Mechanisms for Evolution

1. Mechanisms for Evolution Chapter 15

2. 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

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

4. 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

5. 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

6. 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

7. 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”).

8. 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.

9. 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.

10. 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

11. 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.

12. 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

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

14. 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?

15. 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

16. 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

17. 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

18. 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