Lecture 6: Speciation PDF

Lecture 6: Speciation  Goals Continue discussion of Speciation: Geographic modes of speciation. Reproductive isolation and reinforcement.  The Process of Speciation Speciation is the evolution of biological barriers to gene flow. Study of sp...

Lecture 6: Speciation  Goals Continue discussion of Speciation: Geographic modes of speciation. Reproductive isolation and reinforcement.  The Process of Speciation Speciation is the evolution of biological barriers to gene flow. Study of speciation bridges the evolution of populations and the evolution of taxonomic diversity. Each branching point (node) in the tree of life marks a speciation event (origin of two species from one). Speciation can be considered a tug-of-war between two sets of forces: Selection & Recombination/Gene Flow  Geography of Speciation All three scenarios require reduction of gene flow for new species to evolve. In allopatric speciation, populations diverge while separated by a geographic barrier that that prevents/reduces gene flow between populations. After the two populations become so different that reproductive isolation evolves, the two can coexist without interbreeding. In parapatric speciation, neighboring populations diverge while still exchanging some genes (but not freely). Adjacent populations adapt to different habitats/niches and selection favors divergence. Reduction of gene flow is due to strong selection preventing colonization of the other habitat or the formation of hybrids. In sympatric speciation, two new species emerge from a single ancestor without any geographic isolation and while exchanging genes freely. This requires extremely strong divergent selection. This speciation mode is highly controversial and unlikely to happen in nature.  Sympatric speciation less likely than allopatric speciation: recombination breaks down genetic combinations that might form new species Suppose there are 2 bird morphs: thin bills specialize on insects and stout bills specialize on seeds. Both forms have high fitness and intermediates have lower fitness. Birds select mates on the basis of color: red mates with red, and blue mates with blue. In sympatry, recombination erodes linkage disequilibrium between color and bill loci. In allopatry, geographically isolated populations can diverge in both traits (color and bill size). If the differences are sufficient, the two populations will stay genetically distinct when they come into secondary contact. New species have formed.  Clarification about different types of barriers Geographic and reproductive barriers are not mutually exclusive, and instead often work together during the speciation process. Phylogenetic signature of allopatric speciation Phylogenetic analysis shows that the closest relatives of some Atlantic shrimp species are NOT other Atlantic shrimp species, but rather Pacific species  Allopatric Speciation Widely distributed species may become fragmented into allopatric populations if habitat on which it depends becomes fragmented by climate change An example of Parapatric Speciation Incipient speciation with gene flow: divergence of lizards on the white sand and dark soil habitats shown in the photos at top. Nuclear genes show that genotypes differ between habitats strongly in Holbrookia maculate. Each bar represents the genotype of one individual. The proportion of a bar that is green shows the probability, based on a lizard’s genotype, that the individual belongs to a distinct white sand population, and the proportion that is blue that it belongs to a distinct dark soil population. Bars with intermediate amounts of green and blue indicate that the individual has a mixed genotype.  Stable Coexistence of Species in Sympatry The malaria mosquito (genus Anopheles) is widely distributed in Africa, containing several related species (a messy species complex!). Genomic studies revealed LOTS of introgression between species, but divergent islands in the X chromosome ( 1% of the genome) maintain species boundaries while the rest of the genome is virtually homogenized by occasional hybridization and gene flow. Species boundaries can be porous to introgression The observation of stable coexistence of divergent populations in sympatry (despite introgression) supports the hypothesis that these are real species (“Good species”). Fontaine et al. (2014) Science Reproductive Barriers to Gene Flow Most important distinction of Reproductive Isolation Barriers: Prezygotic and Postzygotic Zimmer & Emlen  Clarification about different types of barriers Geographic and reproductive barriers are not mutually exclusive, and instead often work together during the speciation process.  The evolution of biological barriers to gene flow Pre-mating, Prezygotic isolation (Organisms occur in the same area but don't mate) Potential mates don't meet: Different habitats Different mating seasons/times Potential mates meet but don't mate: Different mating behavior in animals Different pollinators in plants Postmating, prezygotic (Potential mates try to mate but can't form a zygote) Incompatible genitalia Incompatible gametes Postzygotic isolation (Hybrids are formed but have low fitness) "Intrinsic" mechanisms Hybrid lethality Hybrid sterility (physiological) "Extrinsic" mechanisms Ecological: hybrids don't fit into either ecological niche Mate recognition: mating behavior not appropriate for either species Pre-zygotic barriers: pollinator isolation The total degree of reproductive isolation between two species may results from several barriers that act in sequence and some potential barriers may not come into play. In the monkey flower, Mimulus lewisii is distributed at higher elevations in Sierra Nevada than the relative M. cardinalis and both occur at intermediate elevations. Pollinator differences are the second most important barrier. No hybrids found in areas of sympatry, but hybrids can be formed by hand-crosses. Reproductive Isolation (RI) Freq.of heterospecific matings RI = 1 − Freq.of Conspecific matings  Premating Reproductive Isolation: Sexual Isolation in songs Oscillograms of the songs of three morphologically indistinguishable species of green lacewings reveals strong song differentiation. Male and female engage in a duet singing. Mating does not occur unless the female sings back to the male. Females respond more often to the song of males of their own species. http://hydrodictyon.eeb.uconn.edu/people/chenry/Cryptic_songs.html Premating barriers: timing of reproduction Corals reproduce by releasing gametes into the water. Two sympatric species of Monostrea corals remain reproductively isolated by spawning at different times. Speciation consists of the evolution of biological barriers to gene flow Pre-mating, Prezygotic isolation (Organisms occur in the same area but don't mate) Potential mates don't meet: Different habitats Different mating seasons/times Potential mates meet but don't mate: Different mating behavior in animals Different pollinators in plants Postmating, prezygotic (Potential mates try to mate but can't form a zygote) Incompatible genitalia Incompatible gametes Postzygotic isolation (Hybrids are formed but have low fitness) "Intrinsic" mechanisms Hybrid lethality Hybrid sterility (physiological) "Extrinsic" mechanisms Ecological: hybrids don't fit into either ecological niche Mate recognition: mating behavior not appropriate for either species  Postmating, Prezygotic barriers: Gametic incompatibility: sperm or pollen from one species fails to fertilize the egg or ovule of another species Pollen tube growth in plants Speciation consists of the accumulation of various biological barriers to gene flow Pre-mating, Prezygotic isolation (Organisms occur in the same area but don't mate) Potential mates don't meet: Different habitats Different mating seasons/times Potential mates meet but don't mate: Different mating behavior in animals Different pollinators in plants Postmating, prezygotic (Potential mates try to mate but can't form a zygote) Incompatible genitalia. Incompatible gametes. Postzygotic isolation (Hybrids are formed but have low fitness) "Intrinsic" mechanisms Hybrid lethality Hybrid sterility (physiological or behavioral) "Extrinsic" mechanisms Ecological: hybrids don't fit into either ecological niche Mate recognition: mating behavior not appropriate for either species Example of “intrinsic” hybrid sterility/inviability + =  The Dobzhansky-Muller model for the evolution of intrinsic hybrid sterility/inviability Dobzhansky & Muller provided a theoretical solution where reproductive barriers are based on differences at two or more loci that have complementary effects on fitness. Substitutions at two or more loci can generate incompatible genotypes. These new substitutions have never been tested in the background of the other. Such incompatibilities are termed Dobzhansky-Muller Incompatibilities Hybrid Sterility Results from Genetic Incompatibilities Functional divergence between interacting loci yields incompatible interactions in hybrids.  Reinforcement Reinforcement is an adaptation to prevent the production of unfit hybrids. Reinforcement generates selection favoring the evolution of stronger prezygotic reproductive barriers between emerging species. Species may diverge in allopatry and then spread back into secondary contact and isolation is not complete: Alleles that reduce the degree of interbreeding can be favored in the area of sympatry. This is a form of direct selection for reproductive isolation. Character displacement and reinforcement of sexual isolation Chorus frog Pseudacris feriarum in Eastern U.S. partly overlaps with southern P. nigrita  Character displacement and reinforcement of sexual isolation Three steps in the evolution of Reinforcement: 1. Populations diverge in allopatry. 2. The two divergent populations come back in secondary contact. 3. Selection acts against interbreeding, either because of direct costs (e.g. copulatory damage) or because hybrids have lower fitness. Selection favors the evolution of greater reproductive isolation. Important Consideration: Reinforcement can only act on the evolution of pre-zygotic reproductive mechanisms.