Download Population Genetics

Population Genetics
• The science of genetic change in
population.
• Population – all the members of a
species that occupy a particular area at
the same time
• Gene Pool – all the genes in all the
members of a population
Genes and Variation
• Genetics Joins Evolutionary Theory
– Variation is the raw material for natural selection
– Gene pool – consists of all the genes, including all the different alleles
for each gene, that are present in a population
– Relative frequency – the number of times that the allele occurs in a
gene pool, compared with the number of times other alleles for the
same gene occur
– Therefore – evolution is any change in the relative frequency of alleles
in the gene pool of a population over time
• 3 Sources of Genetic Variation
– Mutations
– Genetic Recombination in Sexual Reproduction (Ind assortment and
crossing over)
– Lateral Gene Transfer (conjugation)
• Single-Gene (2 pheno) vs. Polygenic Traits (many pheno/bell curve)
***Natural Selection acts directly on PHENOTYPES not actual
alleles*** some phenotypes are better suited to an environment
than others and they will survive, reproduce and pass on their
genes.
Evolution as Genetic Change in
Populations
How Natural Selection Works – 3 Types
Stabilizing Selection
Individuals with the average form of a trait have the highest
fitness
Represents the optimum for most traits
Results in a similar morphology between most members of the
species
Directional Selection
Individuals that display a more extreme form of a trait have
greater fitness than individuals with an average form of the trait
A shift in one direction
Peppered moth
Disruptive Selection
Individuals with either extreme variation of a trait have greater
fitness than individuals with the average form of the trait
A shift in both direction, away from the center
Shell color (dark rocks and light sand)
Genetic Drift
• Random changes in the frequency of a gene
in the absence of natural selection 
occurs because of CHANCE
• Occurs efficiently in small populations
because small changes affect more
members
• Two examples:
a. Bottleneck effect
b. Founder effect
a. Bottleneck Effect
• Genetic drift (reduction of alleles in a
population) resulting from a disaster that
drastically reduces population size.
– Examples:
1.
2.
Earthquakes
Volcano’s
b. Founder Effect
• Genetic drift resulting from the
colonization of a new location by a small
number of individuals.
• Results in random change of the gene
pool.
• Example:
1. Islands
(first Darwin finch)
Hardy-Weinberg Principle
• Genetic Equilibrium – situation in which allele
frequencies in the gene pool of a population
remain constant
• The concept that the shuffling of genes that
occurs during sexual reproduction, by itself,
cannot change the overall genetic makeup of
a population.
• Shows mathematically and theoretically that
there are situations where evolution DOES
NOT OCCUR
– Seldom achieved in nature
Hardy-Weinberg Principle
• This principle will be maintained in nature
only if ALL five of the following conditions
are met:
1. Very large population
2. Isolation from other populations
3. No net mutations
4. Random mating
5. No natural selection
Hardy-Weinberg Principle
Species
• A group of populations whose individuals have
the potential to interbreed and produce
viable offspring.
Speciation
• The evolution of new species. Species that
occupy an otherwise unoccupied niche face no
competition, they will therefore have a 100%
success rate
Reproductive Isolation
• Any mechanism that impedes two species from
producing fertile and/or viable hybrid offspring
-factor necessary for the formation of a
new species.
• Barriers:
1. Geographic (rivers, mountains)
2. Behavorial - differences in courtship
behavior
3. Temporal - fertile periods (time)
Interpretations of Speciation
• Two theories:
1. Gradualist Model
(Neo-Darwinian):
Slow changes in species
overtime.
2. Punctuated
Equilibrium:
Evolution occurs in
spurts of relatively
rapid change.
Macroevolution
• The origin of taxonomic groups higher
than the species level.
Adaptive Radiation
aka Divergent Evolution
• Emergence of numerous species from a
common ancestor introduced to new and
diverse environments.
• Example:
Darwin’s Finches
Darwin’s Finches an example of
Adaptive Radiation
Convergent Evolution
• Species from different evolutionary branches
may come to resemble one another if they live
in very similar environments.
• Example:
1. Ostrich (Africa) and Emu (Australia).
2. Sidewinder (Mojave Desert) and
Horned Viper (Middle East Desert)
3. Shark and Dolphin
Coevolution
• Evolutionary change, in which one species act
as a selective force on a second species,
inducing adaptations that in turn act as
selective force on the first species.
• Example:
1. Acacia ants and acacia trees
2. Humming birds and plants with flowers
with long tubes