Download Genetics and Evolution

Genetics and Evolution
Chapter 16-2
Standard 8c,d
8c: Students know the effects of genetic
drift on the diversity of organisms in a
population
 8d: Students know reproductive or
geographic isolation affects speciation

Review

Population: A collection of individuals of the
same species in a given area
 Gene pool: The combined genetic information
of all members of a particular population
 Relative frequency: Number of times an allele
occurs in a gene pool compared with the
number of time other alleles for the same gene
occur
 Evolution involves a change in relative
frequencies of alleles in a gene pool of a
population.
Gene Pools
Sample Population
48%
heterozygous
black
16%
homozygous
black
36%
homozygous
brown
Frequency of Alleles
allele for
brown fur
allele for
black fur
Genetic Drift
Remember that mutations always occur.
 Mutations do not always have to be
beneficial in order for evolution to occur
(ie: adaptation not always necessary)
 In other words, evolution can occur in the
absence of natural selection
 Genetic drift: A random change in the
frequency of a gene

Genetic Drift
Sample of
Original Population
Descendants
Founding Population A
Founding Population B
Genetic Drift
Sample of
Original Population
Descendants
Founding Population A
Founding Population B
Genetic Drift
Sample of
Original Population
Descendants
Founding Population A
Founding Population B
Hardy Weinberg
Founder Effect: change of allele
frequency due to the migration of a small
subgroup in a population.
 Hardy-Weinberg Principle: allele
frequency will remain constant (genetic
equilibrium) unless one or more factors
cause frequencies to change.

Genetic Equilibrium

1.
2.
3.
4.
5.

Five conditions are required to maintain
genetic equilibrium
Random mating
Population must be large
No movement in or out of population
No mutations
No natural selection
But does this happen in real life?
Directional Selection
Key
Directional Selection
Low mortality,
high fitness
Food becomes scarce.
High mortality,
low fitness
Stabilizing Selection
Stabilizing Selection
Key
Low mortality, high
fitness
High mortality, low
fitness
Birth Weight
Selection against
both extremes keep
curve narrow and in
same place.
Disruptive Selection
Disruptive Selection
Low mortality,
high fitness
High mortality,
low fitness
Population splits
into two subgroups
specializing in
different seeds.
Beak Size
Number of Birds
in Population
Key
Number of Birds
in Population
Largest and smallest seeds become more common.
Beak Size