General Biology Study Guide: Evolution, Genetics, and Population Biology

Biodiversity

Scales of Biodiversity

Biodiversity refers to the variety of life forms within a given ecosystem, biome, or the entire Earth. It can be studied at different scales:

• Genetic diversity: Variation of genes within species.

• Species diversity: Variety of species within a habitat or region.

• Ecosystem diversity: Diversity of ecosystems in a given place.

Evolution

Principles of Evolution

Evolution is the process by which populations of organisms change over generations. According to Coyne, the six principles of evolution include:

• Evolution occurs over generations.

• All life forms are related by descent from a common ancestor.

• Species change over time (descent with modification).

• Evolution is gradual.

• Speciation occurs, leading to diversity.

• Natural selection is a primary mechanism of evolution.

Additional info: Principles inferred from standard evolutionary theory and Coyne's work.

Theory in Science

In science, a theory is a well-substantiated explanation of some aspect of the natural world, based on a body of evidence and repeatedly tested and confirmed through observation and experimentation.

Natural Selection and Related Concepts

Types of Selection

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

• Artificial selection: The intentional breeding of plants or animals by humans for desired traits.

• Adaptation or adaptive evolution: The process by which a species becomes better suited to its environment.

Fitness is the ability of an organism to survive and reproduce in its environment. Relative fitness compares the fitness of one genotype to others in the population.

Modes of Selection

• Stabilizing selection: Favors intermediate phenotypes and reduces variation.

• Directional selection: Favors one extreme phenotype, causing a shift in population traits.

• Disruptive selection: Favors both extreme phenotypes over intermediate types.

Population Genetics

Key Terms

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

• Allele: Different forms of a gene found at the same locus.

• Locus (plural: loci): The specific location of a gene on a chromosome.

• Genetic structure: The distribution of alleles and genotypes in a population.

• Fixed allele: An allele that is the only variant present in a population.

• Neutral allele: An allele that does not affect fitness.

• Harmful allele: An allele that decreases fitness.

• Homozygote: An individual with two identical alleles at a locus.

• Heterozygote: An individual with two different alleles at a locus.

Genotype and Phenotype

• Genotype: The genetic makeup of an organism.

• Phenotype: The observable characteristics of an organism, determined by genotype and environment.

How are phenotypes determined? Phenotypes result from the interaction of genotype and environmental factors.

Allele Relationships

• Dominant allele: Expressed in the phenotype even if only one copy is present.

• Recessive allele: Expressed only when two copies are present.

• Complete dominance: The dominant allele completely masks the effect of the recessive allele.

• Incomplete dominance: The heterozygote has an intermediate phenotype.

• Co-dominance: Both alleles are fully expressed in the heterozygote.

Quantitative or Continuous Traits

Traits that show a range of phenotypes, often influenced by multiple genes (polygenic inheritance), such as height or skin color.

Mechanisms of Evolution

Mutation

A change in the DNA sequence, which can introduce new genetic variation into a population.

Gene Flow (Migration)

The movement of alleles between populations, which can increase genetic diversity.

Genetic Drift

Random changes in allele frequencies, especially in small populations.

• Population bottleneck: A sharp reduction in population size, leading to loss of genetic diversity.

• Founder effect: When a new population is established by a small number of individuals, leading to reduced genetic variation.

Inbreeding and Related Concepts

• Inbreeding: Mating between closely related individuals.

• Inbreeding depression: Reduced fitness due to increased homozygosity of deleterious alleles.

• Extinction vortex: A downward spiral of population decline due to inbreeding and loss of genetic diversity.

Hardy-Weinberg Equilibrium (HWE)

Definition and Application

The Hardy-Weinberg Equilibrium describes a population in which allele and genotype frequencies remain constant from generation to generation in the absence of evolutionary influences.

• Use allele frequencies to calculate genotype and phenotype frequencies, and vice versa.

• Requirements for HWE: no mutation, random mating, no gene flow, infinite population size, and no selection.

• If a population is not in HWE, it suggests that one or more of these conditions are not met.

Key Equations:

• Allele frequencies: $p + q = 1$

• Genotype frequencies: $p^2 + 2pq + q^2 = 1$

Where p is the frequency of one allele, and q is the frequency of the other allele.

Selection and Sexual Selection

Heterozygote Advantage

Occurs when heterozygous individuals have higher fitness than either homozygote, maintaining genetic diversity.

Sexual Selection

• Intersexual selection: Selection between sexes, often involving mate choice (e.g., peahens choosing peacocks with elaborate tails).

• Intrasexual selection: Competition within the same sex for mates (e.g., male-male competition).

• Direct benefits: Traits that provide immediate advantages to the mate (e.g., food, protection).

• Indirect benefits: Traits that signal good genes or genetic quality.

• Sexual dimorphism: Differences in appearance between males and females of a species.

• Handicap hypothesis: Suggests that costly traits are honest signals of fitness because only high-quality individuals can afford them.

Summary Table: Types of Selection

Type of Selection	Description	Example
Stabilizing	Favors intermediate phenotypes	Human birth weight
Directional	Favors one extreme phenotype	Antibiotic resistance in bacteria
Disruptive	Favors both extremes over intermediates	Beak size in certain finch populations