Download Evolution of Populations

Evolution of Populations
Chapter 16
• The study of genetics helps scientists
understand the relationship between
inheritance and evolution
• Scientists know that…
–genes control traits and that many
genes have at least two forms, or
alleles
-members of all species are
heterozygous for many genes
•
In genetic terms,
evolution is any
change in the relative
frequency of alleles in
a population
• A population is a group of
individuals of the same species
that can interbreed
• Members of a population share a
gene pool
- all the genes, and their
different alleles, in the population
• The number of times the
alleles occurs in a gene
pool compared to the
number of times that other
alleles for the same gene
occur is the relative
frequency of the allele
• The two main sources of
genetic variation are
mutations and gene shuffling
• A mutation is any change in a
sequence of DNA
• Gene shuffling occurs during
gamete formation (crossingover)
• It can produce millions of
different gene combinations,
but doesn’t change the allele
frequency in the population
• Both mutations and gene
shuffling increase genetic
variation by increasing the
number of different genotypes
• The number of phenotypes for
a trait depends on how many
genes control the trait
• A single-gene trait is a trait
controlled by only one gene
• If there are two alleles for the
gene, two genotypes are
possible
• In humans, a single-gene trait is the presence
of a widow’s peak
• Allele for a widow’s peak is dominant over
the allele for hairline with no peak.
• As a result, there are only two phenotypes –
having a widow’s peak or not
• A polygenic trait is controlled by two or
more genes
• Each gene of a polygenic trait may have
more than one allele
• Polygenic traits form many phenotypes
• Variation in a polygenic trait in a
population often forms a bell-shaped
curve with most members near the
middle
• example: height in humans
16.2 Genetic Change
•Natural selection on
single-gene traits can
lead to changes in
allele
frequencies…evolution
Natural Selection on Polygenic Traits
• Directional Selection
Stabilizing Selection
Disruptive Selection
Genetic Drift/Founder Effect
Hardy-Weinberg Principle
• Allele frequencies in a
population will remain
constant unless one or
more factors cause them to
change
• Genetic Equilibrium
5 Conditions to Maintain G.E.
1. Must be random mating
2. Population must be very large
3. No movement into or out of the
population
4. No mutations
5. No natural selection