Evolutionary Processes: Study Guide and Key Concepts

Evolutionary Processes

Introduction

This study guide covers the major evolutionary processes that affect allele frequencies in populations. It includes definitions, models, and examples relevant to general biology, focusing on the Hardy–Weinberg Principle, natural selection, genetic drift, and gene flow.

The Hardy–Weinberg Principle

Definition and Importance

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.

• Population genetics is the study of allele frequency distribution and change under the influence of evolutionary processes.

• The principle assumes no mutation, migration, selection, or genetic drift, and random mating.

Probability in Genetics

• Probability of rolling two sixes on a pair of dice: $1/6 \times 1/6 = 1/36$

• Probability of offspring inheriting a specific allele: If both parents are heterozygous (Aa), the probability of the child getting allele A from the sperm is $1/2$, and from the egg is $1/2$. The probability of the child being AA is $1/2 \times 1/2 = 1/4$.

Alleles, Genotypes, and Phenotypes

• Allele: Different versions of a gene (A or a).

• Genotype: The combination of alleles in an individual (AA, Aa, or aa).

• Phenotype: The physical trait that results (e.g., shell color, size).

Four Evolutionary Processes

These processes can change allele frequencies over time:

Process	Definition
Natural Selection	Changes the frequency of certain alleles if they influence reproductive success in a particular environment.
Gene Flow	Occurs when individuals leave one population, join another, and breed, introducing new alleles.
Genetic Drift	Random changes in allele frequencies due to chance, especially in small populations.
Mutation	DNA sequence changes create new alleles.

Hardy–Weinberg Hypothesis Testing

• Hypothesis: Heritable resistance is evolving in populations.

• Null hypothesis: Heritable resistance is not evolving; allele frequencies do not increase.

Hardy–Weinberg Claims

• Allele frequencies: For two alleles, the frequencies add up to 1 ($p + q = 1$).

• Genotype frequencies: The three possible genotypes (AA, Aa, aa) occur with frequencies $p^2$, $2pq$, and $q^2$.

Statistical Testing in Population Genetics

Statistical tests (such as chi-square) compare observed and expected genotype frequencies to determine if a population is in Hardy–Weinberg equilibrium.

p Value	Conclusion
NS	Not statistically significant
p < 0.05	Statistically significant
p < 0.01	Highly statistically significant
p < 0.001	Extremely statistically significant

Natural Selection

Modes of Natural Selection

Natural selection can act in different ways to shape genetic variation:

Mode	Definition	Example
Directional selection	Occurs when the average phenotype changes in one direction.	Break selection caused increase in a population of finches during a drought.
Stabilizing selection	Genetic variation is reduced, but there is no change in the average value.	Very small and very large babies are more likely to die, favoring a more average birthweight.
Disruptive selection	Eliminates phenotypes near the average value and favors extreme phenotypes.	Whifish with relatively low or high number of gill rakers survive, while intermediate forms do poorly.
Balancing selection	Occurs when no single phenotype has a distinct advantage.	Heterozygote advantage, such as sickle cell disease alleles in malaria regions.

Genetic Drift

Definition and Effects

Genetic drift occurs when allele frequencies of a chosen subset of a population differ from those in the total population, by chance. It is especially significant in small populations.

• Sampling error: Random fluctuations in allele frequencies due to chance events.

• Example: If 400 out of 1000 alleles are A, frequency of A is $400/1000 = 0.4$.

• In a population with two alleles (A and a), if both parents are Aa, the frequency of each allele is $0.5$.

Bottleneck and Founder Effects

Effect	Definition	Example
Bottleneck effect	Sudden reduction in diversity of alleles in a population.	Disease outbreaks, natural catastrophes.
Founder effect	Change in allele frequencies when a new population is established.	Green iguanas founded a new island population.

Gene Flow

Definition and Impact

Gene flow is the movement of alleles between populations due to migration of individuals or gametes. It can increase genetic similarity between populations.

• Gene flow can counteract the effects of genetic drift and selection, leading to more similar allele frequencies across populations.

• Example: Salamander populations separated by unsuitable habitat may become more genetically different over time if gene flow is limited.

Distinguishing Genetic Drift and Gene Flow

• Genetic drift: Random change in allele frequencies.

• Gene flow: Movement of alleles between populations.

Additional info:

• Statistical tests such as chi-square are commonly used to test Hardy–Weinberg equilibrium.

• Non-random mating alone does not cause evolution, but can interact with other processes.

• Mutation is the ultimate source of genetic variation.