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Selection is the process by which organisms that are
better adapted to their environment tend to survive
and breed, while those that are less well adapted
tend not to
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Different
environmental conditions favour different characteristics in the population
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These individuals will be more likely to survive and breed
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Over
time, the mean will then move in the direction of these
individuals
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Some populations of bacteria had developed resistance to antibiotics, it was because of a chance
mutation in the bacteria
...
The population increased at the
one direction form the mean of the populaexpense of the non-resistant population so the frequency
tion
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increased in the population = populations normal distribuSelection may favour average individuals
...
Types of selection
acteristics of a population
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These types of characteristics are
more influenced by the environment than ones determined by a single gene
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Directional Selection:

Stabilising Selection:

If the environmental conditions change, the phenotypes (the observable physical and biochemical characteristics of an organism) that are best suited to the
new conditions are most likely to survive
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These individuals are more likely to pass their
alleles on to the next generation
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Stabilising selection therefore tends to
eliminate the phenotypes at the extremes
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Stabilising selection therefore results in phenotypes
around the mean of the population being selected for
and those at both extremes being selected against
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these can
be:
Anatomical, such as shorter ears and thicker fur in arctic
foxes compared to those in warmer climates
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Behavioural, such as autumn migration of swallows
from the UK to Africa to avoid food shortages in the UK
winter

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