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Evolution of Populations
Chapter 16
Gene and Variation
Although Mendel and Darwin both worked in
the 1800’s, they were not able to share
information as scientists do today. By the
1930’s evolutionary biologists started to
focus on genes as a way to understand
evolutionary change.
Gene and Variation
When the double helix model of DNA was
presented in the 1950s, evolutionary
biologists were able to demonstrate the
molecular nature of mutation and genetic
variation.
Genetic Variation
Genetic variation refers to differences in genes
that produce heritable variation in traits.
(different geneotypes = different phenotypes)
The two main sources of genetic variation are
mutations and the normal shuffling of genes
as a result of sexual reproduction.
Genetic Variation
Most mutations involve only
minor changes in DNA and
do not lead to visible
changes in the phenotype.
Some minor changes in DNA do
have major effects on the
organism.
Genetic Variation
Genetic variation is studied in populations. A
gene pool consists of all the genes, including
the different alleles that are present in a
population.
Genetic Variation
The relative frequency of an allele is the
number of times that the allele occurs in a
gene pool; compared with the number of
times other alleles for the same gene to
occur.
It has nothing to do with whether an allele is
dominant or recessive!
Genetic Variation
In genetic terms, evolution occurs when there
is a change in the relative frequency of alleles
in a population = EVOLUTION!!
Sexual reproduction produces many different
combinations of genes but does not change
the relative frequencies of each type of allele
in the population. (the alleles are still there,
just shuffled!)
Genetic Variation
The number of
phenotypes produced
for a given trait
depends on how
many genes control
that trait. Single-gene
traits have two
alleles– dominant or
recessive.
Genetic Variation
Polygenic traits are controlled by two or more
alleles usually have many genotypes and a
range of phenotypes. Graphing polygenic
traits usually results in a bell shaped curve!
Evolution as Genetic Change
Natural Selection on singlegene traits can lead to
changes in allele
frequencies and thus to
evolution. Organisms of
one color may be more
visible to predators and
therefore less likely to
survive and reproduce
than organisms of
another color.
Evolution as Genetic Change
Natural selection on
polygenic traits are more
complex and can affect
the distribution of
phenotypes in any of
three ways: directional
selection, stabilizing
selection or disruptive
selection.
Evolution as Genetic Change
Think of the bell shaped curve representing a
range of phenotypes, each of these three
changes result in changes in the bell shaped
curve!
Directional Selection
Directional selection is a form of natural
selection in which the entire curve moves.
This occurs when individuals at one end of
the curve have a higher fitness than those at
the other end or in the middle.
Stabilizing Selection
Stabilizing selection is a form of natural
selection by which the center of the curve
remains in its current position because those
in the middle have higher fitness than those
at either end.
Disruptive
Selection
Disruptive Selection is a form of natural selection in
which a single curve splits into two! This occurs
when individuals at the upper and lower ends of a
distribution curve have higher fitness than
individuals near the middle resulting in a
population splitting into two sub groups.
Genetic Drift
Genetic drift is a random change in allele
frequency and may occur when a small group
of individuals or more migrate to colonize a
new habitat.
Genetic Drift
This group may carry different allele frequencies than
the larger population that they came from had. The
cause here is NOT natural selection, but the chance
that certain alleles were in this small subgroup. The
resulting change in allele frequencies is known as
founder effect.
GENETIC DRIFT
Start with five different alleles
Due to random events, only two
different alleles are left after several
generations.
Gene Flow
• Gene flow, or genetic immigration, is the
process in which genes "flow" from one
population to another. This movement of
genes helps maintain genetic diversity, or
genetic variation within populations.
Genetic Equilibrium
Genetic Equilibrium is a situation in which allele
frequencies remain constant and evolution
does not occur. The Hardy-Weinberg
Principle states that allele frequencies in a
population will remain constant unless one or
more factors cause those frequencies to
change.
The five conditions required to maintain genetic
equilibrium from generation to generation are:
1. There must be random mating
2. The population must be very LARGE
3. There can be no movement into or out of the
population
4. No mutations
5. No natural selection
These conditions may be met for long periods of
time, but eventually the equilibrium will be
disrupted and the populations will evolve.