The evolution of Populations

What is a Population? A group of organisms of the same species that live in a specific geographical area and interbreed

Population Size The number of individuals in a population Can affect the population’s ability to survive

Very Small Populations Most likely to become extinct Random events or natural disturbances, such as a fire or flood, endanger small populations Tend to experience interbreeding Only relatives may be available as mates Reduces the population’s fitness

Genetic variation Gene pool: collection of alleles found in all of the individuals of a population Allele frequency: proportion of one allele, compared with all the alleles for that trait, in the gene pool

Genetic variation comes from several sources Mutation: a random change in the DNA of a gene Mutation rates in nature are very slow Genes mutate only about 1-10 times per 100,000 cell divisions Mutation is the source of variation and makes evolution possible Recombination: new alleles that form in offspring during meiosis

Natural selection acts on distributions of traits Normal distribution: distribution in a population in which allele frequency is highest near the mean range value and decreases progressively toward each extreme end

Natural selection can change the distribution of a trait in one of three ways

Directional selection Pathway of natural selection in which one uncommon phenotype is selected over a more common phenotype

Stabilizing selection Pathway of natural selection in which intermediate phenotypes are selected over phenotypes at both extremes

Disruptive selection Pathway of natural selection in which two opposite, but equally uncommon, phenotypes are selected over the most common phenotype

Gene flow is the movement of alleles between populations Increases the genetic variation of the receiving population Caused by the migration of individuals to or from a population New individuals (immigrants) add alleles Departing individuals (emigrants) take away alleles

Genetic drift is change in allele frequencies due to chance Small populations, like small sample sizes, are more likely to be affected be chance Some alleles will likely decrease in frequency and become eliminated Genetic drift causes a loss of genetic diversity in a population

Genetic Drift Example: the cheetah Undergone drastic population declines in the last 5000 years Those alive today are the descendants of only a few individuals, and each cheetah is almost genetically uniform

Bottleneck effect Genetic drift that results from an event that drastically reduces the size of a population Example: northern elephant seals

Founder effect Genetic drift that occurs after a small number of individuals colonize a new area Example: Older Order Amish

Effects of genetic drift Can cause several problems for populations Population loses genetic variation Individuals less likely to be able to adapt to a changing environment Lethal genes could become more common due to chance

Sexual selection occurs when certain traits increase mating success Males Make sperm continuously Value of each sperm is relatively small Females Limited to the number of offspring they can reproduce in each reproductive cycle Each investment is more valuable “The choosy sex”

Sexual selection Selection in which certain traits enhance mating success Traits are passed on to offspring Intrasexual selection: competition among males for the female (i.e. male deer) Intersexual selection: when males display certain traits to attract the female (i.e. male bird of paradise) Some traits that were originally selected for quality and good health are now exaggerated through sexual selection (i.e. the male peacock feathers)

Hardy-weinberg equilibrium describes populations that are not evolving Genotype frequencies stay the same in a population if certain conditions are met: Very large population No emigration or immigration No mutations Random mating No natural selection Real populations rarely meet all 5 conditions

The hardy-weinberg equation is used to predict genotype frequencies in a population p2 + 2pq + q2 p2 = Frequency of individuals that are homozygous for allele A 2pq = Frequency of heterozygous individuals with alleles A and a q2 = Frequency of individuals that are homozygous for allele a Know that: p + q = 1

Hardy-weinberg example

Hardy-Weinberg Principle The principle that states that the frequency of alleles in a population does not change unless evolutionary forces act on the population IMPOSSIBLE IN NATURE!

There are five factors that can lead to evolution Genetic drift Gene flow Mutation Sexual selection Natural selection

Speciation through isolation The isolation of populations can lead to speciation Reproductive Isolation: Final stage in speciation, in which members of isolated populations are either no longer able to mate or no longer able to produce viable offspring

Populations can become isolated in several ways Behavioral isolation: due to differences in courtship or mating behavior Geographic isolation: due to physical barriers Temporal isolation: due to barriers related to time, such as differences in mating periods or differences in the time of day that individuals are most active

Example: Reproductive Isolation Kaibab squirrel North Rim of the Grand Canyon, Arizona Black belly Abert squirrel South Rim of the Grand Canyon, Arizona White belly Have been isolated for about 10,000 years and cannot interbreed Some biologist consider them different species

A New Species Divergence: the accumulation of differences between groups Leads to the formation of new species Speciation: The formation of new species as a result of evolution by natural selection

Subspecies Populations of the same species that differ genetically because of adaptations to different living conditions The members of newly formed subspecies have take the first step toward speciation Eventually, the subspecies may become so different that they can no longer interbreed successfully…they would then be separate species

Subspecies

Evolution through natural selection is not random

Convergent evolution Evolution toward similar characteristics in unrelated species, resulting from adaptations to similar environmental conditions

Divergent evolution Evolution of one or more closely related species into different species; resulting from adaptations to different environmental conditions

Species can shape each other over time Beneficial relationships through coevolution Evolutionary arms race

Species can become extinct Background extinctions Mass extinctions

Speciation often occurs in patterns Punctuated equilibrium: theory that states that speciation occurs suddenly and rapidly followed by long periods of little evolutionary change Adaptive radiation: process by which one species evolves and gives rise to many descendent species that occupy different ecological niches