Population Notes
A population is a collection of individuals of a species in a given area. All members
of a population can interbreed (meaning they can breed together), they share a
common group of genes called a gene pool.
A gene pool contains two or more alleles (or forms) of a certain gene for each trait.
Example: A mouse population may have two alleles for fur color (B for black fur; b for brown fur).
The frequency of these alleles will occur a certain amount of time within the gene pool.
The relative frequency of an allele is the number of times that allele occurs in a gene pool compared with the
number of times other alleles occur.
Genetic Variations- mutations and genetic shuffling
1. Mutations- any change in a sequence of DNA.
 Do not always affect an organism‘s phenotype.
Example: Point mutation of a DNA codon from GGA to GGU will code for the same amino
acid, glycine. This mutation has no effect on the organism’s phenotype.
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Many mutations produce changes in the organism’s phenotype which can
affect its fitness, or its ability to survive and reproduce.
2. Gene shuffling- causes most inheritable differences and occurs during the
production of gametes (sex cells).
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Crossing over can further increase the number of different genotypes that
can appear in offspring.
Sexual reproduction can produce many different phenotypes.
Natural Selection Acting on Traits
Natural selection does not act directly on genes. Instead, it acts on phenotypes.
It determines which alleles are passed on from one generation to the next.
Natural selection can affect the distributions of phenotypes in any of three ways: stabilizing selection,
directional selection, and disruptive selection.
1. Stabilizing Selection- When individuals near the center of the curve have higher fitness than individuals
at either end of the curve.
What happens? This keeps
the center of the curve at its
current position, but
narrows the overall graph.
Example: A mother bird
would overheat small eggs
and wouldn’t keep large
eggs warm enough. The
medium sized babies would
survive.
2. Directional Selection- When individuals at one end of the curve have higher fitness than individuals in
the middle or at the other end.
What happens? The entire
curve moves as the character
trait changes.
Example: When food
became scarce, the finches
with larger beaks were
better able to survive and
reproduce. Thus, the
average size of the beaks
increased.
3. Disruptive Selection- When individuals at the upper and lower ends of the curve have higher fitness
than individuals near the middle.
What happens? Selection
acts most strongly against
individuals of an
intermediate type.
Example: There are only
large nuts for birds with
large beaks and tiny seeds
available for birds with
small beaks.
If disruptive selection is strong enough and long enough, the single curve may split into two; creating two
distinct phenotypes and possibly a different species.