Download Variation and Gene Pools

Variation and Gene Pools
• A gene pool consists of all genes,
including all the different alleles, that are
present in a population.
– Think of a gene pool as a reservoir of all
possible genes within a specific organism.
– The relative frequency of an allele is the
number of times that allele occurs in a gene
pool
– Evolution simply involves the change in the
relative frequency of alleles in a population
Variation and Gene Pools
This diagram shows the gene
pool for fur color in mice. What
does this tell you regarding its
frequency in the mice population?
Sources of Genetic Variation
• The two major sources of genetic variation
are mutations and the genetic shuffling
from sexual reproduction
– Remember, mutations are a change in a
sequence of DNA.
– Genetic shuffling occurs during the formation
of gametes and subsequent fertilization
• Crossing over also leads to genetic shuffling
Single Gene and Polygenic
Traits
• The number of phenotypes for a given trait
depend on how many genes control that
trait
– Single gene traits are controlled by a single
gene with two alleles
– Polygenic traits are controlled by two or
more alleles
Single Gene Traits
The presence of a widow’s peak is controlled by a single gene.
Look at the graph, is this gene dominant or recessive?
Polygenic Traits
Polygenic traits often have two or more alleles. As a result, there
is a wider variation in genotype and phenotype. Look at the graph
and the picture. How does this example differ from a single gene
trait?
Single and Polygenic Alleles
Single Gene Allele
Polygenic Allele
A pair of alleles, for
example, [A] and [a]
control the trait
Multiple alleles, for
example, [A], [B], [C],
and [D] control the trait.
Widow’s peak might
have [W] as the
dominant allele and [w]
as the recessive allele
Allows for a wide variety
of phenotypes and
genotypes (such as AB,
AC, BC, BD, AD, etc.)
Natural Selection’s Effect on
Genes
• Note, natural selection never acts directly
on genes but instead acts on the entire
organism
– Natural selection does not change genes!
• If an individual dies without reproducing,
the individual’s genes does not contribute
to the gene pool.
• If an individual reproduces, its alleles stay
in the gene pool and over time, may
increase in frequency.
Natural Selection’s Effect on
Single Gene Traits
Natural selection on single gene traits can lead to changes
in allele frequency and lead to the evolution of a new
species
This diagram shows you how brown and black alleles have increased in
frequency over time. Meanwhile the red allele has disappeared from the
gene pool!
Natural Selection’s Effect on
Polygenic Traits
On polygenic traits, natural selection can affect the distribution of
phenotypes in three ways
Directional selection occurs when individuals at one end of the curve have higher
fitness than individuals at the other end. Suppose a food shortage causes the
supply of small and medium-sized seeds to run low, leaving only larger seeds. Birds
with large beaks would benefit. Look at the graph, what would then happen with
beak size over time?
Natural Selection’s Effect on
Polygenic Traits
On polygenic traits, natural selection can affect the distribution of
phenotypes in three ways
Stabilizing selection occurs when individuals toward the center of the curve have
higher fitness. Think of human infants and birth mass. Those in the middle are
“healthiest” while very large and very small infants have a higher probability of being
“not healthy”.
Natural Selection’s Effect on
Polygenic Traits
On polygenic traits, natural selection can affect the distribution of
phenotypes in three ways
Disruptive selection occurs when individuals at the extremes have a higher fitness
than those in the middle. For example, suppose middle-sized seeds for birds in an
area suddenly disappear, leaving only small and large seeds. This environment
favors birds with small and large beaks. Over time, two distinct phenotypes (large
and small beaks) appear.