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 p 2 + 2pq + q 2 = 1  p 2 = frequency of homozygous dominant genotype (AA)  2pq = frequency of heterozygous genotype (Aa)  q 2 = 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 variations 2. Limited by historical constraints (acts on existing structures and adaptations) 3. Adaptations are often compromises 4. Chance, natural selection, and the environment interact