Evolution of Populations

Chapter 16

Darwin and MendelGenes control heritable traits

Changes in genes = variationNatural selection works with this variation

16-1 Genes and Variation

Different allelesDominant, recessive, multiple alleles, etc.

Heterozygous genotypesCan pass on dominant OR recessive

Types of Genetic Variation

Combined genetic info. of all members of a population

Relative frequency: # of times an allele appears in a gene pool compared to other allelesHas nothing to do with dominant/recessive

Evolution = change in relative frequency of alleles in a population

Gene Pool

What is the biggest source?1. mutations!2. gene shuffling- ½ from mom and ½ from

dadRandom gamete formation

Sources of Genetic Variation

Single-gene trait: controlled by one gene with 2 allelesEx: widow’s peak (2 possible phenotypes)

Polygenic trait: controlled by 2 or more genesEx: skin color (many possible phenotypes)

Single-gene and Polygenic Traits

Evolutionary fitness- organisms success in passing genes on to offspring

Evolutionary adaptations increase ability to pass on a traitSurvive = pass genes onFail = do NOT pass genes on

Evolution acts on organisms NOT single genes!

Populations evolve NOT organisms

16-2 Evolution as Genetic Change

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

Natural Selection and Phenotypes

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 populationBy 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

Evolution WITHOUT Natural Selection?

Hardy-Weinberg Principle: allele frequency in a population will remain constant as long as the following factors take place:1. random mating2. large population3. no movement in/out of a population4. no mutations5. no natural selection

So, does this ever happen?No, not really

Will evolution ever NOT happen?

Species: group of organisms that breed together and produce offspringShare a gene pool

Speciation: formation of new species

16-3 The Process of Speciation

All different species!

A separation of gene poolsReproductive isolation: separation so that two

organisms cannot breedNo offspring are produced

Speciation occurs with…

behavioral: different mating behaviorsEx: meadowlark’s song

geographic*: physical barrier separatingEx: Abert and Kaibob squirrels

temporal: reproduce at different timesEx: orchid pollen release

3 Types of Reproductive Isolation:

Peter and Rosemary Grant- PrincetonTested Darwin’s hypothesis with finchesNeeded to prove 2 things:

1. natural genetic variations were present2. variations caused a difference in fitness

Success- more support for Darwin!

Testing Natural Selection

Go Darwin!

All species of finch came from 1 founding population:

1. founders arrive from mainland2. geographic isolation (separates gene pools)3. changes to new gene pools- adapt to local

environment4. reproductive isolation- 2 new species can

no longer mate5. ecological competition- 2 species live

together, become more different to increase survival

Descent with modification on the Galapagos