Evolutionary Processes: Study Guide for General Biology

Evolutionary Processes

Introduction

This study guide covers the major evolutionary processes that affect allele frequencies in populations, including the Hardy–Weinberg Principle, natural selection, genetic drift, and gene flow. Understanding these concepts is essential for explaining how populations evolve over time.

The Hardy–Weinberg Principle

Definition and Application

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.

• Allele frequencies: For a gene with two alleles (A and a), the frequencies are represented as p (A) and q (a), where p + q = 1.

• Genotype frequencies: The possible genotypes are AA, Aa, and aa, with expected frequencies $p^2$, $2pq$, and $q^2$ respectively.

Probability in Genetics

• The probability of rolling two sixes on a pair of dice is $1/36$.

• The probability of a child inheriting a specific allele from a parent is determined by the allele frequency in the gametes.

• Example: If the probability of 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 an individual has (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.

Statistical Tests and p Values

• Statistical tests compare observed and expected genotype or allele frequencies.

• p value indicates the probability that the observed difference is due to chance.

• Interpretation of p values:

p Value	Significance
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 populations:

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 of a trait.	Very small and very large babies are more likely to die, leading to a narrower distribution of birthweights.
Disruptive selection	Eliminates phenotypes near the average value and favors extreme phenotypes.	Whiptail fish with relatively low or high number of gill rakers survive, while those with intermediate numbers 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 Examples

Genetic drift refers to random changes in allele frequencies in a population, especially in small populations.

• Sampling error: Occurs when the allele frequencies of a chosen subset of a population are different from those in the total population, by chance.

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

• Coin flip model: Simulates random inheritance of alleles in offspring.

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 between populations.

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

Additional Info

• Non-random mating is not considered a process of evolution on its own, but can interact with other processes.

• Florida panthers: Inbreeding increased the chance of harmful recessive alleles pairing up, causing genetic defects and reduced survival.