Transcript Chapter 17 Evolution of Populations

Chapter 17
EVOLUTION OF POPULATIONS
How does evolution function in genetic terms?
 Natural selection does not act on genes but on the
whole organism
 Population = group of individuals of that same
species that mate and produce offspring
 Share a gene pool (all the genes in a population)
Genes and Variation
 Genotype and Phenotype –
 Through the use of the gene pool, we can evaluate
the allele frequency, or how often a certain trait
appears
 Genetic evolution involves a change in the allele
frequency of a population
 Single trait genes (one gene controls)
 Polygenic trait (more than one gene controls) lead to
changes in
Sources of Genetic Variation
 Three sources:
Mutations
2. Genetic Recombination - when does this occur?
3. Gene Transfer – bacteria
1.
The number of phenotypes depends on how
many genes control a trait.
 Single gene traits – trait only controlled by one gene
 lead to changes in phenotype frequencies
 Polygenic Traits – trait controlled more than one
gene

Lead to change in fitness and result in three types of changes
Natural selection on polygenic traits
 Stabilizing selection: favors the intermediate
 Directional selection: favors one of the extremes
 Disruptive selection: favors individuals at both
extremes, does not favor the intermediate
Genetic Drift and Gene Flow
 Genetic drift: change in the
gene pool of a population due to
chance, 2 causes

Bottleneck effect: where a
natural disaster drastically
reduces population size

Founder effect: small colony
on a new island
 Gene flow: a population loses
or gains alleles when fertile
individuals move out of into a
population
100 greatest discoveries
 http://videos.howstuffworks.com/science-
channel/29786-100-greatest-discoveries-naturalselection-video.htm
Evolution vs Equilibrium
 Genetic
equilibrium: no
evolution means
allele frequencies
stay the
 Helps to predict
frequencies of future
alleles or to see if
evolution is
occurring
Hardy-Weinberg Principle
 States that allele frequencies should remain
constant unless one or more factors cause those
frequencies to change
 Factors:
1. Nonrandom mating (sexual selection =
individuals select mates based on traits) *Video
2. Small population (genetic drift will occur easily)
3. Immigration or Emigration (if individuals leave,
frequencies change!)
4. Mutations (can introduce new alleles)
5. Natural Selection (different fitness levels)
Hardy-Weinberg
 Makes predictions like punnett squares…but for
populations not individuals
 Formula used instead of punnett square
Hardy-Weinberg Equilibrium
 p = dominant allele
 q = recessive allele
p2 = frequency of AA
2pq = frequency of Aa
q2 = frequency of aa
Tay Sachs is a rare genetic disorder that is
recessive and makes up about 4 percent of
the humans population. What would be
the genotype frequency in percentages?
 Blue eyes in humans is due to a recessive allele,
which makes up about 28 % of the population. What
would be the percentages of the homozygous
dominant, heterozygous, and homozygous recessive
conditions of the allele?
 In the human population a recessive genetic disorder
called Down’s Syndrome is seen about 0.6 % of the
time. What would be the allele frequencies for this
disorder in percentages?
What is a species?
Species = A population or group of populations
who can interbreed and produce fertile offspring
Ex: similarity between two species of songbirds
Speciation – the forming of a new species
Reproductive Isolation
 If group members stop interbreeding, reproductive
isolation has occurred = can no longer interbreed
 Three types..



Geographic isolation– different habit, sometimes separated
by mountains or flooded area
Behavioral isolation – differences in courtship
Temporal isolation – differences in mating seasons
 http://dsc.discovery.com/videos/assignment-
discovery-shorts-evolution-adaptation.html
Molecular Evolution
 A molecular clock uses mutation rates in DNA to
estimate the time that two species have been
evolving together.
 Figure 17-18
 How do we know species C is more closely related to
species B than to species A?
Evo-Devo
 Studies relationship of evolution and embryological
development
 Small changes in Hox gene can produce large
changes in adult animals