Chapter 23 - Biology 112

POPULATION GENETICS:

A population's gene pool includes all the alleles of all the individuals in that population.

Allele frequency

the proportion of an allele in the gene pool.

HARDY-WEINBERG EQUILIBRIUM

p² + 2pq + q² = 1 AND p + q = 1

SOURCES OF GENETIC VARIATION

A. Mutation

• Random changes in DNA create new alleles.

  Usually are but the ultimate source of all new genetic variation.

B. Sexual Reproduction

• Independent Assortment (different pairs of chromosomes assort independently into gametes)

• Crossing Over ( Homologous chromosomes exchange segments during meiosis I)

• Random Fertilization (Any sperm can fuse with any egg)

Natural Selection in Populations

Natural selection acts on phenotypes but changes allele frequencies.

Types of Selection

Directional, Stabilizing, Disruptive

Directional

Selection favors one extreme

Stabilizing

Selection favors the average

Disruptive

Selection favors both extremes

GENETIC DRIFT

Random chance causes allele frequencies to change, especially in small populations.

Founder Effect

A new population is started by a small number of individuals from a larger population.

Bottleneck Effect

A large population is drastically reduced in size by a chance event (disease, natural disaster, habitat loss, etc.).

GENE FLOW

Movement of individuals (or gametes) into or out of a population.

MUTATION

Creates new alleles.

PRESERVING GENETIC VARIATION

Diploidy

• having two sets of chromosomes masks the expression of some harmful recessive alleles in heterozygotes

Heterozygote Advantage

• Have higher fitness than either homozygote

Frequency-Dependent Selection

• The advantage of a phenotype depends on how common it is

Negative Frequency Dependent

Rare types have advantage

Positive Frequency dependent

Common types have an advantage

Sexual Selection

A form of natural selection related to mating success rather than survival.

Intrasexual competition

Members of the same sex compete for access to mates.

Intersexual choice

Members of one sex choose mates with certain traits.

WHY EVOLUTION DOESN'T PRODUCE "PERFECT" ORGANISMS

1. Works only on existing variation

2. Constrained by history

3. Adaptations are compromises

4. Chance events matter

MEASURING EVOLUTION

Evolution is detected by a change in allele frequencies over time