Download Causes of Microevolution

Causes of Microevolution
Violations of assumptions of Hardy-Weinberg equilibrium
Conditions for Hardy Weinberg equilibrium:
1. NO net mutations of alleles
2. NO migration of alleles in or out of population
3. Random mating
4. NO genetic drift - large populations needed
5. NO natural selection
Mutations
↓
Multiple alleles → Sexual recombination
↓
New phenotypes
Many mutations are NOT favorable --> death before reproduction
Some mutations are not expressed immediately:
Environmental conditions may change, leading to
expression and an adaptive advantage.
Gene flow: exchange of alleles between populations
increases chances of genetic diversity
Isolation of a population _________
through natural selection. (Each population can evolve
independently)
Natural gene flow __________
decreases genetic diversity - more homogeneous
gene pools (mixing white and chocolate cake batter!)
Artificial gene flow - animal husbandry!
Nonrandom Mating:
Assortive mating: individuals seek out those with
similar phenotypes (e.g., tall people)
Sex selection: males compete for right to reproduce
females select males with specific phenotypes
Genetic Drift
More pronounced in small populations
Random mating may eliminate rare genotypes,
leading to change in allele frequencies
Bottleneck Effect:
Disaster may eliminate
most of population and
only a small minority of
genotypes contribute to
future generations.
reduce genetic diversity
Both ________
Founder effect:
Rare alleles occur at higher frequency in isolated population
because founding individuals carried them.
Central and South American
natives are almost 100%
type O blood (recessive
trait) and the allele for B
is almost nonexistent. We
think this resulted from
migration of a small population
progressively southward.
Percentage
of native
population
with type O
blood
60-70%
70-80%
80-90%
90-100%
and the biggie…..
NATURAL SELECTION
Natural selection REQUIRES:
1. Variation in a population (created by mutations, etc)
2. Inheritance. It is possible for offspring to inherit
these genetic differences.
3. Differential adaptedness. Some differences determine
how well an organism is adapted to its environment.
4. Differential reproduction. Better adapted individuals
are more likely to reproduce.
Natural selection operates on the phenotype, but it changes
the allele frequencies in a population.
Types of Natural Selection:
http://www.evoled.org/images/modes.gif
Extreme phenotype
favored (antibiotic
resistance)
Intermediate
phenotype
favored (infant
birth weight)
Intermediate phenotype
not favored (horned
beetles)
Maintenance of Variations:
Desirable: variations available to adapt to changing environment
HOW?
mutation, gene flow, sexual reproduction,
disruptive selection
↑ variation
natural selection
↓ variation
One way = heterozygous advantage
e.g. A good trait (resistance to malaria) is
linked to a bad one (sickle cell anemia)
Heterozygotes maintain recessive alleles in population
What natural selection is NOT:
Why is it NOT survival of the fittest?
no differential reproduction!
fitness may not be genetic
Why is it NOT predation or phenotypic mortality?
need differential reproduction
Why is it NOT equivalent to evolution?
evolution is a process that includes natural selection