Evolution of Populations
 Darwin and Mendel
 Genes control heritable traits
 Changes in genes = variation
 Natural selection works with this variation
 Different alleles
 Dominant, recessive, multiple alleles, etc.
 Heterozygous genotypes
 Can pass on dominant OR recessive
 Combined genetic info. of all
members of a population
 Relative frequency: # of times an allele appears in a
gene pool compared to other alleles
 Has nothing to do with dominant/recessive
 Evolution = change in relative frequency of alleles in a
population
 What is the biggest source?
 1. mutations!
 2. gene shuffling- ½ from mom and ½ from dad
 Random gamete formation
 Single-gene trait: controlled by one gene with 2 alleles
 Ex: widow’s peak (2 possible phenotypes)
 Polygenic trait: controlled by 2 or more genes
 Ex: skin color (many possible phenotypes)
 Evolutionary fitness- organisms success
in passing genes on to offspring
 Evolutionary adaptations increase
ability to pass on a trait
 Survive = pass genes on
 Fail = do NOT pass genes on
 Evolution acts on organisms NOT single
genes!
 Populations evolve NOT organisms
 Natural selection affects the distribution of
phenotypes (bell curve)
 3 Types:
 1. directional selection: when individuals at one end of
the curve have increased fitness
 2. stabilizing selection: when individuals at the center
of the curve have increased fitness
 3. disruptive selection: when individuals at both ends
of the curve have increased fitness
 *may eventually split into 2 different species
 Genetic drift: random change in allele frequency that
occurs in a small population
 By chance certain alleles are passed on over others
 Founder effect: change in allele frequency due to the
migration of a small subgroup of a population
 Hardy-Weinberg Principle: allele frequency in a
population will remain constant as long as the
following factors take place:





1. random mating
2. large population
3. no movement in/out of a population
4. no mutations
5. no natural selection
 So, does this ever happen?
 No, not really
 Species: group of organisms that breed together and
produce offspring
 Share a gene pool
 Speciation: formation of new species
All different species!
 A separation of gene pools
 Reproductive isolation: separation so that two
organisms cannot breed
 No offspring are produced
 behavioral: different mating behaviors
 Ex: meadowlark’s song
 geographic*: physical barrier separating
 Ex: Abert and Kaibob squirrels
 temporal: reproduce at different times
 Ex: orchid pollen release
 Peter and Rosemary Grant- Princeton
 Tested Darwin’s hypothesis with finches
 Needed to prove 2 things:
 1. natural genetic variations were present
 2. variations caused a difference in fitness
 Success- more support for Darwin!
 All species of finch came from 1 founding population:
 1. founders arrive from mainland
 2. geographic isolation (separates gene pools)
 3. changes to new gene pools- adapt to local
environment
 4. reproductive isolation- 2 new species can no longer
mate
 5. ecological competition- 2 species live together,
become more different to increase survival