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How populations evolve
SECTION 15.2
Allele frequencies
An allele is a form of a gene (usually either dominant or recessive)
Each individual has 2 alleles – homozygous dominant (TT), homozygous
recessive (tt) or heterozygous (Tt)
Within a population we call all the alleles that are present the gene pool
Over time, as populations change, the percentages of these alleles can
also change
If the percentages of alleles change, then we say that the population has
evolved
Hardy-Weinberg Principle
Allele frequencies will NOT change unless evolutionary forces are
acting on the population
Five evolutionary forces:
1. Mutation
2. Gene flow
3. Nonrandom mating
4. Genetic drift
5. Natural selection
Mutation
An allele can be changed by mistakes in copying
Sometimes mutations are caused by chemicals or radiation
in the environment
Most mutations are harmful to the animal
Mutations that make an animal better able to survive will be
passed on to offspring
Gene flow
Animals move into (immigrate) and out of (emigrate)
populations
When they move, the allele frequencies may change or
‘flow’
Nonrandom mating
Most animal species select mates with certain
characteristics (sexual selection)
This may change the allele frequencies to favor these traits
Genetic drift
Small populations may not start out with a lot of variation
If they are isolated from other populations of the same
species, their allele frequencies can change
Natural selection
Individuals with alleles that improve their chances of
survival will reproduce and pass on those alleles to their
offspring
‘survival of the fittest’
How natural selection works
Natural selection only acts on the phenotype of the individual (the
physical expression of alleles)
Heterozygous individuals may still be carriers of harmful alleles
Some traits are caused by combinations of alleles – they are
polygenic
These will show a wide range of phenotypes - example: human
height
We show this range with a graph called a normal distribution
Normal distribution curve
How natural selection works…
Directional selection – the environment favors one
particular allele (either the dominant or recessive) and soon
the other individuals are no longer seen as frequently
Stabilizing selection – the environment favors individuals
with the middle range of phenotypes, so the extremes are
no longer seen as frequently
Directional vs. Stabilizing selection