HARDY-WEINBERG THEOREM

Chapter 23: Population Genetics

Microevolution

Measure of how allele frequencies change over time Allele – different forms of a gene

Ex: beetle color – green or brown

GENETIC VARIATION MAKES EVOLUTION POSSIBLE

23.1

Genetic Variation

Differences in genes among individuals

Sources of Genetic Variation

Mutations Change in DNA sequence Occur more frequently in

asexually reproducing organisms Rapid reproduction = gets

sloppy

Sexual reproduction Crossing over Independent assortment

Only half of genes are passed on; this is random

Random fertilization

THE HARDY-WEINBERG EQUATION CAN BE USED TO TEST WHETHER A POPULATION

IS EVOLVING

23.2

Population

Localized group of individuals belonging to the same species

Species = group of populations whose individuals have potential to interbreed

Gene Pool

Total aggregate of genes in a population at any one time

All alleles at all loci in all individuals

Example: flower population with white and pink flowers Population of 500 individuals

20 white (rr) 320 homozygous pink (RR) 160 heterozygous pink (Rr)

So, 1000 alleles: 800 R alleles, 200 r alleles

Hardy-Weinberg Theorem

Frequency of alleles and genotypes in a population’s gene pool remain constant over generations UNLESS acted upon by agents other than sexual recombination (chance)

Hardy-Weinberg Equilibrium

Allele frequency is constant from generation to generation

Required Conditions for H-W Equilibrium

1. Very large population

Required Conditions for H-W Equilibrium

2. Random mating

3. Isolation from other populations

Required Conditions for H-W Equilibrium

4. No natural selection acting on the populatiom

Required Conditions for H-W Equilibrium

5. No net mutations (changes to the DNA code)

Required Conditions for H-W Equilibrium

Hardy-Weinberg Equation

p + q = 1 p = frequency of dominant allele q = frequency of recessive allele

p2 + 2pq + q2 = 1 p2 = frequency of homozygous dominant genotype (AA) 2pq = frequency of heterozygous genotype (Aa) q2 = frequency of homozygous recessive genotype (aa)

NATURAL SELECTION, GENETIC DRIFT, AND GENE FLOW CAN ALTER ALLELE

FREQUENCIES IN A POPULATION

23.3

Natural Selection & Gene Frequencies

If an allele gives the organism an advantage, it will more likely be passed on and it’s frequency will increase over time

Genetic Drift & Gene Frequencies

Chance events can cause allele frequencies to fluctuate, especially in small populations

The Founder Effect & Gene Frequency

Few individuals isolated from a population start a new population with a different allele frequency than the original population

The Bottleneck Effect & Gene Frequency

Sudden change in the environment reduces the size of the population

By chance alone, certain alleles may be over- or underrepresented or absent in survivors

Gene Flow

Transfer of alleles into or out of a population due to immigration and emigration

NATURAL SELECTION IS THE ONLY MECHANISM THAT CONSISTENTLY CAUSES

ADAPTIVE EVOLUTION

23.4

Relative Fitness

The contribution an individual makes to the gene pool of the next generation relative to the contributions of other individuals A genotype's fitness depends on the

environment in which the organism lives. The fittest genotype during an ice age, for

example, is probably not the fittest genotype once the ice age is over.

Fitness lumps everything that matters to natural selection (survival, mate-finding, reproduction) into one idea. The fittest individual is not necessarily the

strongest, fastest, or biggest. A genotype's fitness includes its ability to

survive, find a mate, produce offspring — and ultimately leave its genes in the next generation.

The brown beetles have a greater fitness relative to the green beetles.

Types of Selection

Stabilizing Favors intermediates

Directional Favors one extreme

Disruptive (diversifying) Favors both extremes

Sexual Selection

Individuals with certain inherited traits are more likely to obtain mates than others

Natural Selection Isn’t Perfection!

1. Only acts upon existing variations2. Limited by historical constraints (acts on

existing structures and adaptations)3. Adaptations are often compromises4. Chance, natural selection, and the

environment interact