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Allopatric Speciation Evolution Biology 4974/597 D.F. Tomback Biology 4974/5974 Evolution Allopatric Speciation And Hybridization Grant and Grant 2002 R.J. Abbott (2003)Science 302: 1189-1190 Learning goals Learning Goals--Know and understand: • The reproductive isolating mechanisms and their functions. • General steps common to allopatric speciation models. • Allopatric speciation models: geographic, parapatric, peripatric. • The speciation mechanisms involving polyploidy and autopolyploidy, and diploid hybridization. • The role of chromosomal rearrangements in speciation. • Punctualism and gradualism as different speciation time frames and models. • The Shifting Balance Theory as a model of speciation. • How speciation in Darwin’s finches may be explained by peak shifts. Reproductive isolating mechanisms Isolating mechanisms prevent hybridization (nonadaptive gene exchange). (Mayr 1963) Premating (prezygotic) isolating mechanisms: • Temporal or habitat isolation • Behavioral or sexual isolation • Mechanical isolation Postmating (postzygotic) isolating mechanisms (these are a by-product of genetic differences): • Gametic mortality • Zygotic mortality • Hybrid inviability • Hybrid sterility Benefits to avoiding hybridization • Preserve adaptation to specific environments. • Prevent disruption of adaptive gene complexes or arrangements. • Prevent investment of energy and gametes if hybrids are inferior or inviable--reduced reproductive success. 1 Allopatric Speciation Evolution Biology 4974/597 D.F. Tomback General steps in allopatric speciation Fig. 22.5 • Starts with geographic isolation between populations. What does this mean? • Next, genetic differentiation between populations occurs over time. • Eventually both populations may come into contact again through overlap (sympatry). • Hybridization can result in “reinforcement” of initial phases of reproductive isolation. • When gene flow no longer occurs, speciation is achieved. • Or, speciation does not occur because hybrids are viable, and genetic introgression (mixing) occurs, and one population emerges again. Other allopatric speciation models Peripatric speciation Speciation in small population outside of a main population. Also called Founder Effect speciation (Mayr 1954). • Small, founding population “buds off” from main species range. • No gene flow occurs between the small population and main range. • Small population then undergoes rapid genetic change through genetic drift and genetic reorganization. • See “peak shift” process later in lecture. Fig. 23.1 b Other allopatric speciation models Parapatric speciation Speciation in contiguous (neighboring) but nonoverlapping populations. • Neighboring populations exist in different environments. • Populations experience reduced gene flow and diverge genetically. aa • Populations become reproductively isolated. AA • “Hybrids” are at a disadvantage, reinforcing reproductive isolating mechanisms. Aa Selection for genetic differences between the two populations must be greater than the rate of gene flow. e.g., The grass, Anthoxanthum odoratum, has diverging populations with heavy metal tolerance. Populations in mine tailings and uncontaminated soil exist side-by-side. Populations with heavy metal tolerance flower at different times and self-pollinate more frequently than other populations. 2 Allopatric Speciation Evolution Biology 4974/597 D.F. Tomback Allopatric speciation models Speciation by polyploidy 40-70% of plants are polyploids (p. 466), and could have arisen from either mechanism below. This mechanisms applies primarily to plants. Autopolyploidy—sympatric speciation Allopolyploidy • Change in ploidy level within a species is a single genetic event. • Speciation is instantaneous. • Results from abnormal reduction division during meiosis, which produces a 2n gamete followed by fertilization. • E.g., triploid (2n + n) or tetraploid (2n + 2n). Allopolyploids—allopatric speciation • Sterile hybrid initially forms. • Chromosomal doubling produces a fertile hybrid.. Fig. 16.2 Allopatric speciation: Diploid hybridization Hybridization is widespread in plants but there are cases of hybridization in animals as well. Hybridization leads to important outcomes: 1.Generating novel genotypes. 2.Founding new evolutionary lineages. Fertile hybrids mediate gene flow from one species to another. •e.g., (Grant and Grant 2008). They note that hybridization between ground finches occurs in about 1% of matings. Most offspring die. •In 1983, after heavy rains, many small seeds were available. Hybrids (two ground finches and ground finch/cactus finch) survived using these seeds. (Inprinted on wrong species’ song.) •Backcrosses survived leading to genetic introgression. (Showed that species not isolated genetically) Hybrids may be better adapted to a unique habitat or environment than either parental species. •e.g., fleabane in Colorado (Apocynum androsaemifolium x A. cannabinum = A. x-floribundum) (Johnson et al. 1998). These hybrids are clonal and represent separate hybridization events. •e.g., hybrids between sunflower species (next slide) 3 Allopatric Speciation Evolution Biology 4974/597 D.F. Tomback Example: the role of hybridization in sunflowers Rieseberg et al. ( 2003): Recreated hybridization events between the sunflowers Helianthus annuus and H. petiolaris. • Believed to produce three hybrid species H. anomalus, H. deserticola, and H. paradoxus. • These natural hybrids formed less than 60 generations ago. • The extreme phenotypes observed in the wild were produced among hybrids in the lab. • These phenotypes were favored by selection when planted in the natural habitats occupied by hybrids in the wild. These hybridization events favored ecological divergence. New species adapted to novel environments. Abbott 2003 Science 301: 1189-1190. Chromosomal rearrangements in speciation Closely related species, and even populations of the same species, may differ in karyotype. Chromosomal differences result from simple rearrangements, such as inversions, translocations, and fusions between chromosomes. • Rearrangements may reduce or prevent gene flow. • Rearrangements may spread within small, isolated populations, the result of Founder Effects or genetic drift. In small populations, they spread by drift, even if deleterious. • Chromosomal rearrangements can lead to reproductive isolation and speciation. Punctualism and gradualism Punctualism and gradualism represent opposing views of the time-frame of speciation, i.e., how fast speciation occurs. • Rates may be fast, such as the radiation of species after a mass extinction or within a new adaptive zone, e.g., cichlid fish. • Or slow, such as in living fossils such as the horseshoe crab (Limulus) or the coelocanth (Latimeria), which have not speciated in hundreds of millions of years. • Gradualism reflects “Descent with modification,” supported by geographic speciation—slow change over time. • Punctualism or punctuated equilibrium (advocated by Mayr and later by N. Eldredge and S.J. Gould) suggests that most speciation occurs sympatrically, often initiated by chromosomal rearrangements, and proceeds quickly. 4 Allopatric Speciation Evolution Biology 4974/597 D.F. Tomback Punctualism and gradualism: graphic models Avers 1989 Wright’s Adaptive Landscape Theory Sewall Wright developed the concepts of Adaptive Landscape and Shifting Balance to explain adaptation within species. The peaks represent genotypes of higher fitness than the genotypes in the valleys, with height directly related to fitness. End Box 19.1 Speciation by Peak Shift A peak shift can lead to speciation. It may involve substantial genetic reorganization and a new phenotype. • Mathematical models suggest that once a population slips off a peak, natural selection rapidly drives it up another adaptive peak. Can culminate with speciation. • The mathematical timeframe provides some support for “punctuated equilibrium.” Avers (1989) Patterns and processes in evolution. Oxford University Press. 5 Allopatric Speciation Evolution Biology 4974/597 D.F. Tomback Example: speciation in Darwin’s finches and the Adaptive Landscape Model • Darwin’s finches speciated faster than other similar groups (e.g., Hawaiian honeycreepers). • Rather than a strict allopatric model, the Grants favor a more complex process involving hybridization and events represented by Wright’s Adaptive Landscape Model. • This involves a dynamic process with changing environments and food resources over time. • As food resources change, the peaks shift, so hybrids can become more successful than parent species. Grant and Grant 2002 Study questions • What are the general steps in allopatric speciation? • What are the main features of each of the following allopatric speciation models? a. Geographic, b. Parapatric, c. Peripatric • What are the key features of speciation by polyploidy, allopolyploidy, and diploid hybridization? • How did the work by Rieseberg et al. (2003) on hybridization in sunflowers illustrate how diploid hybrids arise and can be successful? • Define: punctuated equilibrium and gradualism. • Interpret and understand how punctualism and gradualism differ referring to the graphic models. • How does the Shifting Balance Theory depict a speciation event? • Explain how Grant and Grant (2002) use the Shifting Balance Theory to explain speciation in Darwin’s finches. 6