Evolution of Populations

Population Genetics Natural Selection: nature selects which individuals survive and reproduce- Evolution: occurs as a populations genes and their frequencies change over time Gene Pool: all the genes of a population

Population Genetics If the frequency of the alleles doesn’t change over time, the population is at genetic equilibrium

Population Genetics  When alleles are brought in and out of a population due to migration of individuals, it is called gene flow.

5 Population Genetics Methemoglobinemia

Population Genetics  Gene frequencies in a population can change due to natural selection  3 types of Natural Selection Stabilizing Selection: natural selection favors the average individuals in the population It is best to be average! Cactus with medium number of spines

Stabilizing Selection

Types of Natural Selection Disruptive Selection- natural selection favors both extreme phenotypes of a trait in a population Its not good to be average! Medium color limpets do not survive

Disruptive Selection

Types of Natural Selection Directional Selection-natural selection favors one extreme phenotype of a trait in a population It is best to be different! Antibiotic resistant bacteria

Directional Selection

Practice Which of the three selection types would result in black or white snuzzles but no gray ones? Which of the three selection types would result in medium colored snuzzle being the most frequent in the population rather than light or dark snuzzles? Which selection type might ultimately lead to two separate species unable to mate?

Speciation  Natural selection changes the gene pool and leads to evolution of new species over time.  Speciation occurs when 2 populations of a species changed so much they can no longer interbreed and produce fertile offspring

Speciation is caused by:  Geographic Isolation Physical barrier divides 2 populations- Galapagos species, grand canyon squirrels

Speciation is caused by:  Reproductive Isolation Physical or behavioral changes prevent 2 populations from interbreeding Species of bowerbirds build nests differently to woo females in elaborate mating rituals.

Speciation is caused by:  Change in chromosome number Polyploidy-mutation during meiosis resulting in multiple copies of chromosomes Polyploids can then only mate with other polyploids

Speciation  Can occur rapidly Punctuated Equilibrium  Can occur very slowly Gradualism

Patterns of Evolution  Divergent Evolution: Species that once were similar or closely related become very different and more distinct.

Adaptive Radiation  A type of Divergent Evolution  One ancestral species evolves into a variety of species occupying a variety of habitats  Examples: Galapagos finches Hawaiian honeycreepers

Adaptive Radiation

Patterns of Evolution  Convergent Evolution: Due to similar environmental pressures unrelated organisms evolve similar traits Penguin, dolphin, fish with same body shape