Lecture 15 Objectives PDF

11/21/24 Lecture 15 objectives 1) Genetics of speciation 1) Dobzkansky-Muller incompatibilities 2) Is speciation an incidental effect of adaptation? 3) Reinforcement 4) Sympatric speciation and leaky species boundaries...

11/21/24 Lecture 15 objectives 1) Genetics of speciation 1) Dobzkansky-Muller incompatibilities 2) Is speciation an incidental effect of adaptation? 3) Reinforcement 4) Sympatric speciation and leaky species boundaries 527 527 The Dobzhansky-Muller model of species incompatibility The problem with single-locus models… A1A1 Ancestral population 528 528 1  11/21/24 The Dobzhansky-Muller model of species incompatibility The problem with single-locus models… A1A1 Ancestral population Split followed by allopatric divergence (pop 1) A2A2 A1A1 529 529 The Dobzhansky-Muller model of species incompatibility The problem with single-locus models… A1A1 Ancestral population Split followed by allopatric divergence (pop 1) A2A2 A1A1 Hybridize to make F1 A1A2 Sterile or inviable F1 530 530 2  11/21/24 …which leads to Dobzhansky-Muller model Derived A2 allele not “tested” by natural selection for proper functioning with derived B2 allele, so hybrid may have reduced fitness. Note that this model invokes incompatibilties between alleles at different loci! 531 531 Selection experiments support the idea that incompatibilities are an incidental effect of directional selection Dodd 1989 See Rice and Hostert, Evolution 47:1637-1653 for 532 many additional examples 532 3  11/21/24 Observations from natural populations also support the idea that isolation may be an incidental effect of directional selection willow A population of leaf beetle adapted to willow is sexually isolated from geographically close populations adapted to maple. maple However, maple populations far away from each other show little sexual isolation. maple 533 533 An incidental effect of adaptation to different pollinators is reproductive isolation 534 534 4  11/21/24 Beak adaptation to different foods may influence song and contribute to sexual isolation from Ridley 535 535 For several organisms (fish, birds, frogs, flies, Lepidoptera, angiosperms) the degree of ecological divergence is correlated with the degree of reproductive isolation after statistically removing effects of time (Funk et al, PNAS, 2006). 536 536 5  11/21/24 Three closely related Drosophila species, D. simulans, D. mauritiana, and D. sechellia, share most DNA sequence variation, even though they are reproductively isolated. Many genes contribute to reproductive isolation. Supports the idea that isolation evolves at a rate generally greater than the rate of genetic drift Population genetic analysis of hybrid incompatibility genes reveals evidence of adaptive protein evolution (e.g., McDonald-Kreitman tests) 537 537 Why might speciation rates be higher in some groups than in others? Dispersal and opportunities for allopatry Variation in mating systems or the intensity of selection Clades showing evidence of strong sexual selection tend to be more species rich 538 538 6  11/21/24 539 539 High Rates of Species Accumulation in Animals with Bioluminescent Courtship Displays Current Biology, 2016 540 540 7  11/21/24 Clades using bioluminescence for courtship rather than other functions have more species 541 541 Reinforcement – selection acts directly to increase isolation, as opposed to isolation being an incidental effect of selection on other traits. Ancestral population Allopatric divergence leads to partial isolation - Populations come into contact resulting in partial hybrid unfitness (allows gene flow) - Selection favoring phenotypes associated with assortative mating. - Eventual evolution of increased prezygotic isolation in spite of gene flow. This is reinforcement. Isolation is a direct result of selection on traits promoting assortative mating. Pre-zygotic isolating mechanisms are subject to reinforcement. 542 542 8  11/21/24 Factors working against reinforcement - Gene flow into hybrid zone works against selection - Alleles increasing assortative mating may have deleterious pleiotropic effects outside hybrid zone - Alleles favoring assortative mating may not be strongly genetically associated with alleles reducing hybrid fitness In the race between selection and gene flow, either selection will win (speciation) or gene flow will win (fusion). 543 543 Empirical investigation of reinforcement Data required to support reinforcement - Hybridization occurs in nature - Hybrids have reduced (but non-zero) fitness - Assortative mating is stronger in contact zone Reinforcement is a process/mechanism. In contrast, character displacement is a pattern and can be explained in the absence of reinforcement - Character displacement can occur in species pairs that are completely isolated and therefore, not experiencing reinforcement. 544 544 9  11/21/24 Empirical examples of reinforcement In Hyla cinerea, male songs from sympatric and allopatric populations are different and females from sympatric populations show stronger preferences for males of their own species H. cinerea H. gratiosa 545 545 Drosophila pseudoobscura and D. persimilis occur in a region of sympatry In general, female D. pseudoobscura from sympatric populations (S) discriminate much more strongly against D. persimilis males compared to females from allopatric populations (A) From Noor, 1995, Nature 546 546 10  11/21/24 547 547 See FK, p. 248 for examples from Phlox and flycatchers… 548 548 11  11/21/24 How general is reinforcement? FK 9.26 Each point represents a pair of species and their corresponding genetic distance and degree of sexual isolation. Genetic distance is used as a proxy for time since common ancestor Data from Coyne and Orr, 1989, Evolution, Patterns of speciation in Drosophila 549 549 Sympatric Drosophila species pairs have a strong tendency to show greater isolation at lower genetic distances compared to allopatric species pairs Such data also allow one to use molecular clocks to estimate how long it takes species to evolve in flies. Complete isolation may take about 1 MY for allopatric taxa and only about 0.2 MY for sympatric taxa. 550 550 12  11/21/24 Sympatric speciation - in situ evolution of reproductive isolation 1) Is probably rare 2) Theoretical analysis shows that while possible, is unlikely. Difficulty is that divergence must occur while there is no impediment to gene flow. Requires strong selection and tight linkage required between mate choice genes and performance genes (see FK, p. 242-243). FK, 9.21 3) “Magic traits” that influence ecological and reproductive isolation make sympatric speciation more likely. 4) Many supposed examples of sympatric speciation can alternatively be interpreted as cases of rapid evolution in allopatry followed by range expansion leading to current sympatric distribution 5) Allopatric speciation is a strong null hypothesis 551 551 Apple vs. hawthorne populations in Rhagoletis pomonella used to be considered a good example of sympatric speciation. These types show very low divergence across most of the genome, but it appears that the alleles contributing to the apple vs. hawthorne races could have evolved in allopatry and be associated with a chromosome inversion. 552 552 13  11/21/24 African cichild radiations in Lakes Malawi, Tanganyika and Victoria had been considered plausible examples of sympatric speciation, but historical episodes of allopatry within lakes cannot be ruled out. The crater lake cichlids of Cameroon may be explained by sympatric speciation, but doubts have been raised about this example as well. The cichlid fauna of these two crater lakes are monophyletic and extremely young (about 10,000 yrs). The lakes provide no obvious opportunties for allopatry Schllewen et al., 1994, Nature 553 553 The palms of Lord Howe Island are used as an example of sympatric speciation. But this example may be interpreted in other ways (FK 9.23) 554 554 14  11/21/24 The best examples of sympatric speciation are due to culturally driven reproductive isolation in birds. Indigo birds are nest parasites. Their young are reared by other species and imprint on this other species song 555 555 Speciation associated with hybridization or gene flow… 556 556 15  11/21/24 A rare hybrid speciation event in Galapagos finches? 1) A male G. conirostris male emigrated to Daphne Island and mated with G. fortis. 2) The offspring of this hybrid imprinted on the distinct male song, leading to mating within the family for several generations. See FK, p. 255 for another possible example of hybrid speciation in birds… Catherine E. Wagner Science 2018;359:157-159 Published by AAAS 557 FIGURE 2.12 Possible hybrid origin of some diploid species of sunflowers 558 16  11/21/24 Strong selection can maintain separate species in the face of small amounts of gene flow between species (exception to BSC?). FK. 9.5 Patterns of introgression across hybrid zones formed by secondary contact can provide information on the locations of genes that are incompatible in hybrid genotypes. Genes that do not cross species boundaries are near species incompatibility factors. 559 559 Alternatively, sometimes a strongly favored allele can move through a hybridization event. FK 9.6 560 560 17  11/21/24 Is “speciation with gene flow” more common than strict allopatry? Still unclear, but population genomics results suggest the possibility.. 561 561 The time for speciation may vary dramatically across groups Lake Malawi cichlids of the genus Tropheops may take only about 10,000 years to speciate, as estimated by molecular evolution data (Won et al., PNAS, 2005). Speciation rates in passerine birds may be considerably slower than those in Drosophila. See Lijtmaer et al. 2003, Evolution. Plants may speciate virtually instantaneously by polyploidization. 562 562 18  11/21/24 Many flowering plant species were formed by the hybridization of diploid species to yield a tetraploid (allopolyploid = from two species) descendant. Tetraploids can self or cross with other tetraploids but produce sterile offspring if crossed with diploids. The tetraploid mint Galeopsis tetrahit was formed by hybridization of two diploid mints. Plants resembling G. tetrahit can be created in the laboratory. Polyploid speciation in plants may be facilitated by selfing or vegetative propagation (see FK, p. 253 ). 563 563 The X chromosome has a disproportionate effect on male hybrid sterility in Drosophila This is known as the “large X-effect”. Male fertility in D. pseudoobscura (white)/ D. persimilis (purple) hybrids is greatly affected by the X chromosome 564 Presgraves, Trends in Genetics, 2008 564 19  11/21/24 Figure 2. Distribution of D. mauritiana Introgressions in the D. sechellia Genome red triangle = male sterile introgressions white triangle = male fertile introgressions Masly JP, Presgraves DC (2007) High-Resolution Genome-Wide Dissection of the Two Rules of Speciation in Drosophila. PLoS Biol 5(9): e243. doi:10.1371/journal.pbio.0050243 http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0050243 565 Haldane’s Rule may have multiple explanations: 1) Faster X evolution 2) X-A incompatibilities that are recessive are exposed in the heterogametic sex 566 566 20  11/21/24 In Drosophila, the genes Ods, Hmr, Nup96, and Nup160 contribute to species incompatibility. All show evidence of adaptive protein evolution (MK test). The agents of selection driving adaptive evolution of these genes is a mystery. Recent evidence from Drosophila suggests that genes that contribute to hybrid male sterility may also cause sex-ratio phenotypes in hybrid males (Tao et al. 2001 PNAS, Phadnis and Orr, 2009 Science). Finally, recent evidence in Drosophila supports aberrant regulation of heterochromatin in hybrids as a cause of hybrid breakdown. Ongoing genomic conflicts, for example, sex-ratio meiotic drive in flies, may contribute to speciation. 567 567 21