GENETIC VARIATION
Gene Pools
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Many genes have two alleles, or forms. Animals such as horses, dogs and mice often have several alleles for traits such
as body size or coat color.
Some animals may be heterozygous for as many as 15% of its genes. There is a lot of genetic diversity in life’s
organisms!
Genetic variation is studied in populations. A population is a group of individuals of the same species that interbreed.
Because members of a population interbreed, they share a common group of genes called a gene pool.
A gene pool consists of all genes, including all the different alleles, that are present in a population.
o The relative frequency of an allele is the number of times that allele occurs in the gene pool. It is often
expressed as a percentage.
o The relative frequency of an allele has nothing to do with whether it is dominant or recessive. Sometimes the
recessive allele can occur more frequently than the dominant!
Gene pools are important to evolutionary theory because evolution involves changes in populations over time.
o Evolution is any change in the relative frequency of alleles in a population.
Sources of Genetic Variation:
There are 2 sources of genetic variation:
Mutations:
 Changes in a sequence of DNA
 Most have no effect on phenotype, but those that do, can affect an organism’s fitness, or its ability to survive
and reproduce in its environment.
Genetic Shuffling:
 This occurs during the production of gametes. The 23 pairs of chromosomes found in humans can produce
about 8 million different combinations because they separate independently of one another during the
formation of egg and sperm.
 Crossing-over further increases the number of different genotypes that can appear in offspring.
o When alleles combine during sexual reproduction, they can produce dramatically different
phenotypes.
Single Gene and Polygenic Traits
The number of phenotypes produced for a given trait depends on how many genes control the
trait. There are two types:
o Single gene traits: controlled by a single gene that has two alleles (ex: widow’s peak) –
only two phenotypes are possible – widow’s peak or no widow’s peak
o Polygenic traits: controlled by two or more genes. Has 2 or more alleles. One polygenic
trait can have many possible genotypes and phenotypes! Ex: height
Natural selection does not act directly on genes – it acts on an entire organism, not just a gene, It is
POPULATIONS, not individual organisms, that can evolve over time!
Natural Selection on Single-Gene Traits
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Can lead to changes in allele frequencies and thus to evolution.
o Example: A group of lizards is mostly brown. But mutations lead to some red and black forms.
If red is more visible to predators, they’ll be less likely to survive and reproduce and that red
allele might not become common. Let’s say black lizards are more successful because they can
absorb more sunlight and their higher body temperatures allows them to move faster to feed
and avoid predators. They’ll reproduce and pass that black allele to their offspring. The allele
for black color might increase in relative frequency over time.
Natural Selection on Polygenic Traits
Polygenic traits are more complex and a variety of phenotypes exist. The fitness of individuals with rare phenotypes
is where natural selection can act, because some of these rare phenotypes will HELP them in their environment and
some WON’T help.
GENETIC DRIFT
 Natural selection is not the only source of evolutionary change, though.
 In SMALL populations, an allele can become more or less common simply by change.
The smaller the population is, the farther the results may be from what the laws of
probability for genetics predict. This kind of random change in allele frequency is
called genetic drift.
o In small populations, individuals that carry a particular allele may leave more
descendants than other individuals, just by chance. Over time, a series of chance
occurrences of this type can cause an allele to become common in a population.
 Example: a small population colonizes a new habitat, or a natural disaster
takes place.
o When the allele frequencies change as a result of the migration of a small
subgroup of a population, this is called the founder effect.
 Example: several hundred species of fruit flies found on different
Hawaiian Islands descended from the same original mainland population,
but those species in different habitats on different islands now have allele
frequencies that are different from the original species.
Genetic Equilibrium is reached when NO evolution occurs. There are 5 things that have to happen to maintain genetic
equilibrium from generation to generation:
1. There must be random mating (a.k.a. equal opportunity)
2. The population must be very large (genetic drift has less effect on large populations)
3. There can be no movement into our of the population (new individuals may bring new alleles into a population,
so gene pools of one population must be kept separate from the gene pool of another population)
4. No mutations occur (new alleles might result)
5. No natural selection occurs(no phenotypes can have a selective advantage over another)