Speciation

1. Speciation 26

2. Key Concepts • Speciation occurs when populations of the same species become genetically isolated by lack of gene flow and then diverge from each other due to natural selection, genetic drift, or mutation. • Populations can be recognized as distinct species if they are reproductively isolated from each other, if they have distinct morphological characteristics, or if they form independent branches on a phylogenetic tree.

3. Key Concepts • Populations can become genetically isolated from each other if they occupy different geographic areas, if they use different habitats or resources within the same area, or if one population is polyploid and cannot breed with the other. • When populations that have diverged come back into contact, they may fuse, continue to diverge, stay partially differentiated, or have offspring that form a new species.

4. Introduction • If gene flow ends, allele frequencies in isolated populations are free to diverge—meaning that the populations begin to evolve independently of each other. • Divergence may occur as a result of mutation, natural selection, and genetic drift. • This genetic divergence may eventually lead to speciation, the creation of new species. • Usually, speciation creates two or more distinct species from a single ancestral group.

6. How Are Species Defined and Identified? • A species is defined as an evolutionarily independent population or group of populations. • Biologists commonly use the following three approaches to identify species: • The biological species concept. • The morphospecies concept. • The phylogenetic species concept.

7. The Biological Species Concept • The biological species concept considers populations to be evolutionarily independent if they are reproductively isolated from each other, i.e., they do not interbreed or they fail to produce viable, fertile offspring. • Therefore, no gene flow occurs between these populations. • The biological species concept has disadvantages: • The criterion of reproductive isolation cannot be evaluated in fossils or in species that reproduce asexually. • It can only be applied to populations that overlap geographically.

8. The Biological Species Concept • Biologists categorize the mechanisms that stop gene flow between populations as being either prezygotic or postzygotic. • Prezygotic isolation occurs when individuals of different species are prevented from mating. • Postzygotic isolation occurs when individuals from different populations do mate, but the hybrid offspring produced have low fitness and do not survive or produce offspring.

10. The Morphospecies Concept • Under the morphospecies concept, biologists identify evolutionarily independent lineages by differences in morphological features. • It is based on the idea that distinguishing features are most likely to arise if populations are independent and isolated from gene flow. • It can be widely applied, but it also has disadvantages: • It cannot identify cryptic species that differ in non-morphological traits. • The features used to distinguish species under this concept are subjective.

11. The Phylogenetic Species Concept • The phylogenetic species concept is based on reconstructing the evolutionary history of populations. • On phylogenetic trees, an ancestral population plus all of its descendants is called a monophyletic group or clade. • Monophyletic groups are identified by synapomorphies, homologous traits inherited from a common ancestor that are unique to certain populations or lineages. • Under this concept, a species is defined as the smallest monophyletic group on the tree of life.

15. The Phylogenetic Species Concept • The phylogenetic species concept can be applied to any population, but there are disadvantages: • Phylogenies are currently available for only a tiny (though growing) subset of populations on the tree of life. • Critics point out that it would probably lead to recognition of many more species than either of the other species concepts. • In actual practice, researchers use all three species concepts to identify evolutionarily independent populations in nature.

16. Species Definitions in Action • Subspecies are populations that live in discrete geographic areas and have their own identifying traits but are not distinct enough to be considered a separate species. • Several subspecies of dusky seaside sparrow live along the Atlantic and Gulf Coasts and are physically isolated from one another; scientists believed that there was little or no gene flow between populations. • Based on the biological species and the morphospecies concepts, these subspecies were considered to be separate species.

17. Dusky Seaside Sparrows • Scientists launched a conservation program for one subspecies thought to be nearing extinction, the dusky seaside sparrow. • However, phylogenetic analysis of gene sequences from different seaside sparrow populations showed that only two distinct monophyletic groups of seaside sparrows exist. • The dusky sparrow was shown to be genetically indistinguishable from the other Atlantic Coast sparrows and thus did not need to be individually preserved to preserve the genetic diversity of the species.

21. Isolation and Divergence in Allopatry • Genetic isolation happens routinely when populations become physically separated. Physical isolation, in turn, occurs in one of two ways: dispersal or vicariance. • Dispersal occurs when a population moves to a new habitat, colonizes it, and forms a new population. • Vicariance occurs when a physical barrier splits a widespread population into subgroups that are physically isolated from each other.

22. Isolation and Divergence in Allopatry • Speciation that begins with physical isolation via either dispersal or vicariance is known as allopatric speciation. • Populations that live in different areas are said to be in allopatry. • Biogeography—the study of how species and populations are distributed geographically—can tell us how colonization and range-splitting events occur.

26. Dispersal and Colonization Isolate Populations • Colonization events often cause speciation because the physical separation reduces gene flow, and genetic drift via the founder effect causes the old and new populations to diverge rapidly. • Subsequent natural selection may cause divergence if the newly colonized environment is different from the original habitat.

27. Vicariance Isolates Populations • Vicariance events are thought to be responsible for the origin of many modern species. • Physical isolation of populations via dispersal or vicariance produces genetic isolation, the first requirement of speciation. • When genetic isolation is accompanied by genetic divergence due to mutation, natural selection, and genetic drift, speciation results.

31. Allopatric Speciation

32. Isolation and Divergence in Sympatry • Populations or species that live in the same geographic region (close enough to mate) live in sympatry. • Researchers traditionally believed that speciation could not occur among sympatric populations because gene flow is possible. • The prediction was that gene flow would easily overwhelm any differences among populations created by genetic drift and natural selection.

34. Sympatric Speciation • Under certain circumstances natural selection can overcome gene flow and cause sympatric speciation. • Speciation may occur because even though populations are not physically isolated, they may be isolated by preferences for different habitats.

35. Sympatric Speciation in Apple Maggot Flies • Apple maggot flies feed and mate on apple fruits, and hawthorne flies feed and mate on hawthorne fruits. • Experiments show that each species responds most strongly to its own fruit’s scent. • Even though the two species are often found sympatrically, they do not generally interbreed. • Although they are not yet separate species on the basis of any species concept, apple flies and hawthorn flies are diverging. They are currently in the process of becoming distinct species.

37. How Can Polyploidy Lead to Speciation? • If populations become isolated, it is unlikely that mutation alone could cause them to diverge appreciably. • However, a mutation that results in polyploidy—the condition of having more than two sets of chromosomes—can cause speciation, particularly in plants. • Tetraploid individuals are genetically isolated from wild-type individuals because they produce diploid gametes rather than haploid gametes. • If the two gametes combined, the resulting zygote would be triploid. Triploid individuals produce gametes with a dysfunctional set of chromosomes.

38. How Can Polyploidy Lead to Speciation? • Tetraploid and diploid individuals rarely produce fertile offspring when they mate. As a result, tetraploid and diploid populations are reproductively isolated. • Mutations that result in a doubling of chromosome number produce autopolyploid individuals. • In these individuals, the chromosomes all come from the same species. • Allopolyploid individuals are created when parents that belong to different species produce an offspring in which chromosome number doubles.

42. Autopolyploidy • Researchers found a population of tetraploid (4n) maidenhair ferns. These were the offspring of a parent that produced diploid gametes and then self-fertilized. • Polyploid populations such as these ferns are genetically isolated from wild-type populations, because tetraploid individuals can breed with other tetraploids but not with diploids. • If genetic drift and natural selection cause the wild-type and polyploid populations to diverge, speciation is then under way.

43. Allopolyploidy • New tetraploid species may be created when two diploid species hybridize. • When diploid gametes fuse during self-fertilization, a tetraploid individual results. • Many diploid plant species have closely related polyploid species, supporting the claim that speciation by polyploidy is important in plants.

47. Why Speciation by Polyploidy Is Common in Plants • In plants, somatic cells that have undergone many rounds of mitosis can undergo meiosis and produce gametes. The multiple rounds of mitosis increase the likelihood of tetraploid daughter cells. • The ability of some plant species to self-fertilize makes it possible for diploid gametes to fuse and create genetically isolated tetraploid populations. • Hybridization between plant species is common, creating opportunities for speciation via formation of allopolyploids.

48. Why Speciation by Polyploidy Is Common in Plants • To summarize, speciation by polyploidization is driven by chromosome-level mutations and occurs in sympatry. • Compared to the gradual process of speciation by geographic isolation or by disruptive selection in sympatry, speciation by polyploidy is virtually instantaneous. It is fast, sympatric, and common.

49. Speciation Changes in Ploidy

50. When Isolated Populations Come into Contact • What happens when isolated populations of related species come back into contact depends on many factors, but most importantly on whether the populations have diverged genetically or not. • If two populations have diverged and if divergence has affected when, where, or how individuals in the populations mate, prezygotic isolation exists. • In cases such as this, mating between the populations is rare, gene flow is minimal, and the populations continue to diverge.