Evolutionary Processes and Population Genetics

Ch. 23: Evolutionary Processes

Introduction to Evolutionary Processes

Evolutionary processes are the mechanisms that drive changes in the genetic composition of populations over time. These processes explain the diversity of life and the adaptation of organisms to their environments.

• Evolution is the change in allele frequencies in a population over generations.

• Key mechanisms include natural selection, genetic drift, gene flow, and mutation.

All About Alleles

Definition and Importance of Alleles

An allele is a variant form of a gene. Different alleles can result in different traits, such as eye color, flower color, or fur color in animals.

• Alleles are found at specific locations (loci) on chromosomes.

• Variation in alleles is the basis for genetic diversity within a population.

• Examples: Eye color in humans, flower color in plants, fur color in dogs and rabbits.

Driving Forces of Evolution

Main Mechanisms That Change Allele Frequencies

There are four primary mechanisms that cause evolution by altering allele frequencies in populations:

1. Natural Selection: Increases the frequency of alleles that contribute to reproductive success in a particular environment.

2. Genetic Drift: Causes allele frequencies to change randomly, especially in small populations.

3. Gene Flow: Occurs when individuals move between populations, introducing new alleles and altering frequencies.

4. Mutation: Modifies allele frequencies by continually introducing new alleles into the gene pool.

The Hardy-Weinberg Principle

Population Genetics and Genetic Equilibrium

The Hardy-Weinberg Principle provides a mathematical model to study genetic variation in populations. It predicts how gene frequencies will be inherited from one generation to the next under ideal conditions.

• Developed by G. H. Hardy and Wilhelm Weinberg in 1908.

• Assumes a large, randomly mating population with no evolutionary forces acting (no selection, mutation, migration, or genetic drift).

• The gene pool concept treats all alleles from all gametes as a single group that combines randomly.

Hardy-Weinberg Equation:

$p^2 + 2pq + q^2 = 1$

• $p$ = frequency of the dominant allele

• $q$ = frequency of the recessive allele

• $p^2$ = frequency of homozygous dominant genotype

• $2pq$ = frequency of heterozygous genotype

• $q^2$ = frequency of homozygous recessive genotype

• $p + q = 1$ (the sum of allele frequencies equals 1)

Applications: The Hardy-Weinberg Principle serves as a null hypothesis for detecting evolutionary change. If observed genotype frequencies differ from expected frequencies, at least one evolutionary mechanism is at work.

Summary Table: Evolutionary Mechanisms

Mechanism	Effect on Allele Frequencies	Directionality	Source of Variation
Natural Selection	Increases frequency of beneficial alleles	Non-random	Acts on existing variation
Genetic Drift	Random changes, especially in small populations	Random	Can reduce variation
Gene Flow	Introduces or removes alleles	Random with respect to fitness	Increases variation
Mutation	Creates new alleles	Random	Ultimate source of new variation

Key Terms

• Allele: A variant form of a gene.

• Gene Pool: The total collection of alleles in a population.

• Genotype: The genetic makeup of an individual.

• Phenotype: The observable traits of an individual.

• Population: A group of individuals of the same species living in the same area.

• Evolution: Change in allele frequencies in a population over time.

Example: Calculating Hardy-Weinberg Frequencies

If the frequency of allele A ($p$) is 0.7 and allele a ($q$) is 0.3 in a population, the expected genotype frequencies are:

• AA: $p^2 = (0.7)^2 = 0.49$ (49%)

• Aa: $2pq = 2 \times 0.7 \times 0.3 = 0.42$ (42%)

• aa: $q^2 = (0.3)^2 = 0.09$ (9%)

Conclusion

Understanding evolutionary processes and the Hardy-Weinberg Principle is fundamental to studying how populations change over time. These concepts form the basis for modern evolutionary biology and population genetics.