This document discusses how natural selection and genetic drift can cause evolution by changing allele frequencies in populations over time. It explains that natural selection may cause some alleles to become more common if they increase an individual's chances of survival and reproduction. Genetic drift is the random change in allele frequencies that can occur in small populations by chance. The document also describes the conditions required by the Hardy-Weinberg principle for a population to maintain genetic equilibrium and not evolve.

1. Lesson Overview 17.2 Evolution as Genetic Change in Populations

2. How Natural Selection Works How does natural selection affect single-gene and polygenic traits?

3. How Natural Selection Works Evolutionary fitness is the success in passing genes to the next generation. Evolutionary adaptation is any genetically controlled trait that increases an individual’s ability to pass along its alleles.

4. Natural Selection on Single-Gene Traits Natural selection for a single-gene trait can lead to changes in allele frequencies and then to evolution. For example, a mutation in one gene that determines body color in lizards can affect their lifespan. So if the normal color for lizards is brown, a mutation may produce red and black forms.

5. Natural Selection on Single-Gene Traits: The example of Lizard Color

6. Natural Selection on Single-Gene Traits If red lizards are more visible to predators, they might be less likely to survive and reproduce. Therefore the allele for red coloring might not become common.

7. Natural Selection on Single-Gene Traits Single-Gene Traits: The allele for red coloring might not become common.

8. Natural Selection on Single-Gene Traits Black lizards might be able to absorb sunlight. Higher body temperatures may allow the lizards to move faster, escape predators, and reproduce.

9. Natural Selection on Single-Gene Traits Single-Gene Traits: The allele for black color might become more common.

10. Genetic Drift What is genetic drift?

11. Genetic Drift Genetic drift occurs in small populations when an allele becomes more or less common simply by chance. Genetic drift is a random change in allele frequency.

12. Genetic Bottlenecks The bottleneck effect is a change in allele frequency following a dramatic reduction in the size of a population. For example, a disaster may kill many individuals in a population, and the surviving population’s gene pool may contain different gene frequencies from the original gene pool.

13. The Founder Effect The founder effect occurs when allele frequencies change as a result of the migration of a small subgroup of a population.

14. The Founder Effect Two groups from a large, diverse population could produce new populations that differ from the original group.

15. Evolution Versus Genetic Equilibrium What conditions are required to maintain genetic equilibrium?

16. Evolution Versus Genetic Equilibrium A population is in genetic equilibrium if allele frequencies in the population remain the same. If allele frequencies don’t change, the population will not evolve.

17. The Hardy-Weinberg Principle The Hardy-Weinberg principle describes the conditions under which evolution does not occur. The Hardy-Weinberg principle states that allele frequencies in a population remain constant unless one or more factors cause those frequencies to change.

18. Large Population Genetic drift can cause changes in allele frequencies in small populations. Genetic drift has less effect on large populations, such as the seals shown. Large population size helps maintain genetic equilibrium.

19. No Mutations If mutations occur, new alleles may be introduced into the gene pool, and allele frequencies will change.

20. Random Mating All members of the population must have an equal opportunity to produce offspring. Individuals must mate with other members of the population at random. In natural populations, however, mating is not random. Female peacocks, for example, choose mates on the basis of physical characteristics such as brightly patterned tail feathers. Such non-random mating means that alleles for those traits are under selection pressure.

21. No Movement Into or Out of the Population Individuals who join a population may introduce new alleles into the gene pool. Individuals who leave may remove alleles from the gene pool. Thus, for no alleles to flow into or out of the gene pool, there must be no movement of individuals into or out of a population.

22. No Natural Selection All genotypes in the population must have equal probabilities of surviving and reproducing. No phenotype can have a selective advantage over another.

23. Sexual Reproduction and Allele Frequency Meiosis and fertilization do not change the relative frequency of alleles in a population. The shuffling of genes during sexual reproduction produces many different gene combinations but does not alter the relative frequencies of alleles in a population.