1. Chapter 24 The Origin of Species

2. Introduction Darwin recognized that the young Galapagos Islands were a place for the beginning of new species. –The observation that confirmed this view was the plants and animals that existed nowhere else. Evolutionary theory may also explain macroevolution, the origin of new taxonomic groups (new species, new genera, new families, new kingdoms) Speciation is the process involved in the origination of diversity of higher taxa.

4. Speciation, the origin of new species, is at the focal point of evolutionary theory Evolutionary theory must explain how new species originate and how populations evolve Microevolution consists of adaptations that evolve within a population, confined to one gene pool Macroevolution refers to evolutionary change above the species level

5. Concept 24.1 What is a Species? In 1942 Ernst Mayr devised the biological species concept to divide biological diversity. –A species is a population whose members have the potential to interbreed with each other in nature, and to produce viable, fertile offspring. They cannot produce viable, fertile offspring with members of other species. –A biological species is the largest set of populations in which genetic exchange is possible and is genetically isolated from other populations. –Gene flow between populations holds the phenotype of a population together.

6. Species are based on interfertility, not physical similarity. For example, the eastern and western meadowlarks may have similar shapes and coloration, but differences in song prevent interbreeding between the two species. In contrast, humans have considerable diversity, but we all belong to the same species because of our capacity to interbreed.

7. Reproductive Isolation Reproductive isolation is the existence of biological factors (barriers) that impede two species from producing viable, fertile offspring Hybrids are the offspring of crosses between different species Reproductive isolation can be classified by whether factors act before or after fertilization

8. Habitat IsolationTemporal Isolation Prezygotic barriers Behavioral Isolation Mating attempt Mechanical Isolation (f) (e) (c) (a) (b) (d) Individuals of different species

9. Prezygotic barriers block fertilization from occurring by: –Impeding different species from attempting to mate –Preventing the successful completion of mating –Hindering fertilization if mating is successful

10. Prezygotic barriers Gametic Isolation Fertilization Reduced Hybrid Viability Postzygotic barriers Reduced Hybrid FertilityHybrid Breakdown Viable, fertile offspring (g) (h) (i) (j) (l) (k)

11. Postzygotic barriers prevent the hybrid zygote from developing into a viable, fertile adult: –Reduced hybrid viability –Reduced hybrid fertility –Hybrid breakdown

12. While the biological species concept has had important impacts on evolutionary theory, it is limited when applied to some species. –For example, one cannot test the reproductive isolation of morphologically-similar fossils, which are separated into species based on morphology. –Even for living species, we often lack the information on interbreeding to apply the biological species concept. –In addition, many species (e.g., bacteria) reproduce entirely asexually and are assigned to species based mainly on structural and biochemical characteristics.

13. Other Concepts Several alternative species concepts emphasize the processes that unite the members of a species. The ecological species concept defines a species in terms of its ecological niche, the set of environmental resources that a species uses and its role in a biological community. –As an example, a species that is a parasite may be defined in part by its adaptations to a specific organism.

14. The morphological species concept, the oldest and still most practical, defines a species by a unique set of structural features. A more recent proposal, the phylogenetic species concept, defines a species as a set of organisms with a unique genetic history - one tip of the branching tree of life. –The sequences of nucleic acids and proteins provide data that are used to define species by unique genetic markers. –Each species has its utility, depending on the situation and the types of questions that we are asking.

15. Concept 24.2 Speciation Concepts

16. Allopatric Speciation Several geological processes can fragment a population into two or more isolated populations. –Mountain ranges, glaciers, land bridges, or splintering of lakes may divide one population into isolated groups. –Alternatively, some individuals may colonize a new, geographically remote area and become isolated from the parent population. For example, mainland organisms that colonized the Galapagos Islands were isolated from mainland populations.

17. How significant a barrier must be to limit gene exchange depends on the ability of organisms to move about. –A geological feature that is only a minor hindrance to one species may be an impassible barrier to another. –The valley of the Grand Canyon is a significant barrier for ground squirrels which have speciated on opposite sides, but birds which can move freely have no barrier.

18. The likelihood of allopatric speciation increases when a population is both small and isolated. –A small, isolated population is more likely to have its gene pool changed substantially by genetic drift and natural selection. –For example, less than 2 million years ago, small populations of stray plants and animals from the South American mainland colonized the Galapagos Islands and gave rise to the species that now inhabit the islands.

19. Sympatric Speciation In sympatric speciation, new species arise within the range of the parent populations. –Here reproductive barriers must evolve between sympatric populations –In plants, sympatric speciation can result from accidents during cell division that result in extra sets of chromosomes, a mutant condition known as polyploidy. –In animals, it may result from gene-based shifts in habitat or mate preference.

20. An individual can have more than two sets of chromosomes from a single species if a failure in meiosis results in a tetraploid (4n) individual. This autopolyploid mutant can reproduce with itself (self-pollination) or with other tetraploids. It cannot mate with diploids from the origi- nal population, because of ab- normal meiosis by the triploid hybrids.

21. Another mechanism of producing polyploid individuals occurs when individuals are produced by the matings of two different species, an allopolyploid. –While the hybrids are usually sterile, they may be quite vigorous and propagate asexually. –Various mechanisms can transform a sterile hybrid into a fertile polyploid. –These polyploid hybrids are fertile with each other but cannot interbreed with either parent species.

22. One mechanism for allopolyploid speciation in plants involves several cross-pollination events between two species of their offspring and perhaps a failure of meiotic disjunction to a viable fertile hybrid whose chromosome number is the sum of the chromosomes in the two parent species.

23. The origin of polypoid species is common and rapid enough that scientists have documented several such speciations in historical times. –For example, new species of plants called goatsbeard (Tragopodon), appeared in Idaho and Washington. –They are the results of allopolyploidy events between pairs of introduced European Tragopodon species. Some agriculture plants are the products of polyploidy. –Oats, cotton, potatoes, tobacco, and wheat are polyploid

24. Sympatric speciation –Can also result from the appearance of new ecological niches In cichlid fish –Sympatric speciation has resulted from nonrandom mating due to sexual selection

26. Allopatric and Sympatric Speciation: A Review In allopatric speciation, geographic isolation restricts gene flow between populations Reproductive isolation may then arise by natural selection, genetic drift, or sexual selection in the isolated populations Even if contact is restored between populations, interbreeding is prevented

27. In sympatric speciation, a reproductive barrier isolates a subset of a population without geographic separation from the parent species Sympatric speciation can result from polyploidy, natural selection, or sexual selection

28. Concept 24.3: Hybrid zones provide opportunities to study factors that cause reproductive isolation A hybrid zone is a region in which members of different species mate and produce hybrids

29. Patterns Within Hybrid Zones A hybrid zone can occur in a single band where adjacent species meet Hybrids often have reduced fitness compared with parent species The distribution of hybrid zones can be more complex if parent species are found in multiple habitats within the same region

30. Patterns Within Hybrid Zones A hybrid zone can occur in a single band where adjacent species meet –For example, two species of toad in the genus Bombina interbreed in a long and narrow hybrid zone

32. Hybrids often have reduced fitness compared with parent species The distribution of hybrid zones can be more complex if parent species are found in patches within the same region

33. Fusion: Weakening Reproductive Barriers If hybrids are as fit as parents, there can be substantial gene flow between species If gene flow is great enough, the parent species can fuse into a single species For example, researchers think that pollution in Lake Victoria has reduced the ability of female cichlids to distinguish males of different species This might be causing the fusion of many species

35. Concept 24.4: Speciation can occur rapidly or slowly The fossil record –Includes many episodes in which new species appear suddenly in a geologic stratum, persist essentially unchanged through several strata, and then apparently disappear Niles Eldredge and Stephen Jay Gould coined the term punctuated equilibrium to describe these periods of apparent stasis punctuated by sudden change

36. The punctuated equilibrium model –Contrasts with a model of gradual change throughout a species’ existence

37. Speciation Rates The punctuated pattern in the fossil record and evidence from lab studies suggests that speciation can be rapid The interval between speciation events can range from 4,000 years (some cichlids) to 40,000,000 years (some beetles), with an average of 6,500,000 years

38. Studying the Genetics of Speciation The explosion of genomics is enabling researchers to identify specific genes involved in some cases of speciation Depending on the species in question, speciation might require the change of only a single allele or many alleles

40. From Speciation to Macroevolution Macroevolution is the cumulative effect of many speciation and extinction events

41. You should now be able to: Define and discuss the limitations of the four species concepts Describe and provide examples of prezygotic and postzygotic reproductive barriers Distinguish between and provide examples of allopatric and sympatric speciation Explain how polyploidy can cause reproductive isolation