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