Mechanisms for Evolution

Populations and Evolution A population is a group of organisms that interbreed Each population shares a gene pool (the different alleles present in the population) Each population has a relative frequency of each alleles, or a number of times the allele occurs in the gene pool. The frequency of alleles in a population tend not to change unless there is an outside force causing it. (EQUILIBRIUM)

Evolution occurs because of changes to the equilibrium. There are FIVE mechanisms that can change the allele frequencies in a population: 1. mutation 2. migration 3. genetic drift 4. non-random mating 5. natural selection

Mutations Mutations are inheritable changes to the genotype of an organism Mutations occur randomly and spontaneously within a population Most mutations are harmful, but some are useful Mutations can affect allele frequency in a population by adding new alleles for a trait or changing the amount of each allele present It can take a long time to eliminate a mutation and a long time for a new mutation to become prevalent

Migration Migration Movement into and out of a population can change the allele frequency in a population’s gene pool Immigration can add individuals with variations to the population Emigration can remove individuals with variations from a population Many species encourage migration which can cause more gene flow which is the process of transferring genes among different populations

Genetic Drift Genetic drift is the random changes in allele frequency due to population size. In smaller populations the allele frequency can change more rapidly than in larger populations

Genetic Drift cont’d The founder effect occurs when a few individuals from a larger population colonize a new area. The allele frequency of this population may differ from the larger population because of the limited number of individuals

Non-Random Mating Having a limited number of individuals can also impact mating. It’s a limitation due to lack of genetic diversity within that population!!! Non random mating can influence allele frequencies because: ◦Mates can be limited by geography ◦Mates can be chosen for their traits ◦Mates can be more closely related to one another

Natural Selection process by which individuals more fit for the environment survive and reproduce The interaction of populations and the environment results in changing allele frequencies (ECOLOGY!!) There are different types of natural selection: -stabilizing selection -directional selection -disruptive selection -sexual selection

Stabilizing Selection occurs when individuals with the average form of the trait are most fit for the environment and extreme traits are eliminated the most common form of selection and works in all populations at all times

Example of stabilizing selection Lizard body size: ◦Large lizards are easily seen by predators, but smaller lizards cannot run as fast to escape the predators ◦Mid sized lizards are most fit in the environment, so they survive and reproduce more often, changing the allele frequencies in the population

Directional Selection occurs when individuals with one extreme of variations are the most fit in the environment causes a gradual shift in allele frequency to that extreme

Example of Directional Selection Anteater tongue length: ◦Anteaters with long tongues are most fit because of the depth of the nests of the termites they eat.

Disruptive Selection occurs when both extremes of variations are the most fit There is selection against the middle variations ie: Limpet shells ◦dark limpets blend with bare rocks ◦light limpets blend with barnacle covered rocks ◦tan limpets are visible in both situations and get preyed upon by birds

Sexual Selection Sexual selection is the competition for mates within a population Mates tend to be chosen for their phenotypes and females tend to choose the males The type of sexual selection can be either stabilizing, directional OR disruptive – this is determined by the sexual preferences/practices of that species.

Sexual Selection ◦Male peacocks have large tail feathers that make it difficult to fly and escape from predators ◦Female peacocks choose males based on their tail feather length and fullness ◦Over time males with larger tail feathers reproduce more causing large tails to be selected for reproduction