General Biology: Evolution, Genetics, and Population Study Guide

Evolution and Natural Selection

Natural Selection

Natural selection is a fundamental mechanism of evolution, describing how certain traits become more common in a population due to differential survival and reproduction.

• Definition: The process by which organisms better adapted to their environment tend to survive and produce more offspring.

• Key Points:

  • Variation exists within populations.

  • Some variations confer advantages in survival or reproduction.

  • Advantageous traits become more common over generations.

• Example: The peppered moth (Biston betularia) changed coloration in response to industrial pollution.

Fitness

Fitness refers to an organism's ability to survive and reproduce in its environment.

• Definition: The reproductive success of an individual relative to others in the population.

• Equation: $\text{Fitness} = \frac{\text{Number of offspring}}{\text{Average number of offspring in population}}$

Phylogeny

Phylogeny is the study of evolutionary relationships among species.

• Phylogenetic trees: Diagrams that show evolutionary connections.

• Application: Used to classify organisms and trace lineage.

Genetics and Heredity

Genotype vs. Phenotype

Genotype refers to the genetic makeup of an organism, while phenotype is the observable traits.

• Genotype: The set of genes carried by an organism.

• Phenotype: The physical expression of those genes.

• Example: Eye color is a phenotype determined by genotype.

Sources of Genetic Variation

Genetic variation is essential for evolution and is produced by several mechanisms.

• Mutation: Random changes in DNA sequence.

• Recombination: Exchange of genetic material during meiosis.

• Sexual reproduction: Combines genes from two parents.

Modes of Selection

Selection can act in different ways on populations.

• Directional selection: Favors one extreme phenotype.

• Stabilizing selection: Favors intermediate phenotypes.

• Disruptive selection: Favors both extremes over intermediates.

Population Genetics

Hardy-Weinberg Equilibrium

The Hardy-Weinberg principle describes a population that is not evolving.

• Conditions: No mutation, random mating, no gene flow, infinite population size, no selection.

• Equation: $p^2 + 2pq + q^2 = 1$

• Where:

  • $p$ = frequency of dominant allele

  • $q$ = frequency of recessive allele

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

  • $2pq$ = frequency of heterozygous genotype

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

Calculating Allele, Genotype, and Phenotype Frequencies

Population genetics involves calculating the frequencies of alleles and genotypes.

• Allele frequency: Proportion of a specific allele among all alleles in the population.

• Genotype frequency: Proportion of a specific genotype among all individuals.

• Phenotype frequency: Proportion of individuals with a specific trait.

• Example Calculation:

  • If $p = 0.6$ and $q = 0.4$, then $p^2 = 0.36$, $2pq = 0.48$, $q^2 = 0.16$.

Sexual Selection and Mating Systems

Sexual Selection

Sexual selection is a form of natural selection related to mating success.

• Definition: Selection for traits that increase mating success.

• Male vs. Female Effort: Males often compete for access to females; females may be choosy.

• Example: Peacock tail feathers are selected for by female choice.

Mating Systems

Mating systems describe how individuals pair and reproduce.

• Monogamy: One male and one female pair exclusively.

• Polygamy: Individuals have multiple mates.

• Promiscuity: No exclusive pair bonds.

Summary Table: Modes of Selection

Mode of Selection	Description	Effect on Population
Directional	Favors one extreme phenotype	Shifts average trait value
Stabilizing	Favors intermediate phenotypes	Reduces variation
Disruptive	Favors both extremes	Increases variation, may lead to speciation

Additional info:

• Some content inferred from standard biology curriculum (e.g., Hardy-Weinberg, sexual selection definitions).

• Examples and equations added for completeness and clarity.