Chapter 21 Pearson: Evolution

Chapter 21 Evolution of Populations Study Guide (Section: 21.1, 21.3 and

21.4 only)

1. Be able to explain genetic variation

- differences in DNA sequences and genetic combinations between individual

organisms within species.

2. Know the sources of genetic variation.

- the 3 primary sources are mutations, gene flow, and sexual reproduction.

3. Know phenotype vs. genotype

- genotype is the genetic makeup/code of an organism, and a phenotype is the observable

physical and functional traits of an organism.

4. Understand where mutations occur and which mutations can be passed on to the next

generation.

- mutations occur in genetic material such as DNA and RNA of cells during copying errors

or environmental damage. Only mutations in reproductive cells can be passed to the next

generation.

5. Define natural selection

- a natural process where organisms with traits are best suited in their environment to

survive and reproduce.

6. Know the non-adaptive mechanism of evolution: genetic drift, founder effect, bottleneck

effect, migration (i.e. gene flow), and mutation.

- Non adaptive mechanisms change allele frequencies in a population by change than

improving by survival or reproductive fitness.

- Genetic drift: random shifts in allele frequencies from one generation to the next.

- Founder effect: a special case of genetic drift when small groups break away from larger

populations to establish a new colony while carrying limited samples of original genetic

diversity.

- Bottleneck effect: a sharp reproduction in population sizes caused by random

environmental disasters- Migration (gene flow): the movement of individuals or gametes in or out of a population

- Mutation: a random change in DNA sequence that introduces new alleles into a gene

pool occurring independently.

7. Understand how natural selection can influence the frequency of phenotypes in a

population: directional selection, disruptive selection and stabilizing selection.

- It can influence the frequency of phenotypes by shifting or narrowing trait distributions

based on environmental pressures.

-Directional Selection: when a selection favors one extreme end of a phenotypic range.

Example: lighter skinned human populations became more common in northern climates

due to low sunlight and need for vitamin D.

-Disruptive Selection: a selection that favors both extreme traits while selecting

intermediate or average traits.

Example: an environment where both light and dark phenotypes of an organism are hidden

from predators but intermediate shades are spotted.

-Stabilizing Selection: selection where average or intermediate phenotypes are favored

while extreme variations are selected against.

Example: Babies with average weight have higher survival rates while small babies lose

heat quickly and get sick easily.

8. Understand how natural selection can change the frequencies of alleles: positive

selection, negative selection and balancing selection.

- It changes alleles by favoring or penalizing specific traits based on how well they help

organisms survive and reproduce.

- Positive selection: a process where natural selection drives an advantageous genetic

variant to increase frequency in a population.

Example: an ability to digest milk as an adult evolved through mutations near LCT gene.

-Negative selection: a biological process of removing harmful and unwanted elements

while leaving desired ones untouched.

Example: when immature T-Cells in the thymus that bind strongly to the body’s normal

proteins that receive signals to self-destruct.-Balancing selection: a type of natural selection that keeps multiple versions of a gene in a

population at higher levels than random chances.

Example: sickle-cell trait, one copy of the sickle cell allele have a survival advantage

against malaria.

9. Be able to explain frequency-dependent selection

- a type of natural selection where the reproductive success of a trait depends on how

common or rare it is within a population.

10. Know the difference between natural selection and sexual selection.

- Natural selection focuses on overall survival in an environment while sexual reproduction

focuses on the ability to attract mates.

11. Know the difference between intrasexual selection and intersexual selection.

- Intrasexual selection is the competition between members of the sex while intersexual

selection is a mate choice where one sex selects partners from the other based on traits.

Example: INTRASEXUAL: male deer’s fighting with their antlers to win over a female.

INTERSEXUAL: a female peacock choosing to mate with a male that has the captivating

features.

12. Know the costs and benefits of sexual selection

- The costs are survival and predation risks, energetic and metabolic strain, and time and

energy. The benefits are direct, indirect, and population health.

13. Know which sex is more favored by sexual selection vs. the sex that is more influenced

by natural selection.

- The sex that mostly favors sexual selection is males and the sex that is more influenced by

natural selection is females.