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Biol 121- 2010.03.24- Evolution- Speciation (Ch. 26).docx

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University of British Columbia
BIOL 121

Biol 121 225 Freeman, Ch. 26 Mar 24, 10 Evolution – Speciation What is speciation? -the process by which new species arise -it is where biodiversity comes from -it is the focus of evolutionary theory How does it occur? -mutation and genetic drift cause change in allele fq -gene flow between populations keeps allele fq constant -if gene flow stops, the populations will diverge and at some point, divergence is ”significant” enough that speciation occurs -when gene flow ends, allele fq in the isolated populations are free to diverge and evolve independently -if mutation, selection, and genetic drift cause isolated populations to diverge sufficiently, distinct types, or species form – speciation Speciation creates evolutionarily -large ground finch and medium ground finch are derived from same independent populations (e.g. ground ancestral population finch) -this ancestral population is split into two populations isolated by the lack of gene flow (occupying different islands once the ancestral population splits into separate populations) -because populations began evolving independently, they acquired the distinctive characteristics observed today Barriers that prevent reproduction (ie. -the two main types of barriers that prevent reproduction or prevent gene Prevent gene flow) (2 main types) flow are prezygotic barriers and postzygotic barriers -prezygotic barriers prevent mating/fertilization while postzygotic barriers do not prevent mating/fertilization but offspring might not be fertile, or might not fully develop 1) 1) Prezygotic barriers -prevent mating/fertilization -there are three subtypes of prezygotic barriers a. Habitat isolation -if populations are not in the same place at the same time, then there is no opportunity to mate (or, populations are isolated b/c they breed in different habitats) b. Behavioural isolation -populations do not interbreed because their courtship displays differ -e.g. dances and songs in birds only attract or appeal to mates of one species (they are specific to one species only) c. Temporal isolation -populations are isolated b/c they breed at different times -e.g. Dendrobium – different species of flowers will have pollinators collect pollen on different days -e.g. Bishop pines and Monterey pines release their pollen at different times of the year d. Mechanical isolation -matings fail b/c male and female genitalia are incompatible 1 Biol 121 225 Freeman, Ch. 26 Mar 24, 10 e.g. insects: think of mechanical isolation as lock and key for reproductive parts -sex organs fit together of males and females very specifically -e.g. hummingbirds with long beak will tend to only pollinate deep flowers e. Gametic isolation e.g. coral eggs have coat – if wrong species, the sperm cannot attach to the coat – incompatible -gametic isolation is when there is a barrier that prevents gametes from fusing (note that corals are Animalia that can reproduce sexually) 2) 2) Postzygotic barriers postzygotic barriers do not prevent mating/fertilization but offspring might not be fertile, or might not fully develop -can result in decreased hybrid fertility, or even sterility -can result in decreased hybrid viability (offspring = weakly/sick) a. Reduced hybrid fertility -hybrid offspring are mature but tend not to be fertile (cannot produce offspring) -e.g. horse has diploid number 64, donkey 62, and resulting offspring mule has 63 -this means that one chromosome cannot pair in meiosis, and the mule is thus sterile b. Hybrid viability -hybrid offspring do not develop normally, die as embryos -e.g. when ring-necked doves mate with rock doves, less than 6% of eggs hatch How do new species form? Allopatric speciation -is speciation that begins with physical isolation via either dispersal or vicariance -genetic isolation happens routinely when populations are physically separated -physical isolation occurs in one of two ways: dispersal or vicariance -populations that live in different areas are said to be in allopatry -must study biogeography – geographical distribution Dispersal and colonization -can isolate populations -first, start with one continuous population -then, colonists float to an island on a raft  founder effect -then island population begins to diverge due to drift and selection -finish with two populations (one on mainland, one on island) that are isolated from one another and eventually diverge into two species Vicariance -can isolate populations -first start with one continuous population that are all on one side of a river for example -the river changes course, and now there are two populations b/c this species cannot cross the river -isolated populations (2) begin to diverge due to drift and selection -finish with two populations isolated from each other 2 Biol 121 225 Freeman, Ch. 26
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