Evolution of Populations

I.Genetic Variation A.Review 1.Genes control traits 2.Many genes have more than one version (allele) 3.Many traits are controlled by more than one gene 4.A variation is an inherited trait that makes an individual different from other members of the species and results from a mutation in the organism’s genes.

Variation

B.Important Terms 1.Population = Group of individuals of the same species that can interbreed 2.Gene pool = all the genes, including alleles, present in a population 3.Relative frequency = number of times an allele occurs in a gene pool

Sample Population 48% heterozygous black 36% homozygous brown 16% homozygous black Frequency of Alleles allele for brown fur allele for black fur

Populations and Gene Pool Movie

4. Single-gene trait – trait that is controlled by a single gene that has two alleles a) As a result, only two phenotypes are possible in a population for this gene

5. Polygenic trait – trait that is controlled by two or more genes a) As a result, one polygenetic trait can have many possible genotypes and phenotypes

C. Important Concepts 1. 1.Populations, not individuals evolve over time 2. 2.Microevolution is a generation-to-generation change in a population’s frequencies of alleles and genotype 3. 3.The more variation in a population  the more likely some members of the species will survive if the environment changes

Evolution in Action

II. Sources of Genetic Variation A. Natural Selection – can affect phenotype in 3 ways 1. Stabilizing - environment selects against extremes Key Percentage of Population Birth Weight Selection against both extremes keep curve narrow and in same place. Low mortality, high fitness High mortality, low fitness Stabilizing Selection DRAW

2.Directional – environment selects a phenotype at end of the curve DRAW Directional Selection Food becomes scarce. Key Low mortality, high fitness High mortality, low fitness

3. Disruptive- environment selects against the average trait, so over time there will be 2 new phenotypes at the extremes DRAW Disruptive Selection Largest and smallest seeds become more common. Number of Birds in Population Beak Size Population splits into two subgroups specializing in different seeds. Beak Size Number of Birds in Population Key Low mortality, high fitness High mortality, low fitness

B.Genetic drift - over time, a series of chance occurrences can cause an allele to become more common in a population C.Gene flow - Populations gain or lose alleles due to migration of individuals between populations D.Non-random mating – Inbreeding or selective breeding for specific phenotypes (purebred dogs) E.Mutations  All of these sources of genetic variation cause changes in the gene pool

Gene Pool Changes Movie

III. Speciation “creation of a new species” A. Species –organisms that are similar in physical appearance, have the same number of chromosomes and can interbreed to produce fertile offspring. B. How do you get a new species? 1.You must isolate populations for a long time. 2.Isolation can happen in several ways:

a. Behavioral isolation - 2 populations can interbreeding but have differences in courtship or other reproductive strategies that involve behavior i. Ex: Western and Eastern meadowlarks are the same species, but use different songs to attract mates

b. Geographic isolation - populations are separated by physical barriers (i.e. a river splits a valley in two)

c. Temporal isolation– different mating times i. Ex: 3 similar species of orchid release pollen on different days

d. Anatomical Isolation

3. With time the two isolated populations change independently and eventually become different species.

Speciation Movie

IV. Patterns of Evolution A.Extinction 1.More than 99 % of all species that have ever lived have gone extinct 2.Reasons: competition or environment changes 3.Effect of mass extinctions – disappearance of many species leaves habitats wide open providing opportunity for organisms that survived B.Adaptive radiation 1.Process by which a single species evolves into several different species (ex: Galapagos finches)

Adaptive radiation

C.Convergent evolution 1.Produces unrelated organisms that look similar 2.When organisms face similar environmental changes, such as moving through water, natural selection favors adaptations that have similar functions a.Dissimilar appendages with similar functions: flippers of a dolphin, wings of a penguin, and fins of a shark

Convergent Evolution

D.Co-evolution 1.2 species evolve in response to changes in each other over time a.Ex: flower types only being able to reproduce if their shape, color, and odor attracts the right type of pollinator

E.Punctuated equilibrium 1.Darwin thought evolution was only slow, gradual change (gradualism), but we have evidence this is not always the case 2.Punctuated equilibrium = pattern of long, stable periods interrupted by brief periods of more rapid change a.Ex: surviving organisms evolving rapidly after mass extinctions