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Speciation What are species? How do we identify them? How do they form? **Speciation** • Speciation: A splitting event that creates 2 or more “evolutionarily independent” groups from one ancestral group • Requirements: – Genetic isolation - cessation of gene flow (reproductive isolation) • this is all you really need – Genetic divergence follows- selection, drift, mutation promotes divergence What are species? • Any reproductively cohesive group that is “evolutionarily independent” (reproductively isolated & on different evolutionary trajectory) from other such groups – This forms the basis of our understanding of how life diversified • When species appear similar, how do we identify them as reproductively isolated? How does reproductive isolation develop? • In Sympatry (“together homeland”) – Speciation occurs in same geographical location • In Allopatry (“different homeland”) – Speciation occurs in geographic (or temporal?) isolation, when populations are separated by some barrier Isolation in Allopatry • What puts populations in allopatry? – Vicariant events • Splitting event – Dispersal events • Founding event Causes of vicariance • Plate tectonics – Plate movements; mountain building; vulcanism • Changes in sea level – Land bridge removal; island formation • Glacial cycles – Expanding glaciers intrude • Surround population in refugia; habitat change or loss; ice sheets divide range Plate movements • Ratite distribution – Ostrich, emus, kiwis, rheas, cassowaries • Hypothesized ancestor occurred throughout Gondwana Present 18,000 kya Isolation in Sympatry? • How is this possible, since gene flow stifles incipient species formation? – Strong, multifarious selection – Chromosomal duplications With natural selection • Soapbugs – Host plant switch – Choice of host correlates with beak length – Got reduced gene flow? – Got diversification? 15 10 5 0 Beak length Generation of Polyploids • Many plant species are 4n, 6n, or more…obvious duplications • How do these form? Why? – Reproductive cells have many stages of mitosis prior to meiosis…lots of options for error – Autopolyploid - mutation doubles chromosome number in a single species – Allopolyploid - parents of different species mate; produce polyploid offspring Mating of allopolyploids • Homologous chromosomes aren’t – Early duplication can save it • No pairing during metaphase I – Unequal segregation • Tetraploid individual results Speciation & hybrid formation • Multiple interbreeding populations (gene flow) • Barrier to gene flow separates one population Speciation & hybrid formation • In isolation, the population diverges. • Speciation is imminent 1. Reinforcement: – Hybrid offspring (with intermediate phenotype) have lower fitness than either P – Individuals tending to NOT hybridize leave more offspring = selection for some isolating mechanism – Selected traits reinforce diversification that began in isolation 2. Fusion: Meet & mate successfully. Hybrids and backcrosses are fit & gene flow eradicates divergence 3. Stability: Barrier retreats, gene flow is reestablished in a stable hybrid zone: – Zone of overlap between parent species where hybrids are common & fit, but backcrosses are not. • Long or short-lived, narrow or wide, stationary or moving • What produces these outcomes?...Isolating mechanisms! • Postzygotic isolation prevents multi-generation reproduction • Prezygotic isolation prevents mating What if diverging populations reconnect? • Reinforcement: Hybrids are unfit, so selection for some reproductive isolating mechanism eliminates gene flow. – Isolating mechanisms WILL appear, due to different mutations, genetic drift and selection occurring in each population. • Fusion: Meet, mate successfully, & gene flow eradicates divergence • Stable Hybrid Zone • Speciation via hybridization Types of reproductive isolation • Prezygotic mechanisms - prior to embryo formation – – – – – Habitat Temporal Behavioral Mechanical Gametic • Postzygotic mechanisms -after embryo formation – Hybrid inviability – Hybrid sterility Prezygotic mechanisms • Habitat: parasite infects new host species Prezygotic mechanisms • Temporal: Flies active at different time of day (breed at different temperatures) Prezygotic mechanisms • Behavioral: Courtship sequence differs (sallys) • Mechanical: Bits don’t fit together (hemipenes) • Gametic: Incompatible gametes; sperm cannot penetrate egg; dies early in female repro tract I.D.-ing reproductive isolation • Prezygotic mechanisms - prior to embryo formation • Postzygotic mechanisms -after embryo formation • Requires observation of mating behavior, reproductive success and offspring viability • OK for some contemporary species, but what about: – Fossil species? – Hard to watch or catch species? – Rare or endangered species? Need Operational species definition • Species Concepts – Biological: Are two groups actually or potentially interbreeding? – Morphological: Are two groups (fossils) sufficiently different from each other? – Phylogenetic: Are two groups bound by common ancestry? Do they form monophyletic groups? Morphological • Often applied to newly discovered species, before matings are observed • Applied to fossil remains of same size, shape and bone arrangement • Problem =subjective Phylogenetic • If genetically similar, good chance they are interbreeding…or could, or recently did • Uses monophyletic groups: An ancestor, ALL its descendents & ONLY its descendents – Precise - no guessing about who mated MtDNA break 100 100 0.046 0.022 ~28°