Microevolution: The Forces of Evolutionary Change Part 2 Lecture 23 Outline ● Conditions that cause evolutionary change ● Natural vs artificial selection ● Nonrandom mating and sexual selection ● The role of chance events – Genetic drift – Bottlenecks – Founder effects – Gene flow Conditions that Cause Evolutionary Change in Natural Populations ● Natural selection is an important, but not the only, force that results in biological evolution ● Micro-evolution occurs when the frequency of an allele in a population changes – Natural selection – Artificial selection – Sexual selection, mate choice, and nonrandom mating – Mutation – Genetic drift Review of Natural Selection ● The differential survival and reproduction of organisms whose genetic traits better adapt them to a particular environment ● Change in the number of individuals in a population that carry copies of a specific allele ● Determined by an individual©s phenotype Evolution and Mutation ● Mutation is the source of new alleles ● Evolution is the change in allele frequency over time ● Some other evolutionary mechanism required for new alleles to become more common ● The genetic makeup of populations, and ultimately species, changes as natural selection permits differential survival of genetic variants (mutants) that are better adapted to a particular environment. Disease Evolution ● All organism evolve – One is not ªmore evolvedº than another ● In most examples of disease evolution we are looking at the evolution of the disease rather than of the host: flu, HIV, antibiotic resistant bacteria, cholera, tuberculosis, etc. (exception: balanced polymorphism) ● Our immune system and the drugs we take are part of the disease©s environment natural =variation differential heredity selection reproduction ● Change in allele frequency in a population is called microevolution. ● Can take place over relatively short time period ● Some phenotypes are better adapted to particular environment. ● Natural selection removes some alleles from the population ● Allele frequencies will change in response to environmental change Role of the Environment ● Two fundamental forces are operating: genetic variation and environmental change. ● Those with more adaptive traits survive in a new or changing environment. ● Environment includes – Climate – Abiotic conditions/resources: water, soils, habitat – Parasites, predators, and competitors ● Parasites, predators, and competitors also evolve Role of the Environment ● Natural selection reflects adaptation to a prevailing environmental condition. ● The direction of natural selection can change. ● A phenotype that is adaptive in one set of circumstances may be a liability in another. ● Broad-ranging species often show local adaption among sub-populations – Affects management, conservation, and climate change Reproductive Isolation ● Over time, an isolated sub-population could change so that it can no longer breed with the original group. ● Eventually, a new species would arise. ● Macro-evolution represents accumulated changes in allele frequency in two populations that preclude their interbreeding, leading to the formation of new species (or their extinction). ● Macro-evolutionary changes tend to span many generations, but in some instances can occur in only a few generations. Drivers of Microevolution ● Mutation ● Natural selection ● Artificial selection ● Sexual selection, mate choice, and nonrandom mating ● Genetic drift Artificial Selection: Proof of the Power of Selection ● Also called selective breeding or domestication ● Process of intentional or unintentional modification of a species through human actions which encourage the breeding of certain inherited traits over others ● The breeding potential of individuals who possessed desirable characteristics is intentionally encouraged, whereas the breeding of individuals with less desirable characteristics is discouraged. Artificial Selection ● ªSelection was the keystone of man©s success in making useful races of animals and plantsº -Darwin, 1837 ● No real difference in genetic processes underlying artificial and natural selection ● Underscores the power of selection in generating evolutionary change Non-random Mating ● A variation of Natural Selection ● Completely random matings (where each individual has as equal chance of mating with every other member of the population) are nearly impossible to achieve Sexual Selection & Mate Choice ● Sexual selection: the "natural selection of traits that increase an individual©s reproductive success." ● These traits contribute to attraction, courtship, or mating. ● Most species exhibit some sort of preferences in mate choice ● The alleles for these desired traits will become more common in future generations. Peacock Peahen Bower Bird Sexual selection: male-male competition Genetic Drift ● Changes in allele frequency in a population that result from RANDOM survival or reproduction of individuals with certain characteristics. ● Survival or reproduction of an organism is sometimes just a matter of LUCK or CHANCE, not because of their phenotype or genotype. ● Example: if a Florida Panther is killed by a truck on a highway, that is bad luck. The panther did not get hit because of some allele it carried. ● Genetic Drift contrasts with natural selection. ● In selection the environmental events that affect a population may be random, but the survival or reproduction of the individuals depends on their phenotypes. ● Example of selection: If the panther population is infected with FIV (feline AIDS), individuals with alleles that give them resistance to the disease will survive. Types of random effects ● Gene flow/Migration ● Founder Effects ● Population Bottlenecks ● Genetic drift Gene Flow / Migration ● Gene Flow is the movement of alleles from population to population. ● Immigrating individuals introduce new alleles by mating with members of the population they join. ● Any advantage given to individuals with new alleles will change the population due to subsequent natural selection. ● Because geographic barriers greatly influence migration patterns, allele frequencies may differ between adjacent but separated geographic regions. Founder Effect ● Establishment of a new population (geographically isolated) from a single or very few individuals. ● The gene pool of a founding population only contains a subset of the total genetic diversity of the original population ● Different from gene flow because the migrating individuals are establishing a new population where none existed before. Population Bottleneck ● Occurs when population size is drastically reduced ● The new population has lost much of the genetic diversity that was present in the larger ancestral population as a result of inbreeding. Bottlenecks and Conservation ● Common problem among endangered species ● Florida Panther – Hybrids with Texas pop©n have greater lifespan and fewer genetic problems. – Pop©n has tripled ● Greater Prairie Chicken [Dr. Paige, Animal Bio] Genetic Drift ● Chance events affect which individuals survive and reproduce in a population. ● Small, random changes in allele frequency can "build up" over generations and result in a significant change in allele frequency. Population Size ● Chance changes in allele frequency tend to have small effects on large populations ● Small populations (e.g. endangered species) are much more susceptible ● Tend to be inbred and have little genetic variation anyway. Micro-evolution Recap ● Microevolution (a change in allele frequency in a population) can occur as a result of several factors that fall into two broad categories. ● Factors in which an individual©s chance of survival and reproduction depend on the individual©s phenotype. • Natural Selection • Artificial Selection • Mate Selection, Sexual Selection, Nonrandom Mating – Mutation Micro-evolution Recap 2. Factors in which an individual©s chance of survival and reproduction, are just that, a matter of CHANCE. An individual©s phenotype and genotype has nothing to do with it. – Gene Flow – Founder Effect – Population bottle neck – Genetic Drift .