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Natural Selection







Darwin introduced the idea of the common ancestry of all living things.
He hypothesised that all species have descended from one of a few common ancestors.
Species that share a recent common ancestor tend to resemble one another in many respects.
New species do not arise from independent…

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Coelacanths are considered living fossils, but they are genetically different to the ones living
millions of years ago ­ they haven't changed physically because their environment hasn't changed,
but they are still subject to natural selection.
Humans are no more evolved than any other organism ­ human intelligence is the…

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Oldfield mice live in two different habitats, inland on the dark soil of the Oldfield, and on the
beach.
Hypothesis: natural selection favours a match between coat colour and environmental
background.
Mice with pale fur do well on beaches, and mice with dark fur do well in Oldfield areas.

There…

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With selection the frequency of the aa genotype at the next generation will be q2 (1 ­ s).
2 2
The dominant allele will increase in frequency in each generation by: pq s/(1 ­ q s).
In our example the fitness associated with the dark/light phenotypes was not directly dependent…

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They mate in a face to face position and because of physical constraints, mating in these species
can only take place between individuals whose shells coil in the same direction.
Selection occurs against the rare trait because they find it more difficult to find a mate.
However chirality still occurs…

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As the 1:1 sex ratio is approached, the advantage associated with producing females dies away ­
Evolutionary Stable Strategy (ESS).
Example 3: hostparasite relationship.
The interaction between a host species and a parasite population is expected to be under negative
frequencydependent selection.
A host that is adapted to and therefore…

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Genetic drift causes heterozygosity in a population to decrease over time.
The WrightFisher model predicts that heterozygosity will on average decrease by 1/2Ne in each
generation, where Ne is the effective population size.





Bottlenecks cause drastic shifts in allele frequencies.
Each population starts with the A1 and
A
2 alleles…

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From the 1960s, if there was variation in a population, then explanation for its maintenance
required strong natural selection.
There was not enough selection to account for much of the variation, which led to the Neutral
theory of molecular evolution.
Mootoo Kimura proposed that most DNA or amino acid variation…

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