Mechanisms for Evolution Chapter 15

Populations and Evolution Population – 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 allele, or the number of times the allele occurs in the gene pool. The frequency of alleles in a population tends not to change unless there is an outside force causing the change

Evolution occurs because of changes to the equilibrium There are 5 mechanisms that can change the allele frequencies in a population 1.Natural Selection 2.Genetic Drift 3.Mutation 4.Migration 5.Non-random Mating

1. Natural Selection Natural selection is a process by which individuals who are more fit for their environment survive and reproduce more often Allele frequencies change because of interactions between the population and the environment There are 4 different types of natural selection: a. Stabilizing Selection b. Directional Selection c. Disruptive Selection d. Sexual Selection

1a. Stabilizing Selection Stabilizing selection occurs when individuals with the average form of a trait are most fit for the environment and extreme traits are eliminated This is the most common form of natural selection

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

1b. Directional Selection Directional selection occurs when individuals with one extreme variation of a trait are the most fit in the environment. This causes a gradual shift in allele frequency to that extreme (shift in one “direction”).

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.

1c. Disruptive Selection Disruptive selection occurs when both extreme variations of a trait are the most fit. There is selection against the middle variations. This type of selection often results in new species being created.

Example of Disruptive Selection Ex: 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 more often by birds

1d. Sexual Selection Sexual selection is the competition for mates within a population causing differences to occur in the allele frequencies of the two genders Often results in extreme differences in the physical appearance of males and females. Mates tend to be chosen for their phenotypes and females tend to choose the males.

1d. Sexual Selection cont’d Ex: Peacocks – 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 more common. – Sexual Selection in Peacocks Sexual Selection in Peacocks

2. Genetic Drift Genetic drift describes changes in allele frequency due to population size. Allele frequencies can change more rapidly in smaller populations

Genetic Drift Demo 1. What do the marbles in the bottle represent? 2. Describe the allele frequencies of the original “population” in the bottle. 3. How are the allele frequencies in the final population different from the original population? 4. What sorts of events might cause genetic drift to occur?

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

3. Mutations Mutations are inheritable changes to the genotype of an organism Mutations occur randomly and spontaneously within a population Many mutations are harmful, but some are useful Mutations can affect allele frequency in a population by 1. Adding new alleles for a trait 2. 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

4. Migration Movement into and out of a population can change the allele frequency in a gene pool – Immigration can ADD individuals with variations to the population – Emigration can REMOVE individuals with variations from a population Gene flow which is the process of transferring genes among different populations

5. Non-Random Mating Having a limited number of individuals can also impact mating. 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