speciation is highly debated and controversial. Models of sympatric speciation propose various evolutionary forces and processes as the driving force behind the genetic divergence required for reproductive isolation in sympatry, including diversifying selection (a form of natural selection) and polyploidy (when multiple, duplicate copies of the genome are present within individuals).
When populations inhabit environments with multiple resources and microhabitats, some individuals may possess traits or characteristics that allow them to use one of the resources and/or microhabitats more efficiently. Over time, diversifying selection can cause the population to split into genetically distinct groups that are adapted to discreet niches or the use of different resources within the environment. If selection pressures are strong enough to overcome gene exchange in the population, speciation can occur in sympatry.
An example of sympatric speciation resulting from diversifying selection may be found in flies of the genus Rhagoletis. These flies exhibit strong fidelity to the host plants in which they mate and leave their offspring to develop. Until the mid-nineteenth century, Rhagoletis in the northeastern United States used hawthorns exclusively as their host plant. However, when apples were introduced in some areas approximately 150 years ago, a new race of Rhagoletis appeared that inhabits apples rather than hawthorns. Because hawthorns and apples are often found in the same geographic area, the hawthorn and apple-maggot flies can exist in trees that are only yards apart. Many consider that the hawthorn flies are the parent species of the apple flies and that the speciation event was initiated by genetic variations that caused some members of the original population to be attracted to new hosts. This argues for sympatric speciation in Rhagoletis. However, this claim has come under debate. For example, some argue that the hawthorn and apple-maggot flies descended from distinct, independent lineages, through allopatric speciation.
References:
Campbell, N.E. & Reece, J.B. (2002). Biology (6th ed.). San Francisco: Benjamin Cummings.
Coyne, J.A. & Orr, H.A. (2004). Speciation. Sunderland, MA: Sinauer Associates.
Futuyma, D.J. (2005). Evolution. Sunderland, MA: Sinauer Associates.
Image Reference:
Wikimedia Commons. (2005). Common Hawthorn flowers (Sannse). Retrieved 2-3-09, from http://commons.wikimedia.org/wiki/File:Common_hawthorn_flowers.jpg.Sympatric Speciation
In sympatric speciation, reproductive isolation is said to arise within a single, freely, and randomly interbreeding population in the absence of any spatial segregation. Thus, gene flow is initially restricted by biological features of organisms rather than by geography or distance. Because even low levels of gene exchange can swamp out the build up of genetic differentiation that is required for speciation, the occurrence of sympatric speciation is highly debated and controversial. Models of sympatric speciation propose various evolutionary forces and processes as the driving force behind the genetic divergence required for reproductive isolation in sympatry, including diversifying selection (a form of natural selection) and polyploidy (when multiple, duplicate copies of the genome are present within individuals).
When populations inhabit environments with multiple resources and microhabitats, some individuals may possess traits or characteristics that allow them to use one of the resources and/or microhabitats more efficiently. Over time, diversifying selection can cause the population to split into genetically distinct groups that are adapted to discreet niches or the use of different resources within the environment. If selection pressures are strong enough to overcome gene exchange in the population, speciation can occur in sympatry.
An example of sympatric speciation resulting from diversifying selection may be found in flies of the genus Rhagoletis. These flies exhibit strong fidelity to the host plants in which they mate and leave their offspring to develop. Until the mid-nineteenth century, Rhagoletis in the northeastern United States used hawthorns exclusively as their host plant. However, when apples were introduced in some areas approximately 150 years ago, a new race of Rhagoletis appeared that inhabits apples rather than hawthorns. Because hawthorns and apples are often found in the same geographic area, the hawthorn and apple-maggot flies can exist in trees that are only yards apart. Many consider that the hawthorn flies are the parent species of the apple flies and that the speciation event was initiated by genetic variations that caused some members of the original population to be attracted to new hosts. This argues for sympatric speciation in Rhagoletis. However, this claim has come under debate. For example, some argue that the hawthorn and apple-maggot flies descended from distinct, independent lineages, through allopatric speciation.
References:
Campbell, N.E. & Reece, J.B. (2002). Biology (6th ed.). San Francisco: Benjamin Cummings.
Coyne, J.A. & Orr, H.A. (2004). Speciation. Sunderland, MA: Sinauer Associates.
Futuyma, D.J. (2005). Evolution. Sunderland, MA: Sinauer Associates.
Image Reference:
Wikimedia Commons. (2005). Common Hawthorn flowers (Sannse). Retrieved 2-3-09, from http://commons.wikimedia.org/wiki/File:Common_hawthorn_flowers.jpg.
21. Polyploidy Sympatric speciation can occur through polyploidy.
Polyploids:
have more than two sets of chromosomes;
are reproductively isolated from their non-polyploid parent species; and
are rare among animals but common among plants. Polyploidy
While many models of sympatric speciation remain somewhat controversial among evolutionary biologists, most (if not all) agree that sympatric speciation can occur through polyploidy, the inheritance of more than two basic genome copies as a result of errors during cell division. Polyploids (organisms with more than two duplicate sets of chromosomes) are named according to the number of chromosome sets they have within their nuclei: a triploid has three sets of chromosomes (3n), a tetraploid has four sets (4n), etc.
Polyploid organisms are immediately reproductively isolated from their diploid parent species. For example, a failure during meiosis can cause diploid (2n) parent species to produce tetraploid (4n) offspring. However, crosses between a diploid and a tetraploid will result in inviable or sterile offspring due to abnormal chromosomal pairings during mitosis or meiosis. In fact, for the tetraploid organism to successfully reproduce, it must fertilize itself or mate with another tetraploid. Thus, polyploidy can result in instantaneous speciation in a single genetic event.
Polyploidy is very common in plant species. It is estimated that 35% of flowering plant species are polyploid. While relatively rare, polyploidy is also found in animal species, such as the frog Hyla versicolor, a tetraploid derived from the diploid Hyla chrysoscelis. Polyploidy also provides an immediate way for hybrids of otherwise incompatible parent genomes to produce fertile offspring.
Reference:
Campbell, N.E. & Reece, J.B. (2002). Biology (6th ed.). San Francisco: Benjamin Cummings.
Coyne, J.A. & Orr, H.A. (2004). Speciation. Sunderland, MA: Sinauer Associates.
Futuyma, D.J. (2005). Evolution. Sunderland, MA: Sinauer Associates.
Image Reference:
Wikimedia Commons. (2007). Grey Tree Frog, Hyla versicolor (L.A. Dawson). Retrieved 2-3-09, from http://commons.wikimedia.org/wiki/File:Hyla_versicolor.jpg.Polyploidy
While many models of sympatric speciation remain somewhat controversial among evolutionary biologists, most (if not all) agree that sympatric speciation can occur through polyploidy, the inheritance of more than two basic genome copies as a result of errors during cell division. Polyploids (organisms with more than two duplicate sets of chromosomes) are named according to the number of chromosome sets they have within their nuclei: a triploid has three sets of chromosomes (3n), a tetraploid has four sets (4n), etc.
Polyploid organisms are immediately reproductively isolated from their diploid parent species. For example, a failure during meiosis can cause diploid (2n) parent species to produce tetraploid (4n) offspring. However, crosses between a diploid and a tetraploid will result in inviable or sterile offspring due to abnormal chromosomal pairings during mitosis or meiosis. In fact, for the tetraploid organism to successfully reproduce, it must fertilize itself or mate with another tetraploid. Thus, polyploidy can result in instantaneous speciation in a single genetic event.
Polyploidy is very common in plant species. It is estimated that 35% of flowering plant species are polyploid. While relatively rare, polyploidy is also found in animal species, such as the frog Hyla versicolor, a tetraploid derived from the diploid Hyla chrysoscelis. Polyploidy also provides an immediate way for hybrids of otherwise incompatible parent genomes to produce fertile offspring.
Reference:
Campbell, N.E. & Reece, J.B. (2002). Biology (6th ed.). San Francisco: Benjamin Cummings.
Coyne, J.A. & Orr, H.A. (2004). Speciation. Sunderland, MA: Sinauer Associates.
Futuyma, D.J. (2005). Evolution. Sunderland, MA: Sinauer Associates.
Image Reference:
Wikimedia Commons. (2007). Grey Tree Frog, Hyla versicolor (L.A. Dawson). Retrieved 2-3-09, from http://commons.wikimedia.org/wiki/File:Hyla_versicolor.jpg.
