Speciation PDF

11/19/24 Speciation (Ch. 9) In reply to theologians who inquired if there was anything that could be concluded about the Creator from the study of Nature, JBS Haldane supposedly replied, “He has an inordinate fondness for beetles”...

11/19/24 Speciation (Ch. 9) In reply to theologians who inquired if there was anything that could be concluded about the Creator from the study of Nature, JBS Haldane supposedly replied, “He has an inordinate fondness for beetles” 494 494 FIGURE 9.1 Examples of the diversity of cichlid fishes in Lakes Tanganyika (at left) and Malawi (at right) 495 1  11/19/24 What are species? What is the geography of speciation? Why do some groups speciate more than others? What is the genetics of speciation? What is the time scale of speciation? 496 496 What are species? The biological species concept (BSC) - Groups of actually or potentially interbreeding populations, which are reproductively isolated from other such groups - Gene flow (reproductive isolation) is the focus, rather than phenotypic differences - Cryptic species - isolated but with little phenotypic differentiation Problems with BSC Asexuals Extinct groups Gene flow between “good” species Breeding experiments impractical 497 497 2  11/19/24 FIGURE 9.2 Can you distinguish the species? 498 FK, Fig 9.4 499 499 3  11/19/24 Other species concepts… The phylogenetic species concept - Two populations are species if they are “diagnose-ably” different (morphology, DNA) More species concepts Evolutionary Recognition/cohesion Genealogical The search for a single species concept that applies unambiguously and in all situations is doomed to fail. EVOLUTION IS A PROCESS, so we should expect intermediate stages and fuzzy boundaries. 500 500 Two species may remain isolated through much of their range but still interbreed in a small number of geographic regions 501 501 4  11/19/24 FIGURE 9.3 Hybrids usually show mixtures of several features that distinguish species 502 In spite of interesting, ambiguous cases, species are often reproductively isolated AND phenotypically diverged. 503 503 5  11/19/24 Cryptic species are frequently revealed by DNA sequence divergence Ten species in one: DNA barcoding reveals cryptic species in the neotropical skipper butterfly, Astraptes fulgerator Hebert et al. PNAS, 2004 504 504 Giraffes 505 505 6  11/19/24 Mechanisms of reproductive isolation (reproductive isolating barriers) are often divided into two main categories: Pre-zygotic Post-zygotic 506 506 507 7  11/19/24 Ecological isolation Cicada (e.g., 13 vs. 17 yr) Rhagoletis (apple vs. hawthorne) 508 508 Behavioral/sexual isolation Photinus (firefly) Lacewings (Chrysoperla) 509 509 8  11/19/24 Pollinator specialization generates strong isolation even though hybrids made in the greenhouse are vigorous and fertile Mimulus lewisii and M. cardinalis 510 Barton et al. 510 In many cases, multiple isolating barriers operate 511 511 9  11/19/24 Extrinsic (environment dependent) sexual isolation These closely related African cichlids are behaviorally isolated in clear water, but isolation breaks down in murky water They show no evidence of postzygotic isolation 512 Ridley 512 Another possible example of extrinsic isolation from Ridley Hybrid viability is similar to pure species viability in El Nino years, but lower than pure species viability in typical years in Darwin’s finches 513 513 10  11/19/24 In many animals, males show much greater phenotypic divergence between species than do females. Does sexual selection contribute to the evolution of reproductive isolation? 514 514 An example of post-mating, pre-zygotic isolation Data from the beetles Callosobruchus maculatus and C. subinnotatus. P2 is proportion offspring sired by second male. All females are C. subinnotatus Rugman-Jones and Eady, 2007. Gametic incompatibilities in marine invertebrates are another example 515 515 11  11/19/24 Post-zygotic isolation Hybrid inviability Hybrid sterility Haldane’s Rule - when hybrid breakdown is differentially expressed in the two sexes, the heterogametic sex is usually affected more severely Barton et al. Haldane’s Rule is very general, with only few exceptions. Hybrids often exhibit many deficits relative to pure species 516 Hybrid sterility often evolves before hybrid inviability 516 The geography of speciation Allopatric speciation is the most common mode of speciation 517 517 12  11/19/24 Empirical evidence for the importance of allopatry FK 9.19 518 518 Gradual divergence of sexual compatibility in Laupala cerasina 519 519 13  11/19/24 from Ridley 520 520 FIGURE 18.10 A pair of species that formed by vicariance 521 14  11/19/24 Sister species of snapping shrimp (Alpheus) are often on opposite sides of the Isthmus of Panama 522 522 Shared geography of hybrid zones across species pairs Grosbeak(a), flicker (b), bunting(c), oriole(d) Swenson (2006) 523 523 15  11/19/24 Island endemics Male genitalia of Drosophila simulans and two island endemics, D. sechellia, and D. mauritiana 524 524 FIGURE 9.18 Allopatric speciation is illustrated by populations that are isolated on different islands 525 16  11/19/24 Sister species are rarely sympatric (Bolnick and Fitzpatrick 2007) Speciation is much less likely to occur on small islands (Kisel and Barrowclough 2010) FK, 9.20 526 526 The Dobzhansky-Muller model of species incompatibility The problem with single-locus models… A1A1 Ancestral population 527 527 17  11/19/24 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 528 528 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 529 529 18  11/19/24 …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! 530 530 19