- When the allele frequencies in a population remain constant from generation to generation, the population is said to be in genetic equilibrium or Hardy-Weinberg Equilibrium
- At genetic equilibrium, there is no evolution
- But allele representation in generations might differ
- In most natural populations, the conditions of hardy-Weinberg equilibrium are not obeyed
- But calculations serve as starting point that reveals how allele frequencies are changing, which equilibrium conditions are being violated, and what mechanisms are driving the evolution of a population
- Population: a group of individuals of the same species that live in the same area and interbreed, producing fertile offspring
- Gene Pool: consists of all copies of every type of allele at every locus in all members of the population.
- At genetic equilibrium, there is no evolution
Conditions for Hardy-Weinberg Equilibrium
- If allele frequencies are changing then one of these are likely being violated
- No natural selection (all traits are selectively neutral)
- No mutations
- Gene pool modified by mutations
- No gene flow (the population must be isolated from other populations)
- Moving alleles in and out of a population can alter allele frequency
- No genetic drift (The population is large)
- In small populations, allele frequencies fluctuate by chance
- No sexual selection (mating must be random)
Values of Genetic Equilibrium
- Allele frequencies for each allele (p,q)
- Frequency of homozygous dominant (p^2) and homozygous recessive ( q^2)
- Frequency of heterozygous [2 diff alleles] (2pq)
2 Equations
- p + q = 1 (all alleles sum to 100%)
- p^2 + 2pq + q^2 = 1 (all individuals sum to 100%)
Steps
- Find percentage of homozygous recessive
- Square root q% (as decimal) → q
- 1 – q = p
- Can use q and p to find homozygotes and heterozygotes
Causes for Changes in Allele Frequencies
- Multiple factors, together with natural selection, cause evolution
- Natural Selection: increases or decreases allele frequencies bcuz of impact of environment
- Mutations introduce new alleles that may provide selective advantage
- WEAK force for changing allele frequencies; STRONG force for creating new alleles
- Gene Flow: the transfer of alleles between populations
- Movement of individuals between populations resulting in the removal of alleles when they leave (emigration) or introduction of new alleles when they enter (immigration) the population
- Ex: pollen transferred from one population to another
- Tends to reduce the genetic differences between populations
- Genetic Drift: random increase or decrease of alleles
- Especially in small (usually <100) chance events can cause allele frequencies to fluctuate and an allele to be disproportionately over or underrepresented in the next generation
- Decreases genetic variation & evolutionary adaptability and increases homozygosity
- Founder Effect: “When a few individuals become isolated from a larger population, this smaller group may establish a new population whose gene pool differs from the source population”
- Ex: founding fathers contain mutated allele and established community → reproductive isolation cause mutation to be concentrated in that area
- Population tends to have reduced genetic diversity
- Bottleneck: when the population undergoes dramatic decrease in size (predation, catastrophe, disease) → becomes susceptible to genetic drift
- Nonrandom Mating: when individuals choose mates based upon their particular traits
- Ex: always choose mates with traits similar to their own or different from their own; only nearby individuals
- Inbreeding: individuals mate with relatives
- Sexual Selection: process in which individuals with certain inherited characteristics are preferred as mates
- Intrasexual Selection: Individuals of one sex compe2te directly for mates of the opposite sex.
- Intersexual Selection (mate choice): females choose males based on attractive appearance or behaviour
- Leads to sexual dimorphosim:
Extra Notes
- Fixation: when one allele goes extinct and only one remains (becomes fixed)
→ all individuals will be homozygous for allele