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Entomology 601 2015 Lecture 27 Allopatric Speciation Species When we trade in ideas in population biology, species constitute our currency. Their peculiar significance was long ago appreciated by Darwin’s mentor, Sir Charles Lyell: “The ordinary naturalist is not sufficiently aware that when dogmatizing about what species are, he is grappling with the whole question of the organic world and its connection with a time past and with man, that it involves the question of man and his relation to the brutes, of instinct, intelligence and reason, of Creation, transmutation, and progressive improvement or development. Each set of geological questions and of ethological and zoological and botanical are parts of the great problem, which is always assuming a new aspect” (H.E.H. Paterson) Speciation • Perhaps if we examine the processes by which species are formed, we can better understand the endpoints anagenetic change Study of Speciation We are interested in events that happened in this time frame anagenetic change A B We must reconstruct these events by studying attributes of the products, A and B anagenetic change A B But the differences between A and B include: 1- differentiation that occurred during speciation 2- differentiation in each species that occurred after speciation anagenetic change A B 2- differentiation in each species that occurred after speciation 1- differentiation that occurred during speciation Perhaps if we look at enough cases, we can determine commonalities • Is differentiation in any set of attributes always associated with speciation? • Unfortunately, review of literature does not provide conclusive answer • Differentiation in morphology, chromosomes, DNA sequences, biology, behavior, ecology etc. may or may not be found in apparent sister species Neodarwinian view of species • Species are a reproductive community • Composed of number of populations • Gene pool of species contains substantial genetic variation • Gene flow within and among populations is a strong cohesive force Neodarwinian view of species • Speciation involves extensive replacement and reorganization within gene pool • “Genetic revolution” • Old groups of co-adapted allele complexes broken up • New co-adapted allele complexes formed in response to new selective pressures • This also results in reproductive isolation and speciation How about genetics of reproductive isolation? • Templeton (1981*) reviewed literature on hybrid incompatibility • Perhaps these will reveal critical genetic changes that occur during speciation • In some cases, only few specific loci involved in hybrid incompatibility • Hybrid incompatibility due to a breakdown in some specific system involved in growth and life history * Ann. Rev. Ecol. Syst. 12:23-48 Genetic Basis for Hybrid Incompatibility • Type I = polygenic – Many segregating units – Each with a small effect • Type II = one or a few major segregating units • Type III = complementary or duplicate pairs of loci • Apparently, notion that speciation ALWAYS involves massive genetic change (“genetic revolution”) is not supported by the evidence Coyne and Orr (1998) Phil. Trans. R. Soc. Lond. B 353:287-305 Coyne and Orr (1998) Phil. Trans. R. Soc. Lond. B 353:287-305 Geographical Distributions • • • • Allopatric Sympatric Parapatric Peripatric – Central population surrounded by peripheral populations • Allochronic Neodarwinian Model of Allopatric Speciation Neodarwinian Model of Allopatric Speciation • Gene flow between pops interrupted • Causal agent is geographical isolation • May occur over long period of time • Two sets of populations exposed to different selective regimes • Stochastic genetic changes may also occur Stage One Stage Two • Contact is resumed between populations • Two possible outcomes: • 1- A single gene pool is re-established • 2- two groups continue to diverge due to reproductive isolating mechanisms that evolved during stage I Speciation! “Biological” Species Concept • Species are groups of populations that are actually, or potentially, reproductively isolated from other such groups (Ernst Mayr) Mechanisms for Pre-Mating (Pre-Zygotic) Reproductive Isolation • Differences in mate recognition system – Courtship – Pheromones – Coloration • • • • Ecological or habitat differences Seasonal or temporal isolation Mechanical factors - genitalia Sperm competition or incompatibility Mechanisms for Post-Mating (Post-Zygotic) Reproductive Isolation • Hybrid inviability • Hybrid sterility • Hybrid breakdown “Biological” Species Concept • Species are groups of populations that are actually, or potentially, reproductively isolated from other such groups • Why potentially? (Ernst Mayr) Stage Two is seen as a necessary part of the process • Species are “tested” • Reinforcement of post-mating reproductive isolation by selection for premating RI • Allopatric distributions are always a problem A Common Assumption • Post-mating isolating mechanisms may often be present at end of Stage One • Pre-mating isolating mechanisms may or may not be present at end of Stage One • But if pre-mating R.I. is not present, hybrid matings result in genetic death • This REINFORCES pre-mating R.I. An important distinction • REINFORCEMENT = evolution of increased reproductive isolation resulting from selection against hybrids • CHARACTER DISPLACMENT = interactions between sympatric species causing divergence in some characters – REPRODUCTIVE CHARACTER DISPLACMENT = character displacement involving mate recognition, etc. Demonstration of reinforcement has been difficult • Difficult to separate from reproductive character displacment • Difficult to model, genetically • Excellent recent review in Coyne and Orr’s book Reinforcement A A+B B Selection occurs only here If hybrids inviable, generally A or B will win out Other form goes extinct Reinforcement A A+B B Selection occurs only here If hybrids viable, A/B backcrosses lead to complex genetic backgrounds Outcome is a cline or spectrum of genotypes Models of Reinforcment • Requires substantial postzygotic reproductive isolation and • low errors in mate choice Models of Reinforcment • Extinction of one or the other also a possibility Coyne and Orr (1989) Evolution 43(2):362-381 • 119 pairs of Drosophila species • Known geographic ranges (sympatric or allopatric) • Known genetic distances • Known mating discrimination – pre-zygotic reproductive isolation • Known sterility and inviability – post-zygotic reproductive isolation Total RI vs. genetic distance Genetic distance and reproductive Isolation • Substantial prezygotic RI can occur with very low genetic distance Sympatry vs. Allopatry • Much stronger prezygotic RI in sympatry! • Is this reinforcement? • Problem: appears that generally there is not much postzygotic RI to reinforce Sexual Selection • Competition for mates • One sex (often the female) preferentially chooses among many possible mates • Choice based on male phenotype • Strong selection for genetics underlying the traits in successful males • May or may not have anything to do with fitness of progeny Sexual Selection The Tufted Coquette Lophornis ornatus (male on right) Sexual Selection Sexual Selection Sexual Selection • May involve traits of mate recognition systems • Evolution in such systems may be very rapid