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Unformatted text preview: Speciation and Phylogeny Homework problem what happens when homozygous recessive individuals are sterile or cannot mature? What will the allele frequency be in the next generation if p = 0.8 and q= 0.2 in starting generation. We worked the problem in class but the main point is assume you have 100 individuals in the next generation. How many total alleles (200 = 100 individuals X 2 alleles each). However if the homozygous recessives are unable to reproduce we will only have 96 individuals effectively so 192 alleles. If we have BB = 64 individuals, and Bb = 32 individuals there are total of 64x2 + 32 “B” alleles for those individuals. So 160 total B alleles out of 192 total alleles is frequency of 0.83. So p = 0.83 in the next generation. You can either work it out that there will be 32 b alleles (only the heterozygotes have them) divided by 192 total alleles = 0.17. Remember that p+q = 1 so if you find p you can also find q easily as 1 – p = q Speciation is macroevolutionary study. Anagensis and cladogenesis describe formation of new species. Cladogenesis is where a new species forms from splitting of ancestral population. Biological species concept – if two individuals can mate and produce fertile offspring they are same species, if they can’t, then are different species. So how does this happen? Reproductive isolation. Two types Prezygotic ‐ habitat isolation (upper part of the forest canopy and ground) ‐ behavioral isolation ( courtship rituals – don’t do the right dance to entice a mate) ‐ temporal isolation (are fertile at non‐overlapping times, ie flowers that bloom at different times of year) ‐ mechanical isolation – morphological differences can prevent interbreeding (mimulus flowers attract different pollenators) ‐ gametic isolation – sperm of one species cannot fertilize eggs of another (ie unrecognized sperm protein coat doesn’t have the correct ‘password’ to get into the egg) Postzygotic ‐ reduced hybrid viability (hybrids are sick & frail) ‐ reduced hybrid fertility (hybrids are sterile ie horse+donkey = mule which is sterile) ‐ hybrid breakdown – F1 is successful mating, but subsequent generations feebile or sterile due to recessive alleles that accumulate. Gave example of rice strains. Remember biological species concept doesn’t apply to asexual organisms, dead organisms with only fossil records, or organisms where little is known or can be studied about reproduction. How can speciation happen – when gene flow is restricted can lead to formation of new species. (microevolutionary force is gene flow ‐‐ where gene flow stop occurring between individuals in the parts of populations making them two separate populations because there is no gene flow and eventually this leads to new species forming). ‐ Allopatric speciation ‐ Geographic boundary prevents gene flow. (ie they live in different places, their populations were split by a river, ravine, ocean, etc). ‐ Sympatric speciation – Live in the same area but gene flow is restricted by another means. ...
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- Spring '08