Chapter 17: Evolution of Populations Evolution as Genetic Change in Population

How Natural Selection Works Natural Selection on Single Gene Traits – When controlled by only one gene – Natural Selection will favor one trait over another – As time goes by, that trait will appear more frequently in the population Natural Selection on Polygenic Traits – Three different patterns appear Directional Selection Stabilizing Selection Disruptive Selection

Natural Selection on Polygenic Traits Directional Selection – Favors organisms at one end of a bell curve – These individuals have a higher fitness level

Natural Selection on Polygenic Traits Stabilizing Selection – Favors the average organism – Individuals in the center of bell curve have highest fitness level

Natural Selection on Polygenic Traits Disruptive Selection – Organisms with most extreme traits most likely to survive – This will create 2 distinct phenotypes

Genetic Drift Sometimes chance events can cause evolution These will cause random changes in the frequency of alleles in a gene pool – Examples: Bottleneck Effect (lack of diversity in population)

Genetic Drift Example: – Founder Effect (Small group starting new population)

Genetic Equilibrium Population is not evolving and allele frequency in gene pools don’t change Hardy-Weinberg Principle – 5 conditions can upset equilibrium 1. Nonrandom Mating 2. Small Population 3. Movement into or out of population 4. Mutations 5. Natural Selection – Since all these actually happen, equilibrium is impossible – Validates Darwin’s theory

Chapter 17: Evolution of Populations Process of Speciation

Isolating Mechanisms What is speciation? – Formation of a new species – If members of a population can’t mate, a new species evolves – Reproductive Isolation Different populations can’t mate and produce offspring Seen in 3 different ways 1. Geographic Isolation – Occurs when populations are separated by a barrier – Ex: mountain range, river, ravine 2. Behavioral Isolation – Occurs when different populations develop different breeding habits – Ex: different mating dances or calls 3. Temporal Isolation – Occurs when populations in a habitat reproduce at different times in the year

Speciation in Darwin’s Finches 1. Founders Arrive – Finches from South America arrive to Galapagos – Reproduce and change allele frequency 2. Geographic Isolation – Finches usually don’t fly over water – Become stranded on different islands 3. Changes in Gene Pool – Different islands have different environments causing changes 4. Behavioral Isolation – Finches now mate with only similar finches based on calls 5. Competition – Competition for resources leads to new resources

Chapter 17: Evolution of Populations Molecular Evolution

Molecular Clocks – We can look at mutation rates in DNA to see how long ago 2 organisms shared a common ancestor – Mutations happen at different rates – Some mutations are harmful – Some mutations are good