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Transcript
The Evolution of
Populations
Chapter 21
Microevolution

Evolutionary changes within a population



Changes in allele frequencies in a population over
generations
Population – all members of a species living
in the same area, interbreed, produce fertile
offspring
Example – industrial melanism and the
peppered moth
Genetic Variation

Differences among individuals in the
composition of the genes

Single gene influence (Mendel) or polygenic

Phenotype – physical traits, can be inherited
or influence by environment.
Sources of Genetic Variation

Formation of new alleles


Altering Gene number or position



Mutation
Chromosomal changes – deletion, translocation,
inversion and duplication
Rapid reproduction – prokaryotes
Sexual Reproduction

Crossing over, independent assortment and
random fertilization
Hardy-Weinberg principle
p2 +2pq + q2
Used to calculate the genotype and gene
frequencies of a population
States: equilibrium of allele frequencies in a gene
pool will remain in effect in each generation of
sexually reproducing populations as long as:



1.
2.
3.
4.
5.
No mutations
No gene flow
Random mating
No genetic drift
No selection
Hardy-Weinberg principle

Tells us what factors cause evolution




The 5 conditions are hardly ever met
Allele frequencies do change from one generation
to another
Evolution can be detected by seeing any deviation
from a Hardy-Weinberg equilibrium
Practice problems p.406
Causes of microevolution



Opposite of HWP
Genetic mutations – cause for multiple
alleles, can be adaptive and include
favorable phenotypes
Nonrandom mating – inbreeding or
breeding between relatives, decreases the
heterozygote
Causes of microevolution that alter
allele frequency directly
Genetic Drift – change in allele frequencies
due to chance
Bottleneck effect – natural disaster, reduce in
population prevents the majority of genotypes from
participating in the production of the next generation
Founder effect – rare alleles occur at a higher
frequency in a population isolated from a general
population ex. amish
Microevolution

Gene Flow – transfer of alleles into or out of a
population due to the movement of fertile
individuals or their gametes.
Natural selection
Not random – adaptive evolution
Most traits are polygenic, see bell curve in
allele frequency
3 major types of selection -



Directional – extreme phenotype favored


Stabilizing – intermediate phenotype is favored



Resistance to antibiotics and pesticides, malaria
Birth weight survival, sickle cell trait
Disruptive – 2 or more extreme phenotypes are
favored
Sexual selection

Natural selection in which individuals with
certain inherited characteristics are more
likely than others to obtain mates.

Sexual dimorphism – differences in males and
females (i.e. size, color, …)
Balancing selection

Natural selection maintains two or more
forms in a population.


Heterozygote advantage – Malaria and sickle cell
anemia
Frequency-dependent selection – scale eating
fish. Right and left mouthed
Why doesn’t Natural Selection create
perfect organisms?




Selection can act only on existing variations
Evolution is limited by historical constraints
Adaptations are often compromises
Chance, natural selection and the
environment interact