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Study notes for Assessment #4 Fall 2011

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Western University
Biology 1001A
Tom Haffie

Lecture 17 PhylogenySpeciationnew species reproductive isolation gives rise to tree of life diversityDefinition of speciationhow do we define species concepts again Mostly biological species concept ie reproductive isolation ie different species do not reproduce with each otherAllopatryseparation due to geographyphysical barrier ie glacier or river or just distance a founder eventcolonizing an islandGenetic isolation leads then to genetic divergence differences arise due to different selective forces genetic driftIsolation and Divergence cause Speciation new noninterbreeding speciesrd3 thing Secondary Contactwe can test whether the species can interbreed or not but are they hybrids nowReinforcementIslandshotbeds of allopatry and speciationphysically isolated from other populations by watersmall population sizeallows easier genetic drift stronger founder effectsdifferent selection pressures on islands versus the mainlandgenetic divergence selection pressuresex Darwins finches14 or 15 species all derived within a few million years from a single finch species from Ecuadorex Hawaiidrosophilaall descended from common fruitfly to 100s of new speciesSecondary Contactmay never happen2 possible outcomes 1 shortlived separation little selection pressure little divergencepopulations resume interbreeding Genetic diversity still increased Single population again2 Long time separation strong genetic drift or selection on populations large genetic divergence between population Are chromosomes now compatible or incompatible Postzygotic isolation mechanisms now come into play Offspring could be sterile or have low fitness Members that do interbreed are selected against since offspring have low fitnessReinforcementeasier to avoid mating with other species in the first place 2 pops in secondary contact selection to save energy favours development of prezygotic isolating mechanisms speciation may speed up due to these prezygotic isolating mechanismsParapatry no physical separationless powerful than allopatrybut speciation may still occurneighbouring populations Still some gene flow Intermediate region where hybrids thrivebut if hybrids are less fit and selected against then development of prezygotic isolating mechanisms is favouredSympatric Speciationoverlapping rangesrare in vertebratesforms in many plantsforming polyploid gametes by accident and thereby polyploid diploid rather than haploid offspringhappens very fast within a single generationmay happen when a new food source becomes available and mating always occurs on the food source ex Hawthorn berry maggotsmigrated to feed on applesspecies became reproductively isolatedSpeciation is reversiblego back to hybrid swarmif isolating mechanisms break downrecently diverged speciesconsider a behavioural isolating mechanismie female selection based on colour yellow or blue desireable But now the environment has changed cant tell colour apart and since postzygotic isolating mechanisms havent evolved yet the species are blending females pick yellow AND blue mates nowhybrid swarmPhylogeniesclosest common ancestorsreconstruct phylogenies based on similarities and differences between species and groupssimilarities SHOULD reflect common ancestry similar genetic materialex pigs bats and dolphins all have 4 bones in their forelimb fingers although each has much different morphology they are all relatedConvergenceunrelated species that appear very similar have similar traits due to inhabiting the same environmentsniches homoplasyDivergencesrelated species appear different homoplasyHomologysimilarity due to common ancestryHomoplasyproblem in phylogenetics
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