Unit 7 (Natural and Artificial Selection + Population Genetics)

Natural Selection Statements

1. Acts on genetic variations that lead to survival and reproduction

2. Only acts on traits that lead to survival and reproduction

• Just because a trait leads to survival doesn’t mean it’s good for the species

  • Ex. An organism has a trait that makes it the best predator, but it may run out of a food source and die

3. Acts on random mutations that have lead to a genetic variation in the population

4. The fittest organism refers to the organism that can survive and reproduce

Evolution

• Descent with modification → evolution

• A change in the genetic makeup of a population over time descent with modification

• Heritable traits from generation to generation

• Darwin used natural selection to explain the pattern of evolution

Natural Selection

A process in which individuals that have certain traits tend to survive and reproduce at higher rates than other individuals because of those traits

• Acts on phenotype variation in population

• Some phenotypes will increase or decrease an organism’s fitness.

  • Measured by reproductive success

• Environments can change, causing selective pressure to populations

The Theory of Natural Selection

1. Traits are heritable

• Characteristics can be passed from parent to offspring

• Inherited characteristics of organisms that enhance their survival and reproduction

2. More offspring are produced than can survive

• Leads to competition for limited resources, which results in differential survival

  • The traits that lead to survival (“favorable” traits) will accumulate in the population

  • Populations evolve, NOT individuals

Artificial Selection

The selective breeding of domesticated plants and animals to encourage the occurrence of desirable traits

Natural selection vs Artificial selection

• Natural Selection: Nature “selects” traits that are better for survival and reproduction

• Artificial Selection: Humans select desirable traits

  • Domestication of plants and animals

• Both can lead to evolutionary change in the organism, but natural selection occurs in nature without the influence of humans

Population

A group of individuals of the same species that live in the same area and interbreed to produce fertile offspring

Gene pool

A population’s genetic makeup

• Consists of all copies of every type of allele

• Many fixed alleles = less genetic diversity

• A population’s allele frequencies will change over time

  • Populations evolve, NOT individuals

Microevolution

Small scale genetic changes in a population

• Evolution is driven by random occurrences

  • Mutations

  • Genetic DNA

  • Migration/gene flow

  • Natural selection

Mutations

A change in the genetic makeup of an organism/alteration of DNA

• Can form new alleles

• Natural selection can act on varied phenotypes

• Mutation rates tend to be slow in plants and animals and fast in prokaryotes to due a faster generation time

• Mutations can be harmful, neutral, or beneficial. Most are neutral to harmful range

• Not all mutations lead to evolution

Genetic Drift

Chance events that cause a change in allele frequency from one generation to the next

• Most significant to small populations

• Can lead to a loss of genetic variation

• Can cause harmful alleles to become fixed

• Does not produce adaptations

• Ex. Bottleneck affect and Founder effect

Bottleneck effect

When a large population is drastically reduced by a non-selective disaster

• Ex. Floods, famine, fires, hurricanes, hunting, etc.

• Some alleles may become overrepresented, underrepresented, or absent

Founder effect

When a few individuals become isolated from a large population and establish a new small population with a gene pool that differs from the large population

• Lose genetic diversity

Gene flow

The transfer of alleles into or out of a population due to fertile individual or gametes

• Alleles can be transferred between populations

• Ex. Pollen being blown to a new location

Relative fitness

• Measures reproductive success

• The number of surviving offspring that an individual produces compared to the number left by others in the population

• The effects of natural selection can be measured by examining the changes in the mean of phenotypes

• Modes of natural selection: Directional, Stabilizing, Disruptive

Directional Selection

Selection towards one extreme phenotype

Stabilizing Selection

Selection towards the mean and against the extreme phenotypes

Disruptive Selection

Selection against the mean. Both phenotype extremes have the highest relative fitness

Sexual Selection

A type of natural selection that explains why many species have unique/showy traits

• Males often has useless structures simply because females chose that trait

• Can produce traits that are harmful to survival

  • Ex. Color feathers of male peacocks make them easier to spot by predators

Hardy Weinberg Equillibrium

A model used to assess whether natural selection or other factors are causing evolution at a particular locus

• Determines what the genetic makeup of the population would be if it were not evolving

  • If there are no differences, then the population is not evolving

  • If there are differences, then the population may be evolving

• The frequencies of alleles and genotypes in a population will remain constant from generation to generation, provided that only Mendelian segregation and recombination of alleles are at work.

Five conditions must be met to be in Hardy Weinberg Equilibrium

1. No mutations

2. Random mating

3. No natural selection

4. Extremely large population size

5. No gene flow - no migration

Hardy Weinberg Formulas

p + q = 1

• p: frequency of the dominant allele in a population

• q: frequency of the recessive allele in a population

p² + 2pq + q² = 1

• p²: percentage of the homozygous dominant individuals

• 2pq: percentage of the heterozygous individuals

• q²: percentage of the homozygous recessive individual

• Allele frequencies→ p and q.

• Information about individual organisms or populations,→ p²,2pq, and a²

• Most times, you will need both formulas