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Explore the concept of species and its various definitions, including the biological, morphological, ecological, and phylogenetic species concepts. Learn about the reproductive barriers that keep species separate and contribute to the origin of biological diversity.
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14.1 The origin of species is the source of biological diversity • Darwin was eager to explore landforms newly emerged from the sea when he came to the Galápagos Islands. • He noted that these volcanic islands, despite their geologic youth, were teeming with plants and animals found nowhere else in the world. • He realized that these species, like the islands, were relatively new.
14.1 The origin of species is the source of biological diversity Microevolution is the change in the gene pool of a population from one generation to the next. Speciation is the process by which one species splits into two or more species. Each time speciation occurs, the diversity of life increases.
14.1 The origin of species is the source of biological diversity • Over the course of 3.5 billion years, • an ancestral species first gave rise to two or more different species, • which then branched to new lineages, • which branched again, • until we arrive at the millions of species that live, or once lived, on Earth.
14.2 There are several ways to define a species The word species is from the Latin for “kind” or “appearance.” Although the basic idea of species as distinct life-forms seems intuitive, devising a more formal definition is not easy and raises questions. In many cases, the differences between two species are obvious. In other cases, the differences between two species are not so obvious.
14.2 There are several ways to define a species The biological species concept defines a species as a group of populations whose members have the potential to interbreed in nature and produce fertile offspring (offspring that themselves can reproduce). Thus, members of a biological species are united by being reproductively compatible, at least potentially.
14.2 There are several ways to define a species • Reproductive isolation • prevents genetic exchange (gene flow) and • maintains a boundary between species. • But there are some pairs of clearly distinct species that do occasionally interbreed. • The resulting offspring are called hybrids. • An example is the grizzly bear (Ursusarctos) and the polar bear (Ursusmaritimus), whose hybrid offspring have been called “grolar bears.”
Figure 14.2c-0 Grizzly bear Polar bear Hybrid “grolar” bear
14.2 There are several ways to define a species • There are other instances in which applying the biological species concept is problematic. • There is no way to determine whether organisms that are now known only through fossils were once able to interbreed. • Reproductive isolation does not apply to prokaryotes or other organisms that reproduce only asexually. • Therefore, alternate species concepts can be useful.
14.2 There are several ways to define a species • The morphological species concept • classifies organisms based on observable physical traits and • can be applied to asexual organisms and fossils. • However, there is some subjectivity in deciding which traits to use.
14.2 There are several ways to define a species • The ecological species concept • defines a species by its ecological niche and • focuses on unique adaptations to particular roles in a biological community. • For example, two species may be similar in appearance but distinguishable based on what they eat or the depth of water in which they are usually found.
14.2 There are several ways to define a species • The phylogenetic species concept defines a species as the smallest group of individuals that share a common ancestor and thus form one branch of the tree of life. • Biologists trace the phylogenetic history of a species by comparing its • morphology, • DNA sequences, or • biochemical pathways. • However, agreeing on the amount of difference required to establish separate species remains a challenge.
14.3 VISUALIZING THE CONCEPT: Reproductive barriers keep species separate • Reproductive barriers • serve to isolate the gene pools of species and • prevent interbreeding. • Depending on whether they function before or after zygotes form, reproductive barriers are categorized as • prezygotic or • postzygotic.
14.3 VISUALIZING THE CONCEPT: Reproductive barriers keep species separate • Five types of prezygotic barriers prevent mating or fertilization between species. • In habitat isolation, there is a lack of opportunity for mates to encounter each other. • In temporal isolation, there is breeding at different times or seasons.
14.3 VISUALIZING THE CONCEPT: Reproductive barriers keep species separate • In behavioral isolation, there is failure to send or receive appropriate signals. • In mechanical isolation, there is physical incompatibility of reproductive parts. • In gametic isolation, there is molecular incompatibility of eggs and sperm or pollen and stigma.
PREZYGOTIC BARRIERS Habitat isolation (different habitats) Temporal isolation (breeding at different times) Behavioral isolation (different courtship rituals) Figure 14.3-0 Mechanical isolation (incompatible reproductive parts) Gametic isolation (incompatible gametes) POSTZYGOTIC BARRIERS Reduced hybrid vitality (short-lived hybrids) Reduced hybrid fertility (sterile hybrids) Hybrid breakdown(fertile hybrids withsterile offspring)
14.3 VISUALIZING THE CONCEPT: Reproductive barriers keep species separate • Three types of postzygotic barriers operate after hybrid zygotes have formed. • In reduced hybrid viability, interaction of parental genes impairs the hybrid’s development or survival. • In reduced hybrid fertility, hybrids are vigorous but cannot produce viable offspring. • In hybrid breakdown, hybrids are viable and fertile, but their offspring are feeble or sterile.
14.4 In allopatric speciation, geographic isolation leads to speciation • A key event in the origin of a new species is the separation of a population from other populations of the same species. • With its gene pool isolated, the splinter population can follow its own evolutionary course. • Changes in allele frequencies caused by natural selection, genetic drift, and mutation will not be diluted by alleles entering from other populations (gene flow).
14.4 In allopatric speciation, geographic isolation leads to speciation • In allopatric speciation, the initial block to gene flow may come from a geographic barrier that isolates a population.
14.4 In allopatric speciation, geographic isolation leads to speciation • Several geologic processes can isolate populations. • A mountain range may emerge and gradually split a population of organisms that can inhabit only lowlands. • A large lake may subside until there are several smaller lakes, isolating certain fish populations. • Continents themselves can split and move apart. • Allopatric speciation can also occur when individuals colonize a remote area and become geographically isolated from the parent population.
14.4 In allopatric speciation, geographic isolation leads to speciation • How large must a geographic barrier be to keep allopatric populations apart? • The answer depends on the ability of the organisms to move. • Birds, mountain lions, and coyotes can easily cross mountain ranges. • In contrast, small rodents may find a canyon or a wide river a formidable barrier. The Grand Canyon and Colorado River separate two species of antelope squirrels.
Figure 14.4a-0 North rim South rim A. harrisii A. leucurus
14.4 In allopatric speciation, geographic isolation leads to speciation • Thirty species of snapping shrimp in the genus Alpheus live off the Isthmus of Panama, the land bridge that connects South and North America. • Morphological and genetic data group these shrimp into 15 pairs of species, with the members of each pair being each other’s closest relative. • In each case, one member of the pair lives on the Atlantic side of the isthmus, while the other lives on the Pacific side. • This strongly suggests that geographic separation of the ancestral species of these snapping shrimp led to allopatric speciation.
A. formosus A. nuttingi Figure 14.4b ATLANTIC OCEAN Isthmus of Panama PACIFIC OCEAN A. panamensis A. millsae
14.5 Reproductive barriers can evolve as populations diverge • How do reproductive barriers arise? • The environment of an isolated population may include • different food sources, • different types of pollinators, and • different predators. • As a result of natural selection acting on preexisting variations (or as a result of genetic drift or mutation), a population’s traits may change in ways that also establish reproductive barriers.
14.6 Sympatric speciation takes place without geographic isolation • Sympatric speciation occurs when a new species arises within the same geographic area as its parent species. • How can reproductive isolation develop when members of sympatric populations remain in contact with each other? • Gene flow between populations may be reduced by • polyploidy, • habitat differentiation, or • sexual selection.
14.6 Sympatric speciation takes place without geographic isolation Many plant species have originated from sympatric speciation that occurs when accidents during cell division result in extra sets of chromosomes. New species formed in this way are polyploid, in that their cells have more than two complete sets of chromosomes.
14.6 Sympatric speciation takes place without geographic isolation • Sympatric speciation can result from polyploidy • within a species (by self-fertilization) or • between two species (by hybridization).
14.7 EVOLUTION CONNECTION: The origin of most plant species can be traced to polyploid speciation Plant biologists estimate that 80% of all living plant species are descendants of ancestors that formed by polyploid speciation. Hybridization between two species accounts for most of these species, perhaps because of the adaptive advantage of the diverse genes a hybrid inherits from different parental species.
plums, apples, sugarcane, coffee, and wheat. 14.7 EVOLUTION CONNECTION: The origin of most plant species can be traced to polyploid speciation • Polyploid plants include • cotton, • oats, • potatoes, • bananas, • peanuts, • barley,
14.7 EVOLUTION CONNECTION: The origin of most plant species can be traced to polyploid speciation • Wheat • has been domesticated for at least 10,000 years and • is the most widely cultivated plant in the world. • Bread wheat, Triticumaestivum, is • a polyploid with 42 chromosomes and • the result of hybridization and polyploidy.
14.8 Isolated islands are often showcases of speciation Isolated island chains are often inhabited by unique collections of species. Islands that have physically diverse habitats and that are far enough apart to permit populations to evolve in isolationbut close enough to allow occasional dispersions to occurare often the sites of multiple speciation events. The evolution of many diverse species from a common ancestor is known as adaptive radiation.
14.8 Isolated islands are often showcases of speciation • The Galápagos Archipelago • is located about 900 km (560 miles) west of Ecuador, • is one of the world’s great showcases of adaptive radiation, • was formed naked from underwater volcanoes from 5million to 1 million years ago, • was colonized gradually from other islands and the South America mainland, and • has many species of plants and animals found nowhere else in the world.
14.8 Isolated islands are often showcases of speciation • The Galápagos Islands currently have 14 species of closely related finches, called Darwin’s finches, because Darwin collected them during his around-the-world voyage on the Beagle. • These birds • share many finchlike traits, • differ in their feeding habits and their beaks, specialized for what they eat, and • arose through adaptive radiation.
Cactus-seed-eater (cactus finch) Figure 14.8-0 Tool-using insect-eater(woodpecker finch) Seed-eater (large ground finch)
14.9 SCIENTIFIC THINKING: Lake Victoria is a living laboratory for studying speciation We can see speciation occurring. The species living today represent a snapshot, a brief instant in this vast span of time. The environment continues to change, sometimes rapidly due to human impact, and natural selection continues to act on affected populations. Researchers have documented at least two dozen cases in which populations are diverging as they exploit different food resources or breed in different habitats.
14.9 SCIENTIFIC THINKING: Lake Victoria is a living laboratory for studying speciation Sexual selection is a form of natural selection in which individuals with certain traits are more likely to obtain mates.
14.9 SCIENTIFIC THINKING: Lake Victoria is a living laboratory for studying speciation • Biologists can also test hypotheses about the process of speciation by studying species that arose recently. • Cichlids are a family of fishes that live in tropical lakes and rivers. • They come in all colors of the rainbow. • They are renowned for the spectacular adaptive radiations that stocked the large lakes of East Africa with more than a thousand species of cichlids in less than 100,000 years. • In the largest of these lakes, Lake Victoria, roughly 500 species evolved in about 15,000 years.
Uganda Kenya Figure 14.9a LakeVictoria IndianOcean Tanzania
14.9 SCIENTIFIC THINKING: Lake Victoria is a living laboratory for studying speciation • In Lake Victoria, there are pairs of closely related cichlid species that differ in color but nothing else. • Breeding males of Pundamilianyererei have a bright red back and dorsal fin. • Breeding males of Pundamiliapundamilia males are metallic blue-gray.
Figure 14.9b Pundamilianyererei Pundamiliapundamilia
14.9 SCIENTIFIC THINKING: Lake Victoria is a living laboratory for studying speciation • Pundamilia females prefer brightly colored males. • Mate-choice experiments performed in the laboratory showed that • P. nyererei females prefer red males over blue males, • P. pundamilia females prefer blue males over red males, • the vision of P. nyererei females is more sensitive to red light than blue light, and • the vision of P. pundamilia females is more sensitive to blue light than red light. • Researchers also demonstrated that this color sensitivity is heritable.
14.9 SCIENTIFIC THINKING: Lake Victoria is a living laboratory for studying speciation As light travels through water, suspended particles selectively absorb and scatter the shorter (blue) wavelengths, so light becomes increasingly red with increasing depth. Thus, in deeper waters, P. nyererei males are pleasingly apparent to females with red-sensitive vision but virtually invisible to P. pundamilia females.
14.9 SCIENTIFIC THINKING: Lake Victoria is a living laboratory for studying speciation • When biologists sampled cichlid populations in Lake Victoria, they found that • P. nyererei breeds in deep water, while • P. pundamilia inhabits shallower habitats where the blue males shine brightly. • As a consequence of their mating behavior, the two species encounter different environments that may result in further divergence.
14.10 Hybrid zones provide opportunities to study reproductive isolation What happens when separated populations of closely related species come back into contact with each other? Biologists try to answer such questions by studying hybrid zones, regions in which members of different species meet and mate to produce at least some hybrid offspring.
14.10 Hybrid zones provide opportunities to study reproductive isolation • Pollution caused by development along the shores of Lake Victoria has turned the water murky. • What happens when P. nyererei or P. pundamilia females can’t tell red males from blue males? • The behavioral barrier crumbles. • Many viable hybrid offspring are produced by interbreeding. • The once isolated gene pools of the parent species are combining, with two species fusing into a single hybrid species.
