Natural Selection and Genetic Drift

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  • Created on: 21-05-16 11:59
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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 acts of creation or spontaneous generation but from
preexisting species.
This process generates a branching pattern of ancestry that connects all life (phylogenetic tree).
Natural selection is limited in what it can do ­ it is constrained by the laws of physics which
influence the form and structure of living organisms.
There are physical constraints, for example an elephant cannot grow long legs because it is limited
by the size of the femur.
Evolution has no foresight and no purpose ­ what works now is what matters.
Even if there is variation in a given character, selection may be unable to act if the genes involved
have effects on other characters that are also under selection.
Natural selection has no way to anticipate the future.
It favours changes which are immediately beneficial, not changes that may be useful in the future.
Consequently there is no such thing as progress in evolution.
Natural selection adapts populations of organisms only to the specific local circumstances
encountered by each population.
This may produce some shortterm improvement and directional change as the organism adapts
better to local conditions, or as the environment changes.

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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 equivalent of a
snake's venom.
It is possible to get evolution without natural selection.
It can occur by random genetic drift.…read more

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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 are two major genes responsible for the coat colour.
The melacortin1 receptor gene (Mc1R) is known to influence coat colour in many species of
mammals and birds.…read more

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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
on the frequency of the trait in the population.
This is frequencyindependent selection.…read more

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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 because it is linked to another gene.
PFS may favour genetic differentiation and in the long term possibly speciation.…read more

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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 resistant to the parasite will have the highest fitness.
But a parasite population that is of a rare type will have an advantage, and would be expected to
spread.…read more

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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 each at frequency
50%.
Allele frequencies drift gradually until the population bottleneck,
at which point the drift accelerates dramatically, causing large
changes in allele frequencies.…read more

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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 is neutral with respect to fitness.
This serves as a null model against which to test models of natural selection.…read more

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