Understanding Evolution: Mechanisms and Genetic Variation

Evolutionary Biology Fundamentals

Charles Darwin and the Voyage of the Beagle

Charles Darwin, born in 1809, conducted the Voyage of the Beagle (1831–1836), which led to his foundational observations on biological diversity.

Microevolution vs. Macroevolution

• Microevolution: Changes in a single gene within a population over time.
• Macroevolution: The formation of new species or groups of related species.

Genetics and Inheritance

• Phenotype: The physical expression of an organism’s genes.
• Genotype: The two copies (alleles) of a gene an individual possesses.
• Inheritance: Each individual has two copies of each gene (one from each parent).
• Alleles: Can be dominant or recessive.
• Heterozygous: Individuals with one dominant and one recessive allele (carriers).
• Gene Pool: The total collection of genes present in a given population.

Natural Selection and Fitness

The mean fitness of a population is defined by the average reproductive success of its members.

• Natural Selection: The struggle for survival.
• Heritable Variation: Survival depends partly on inherited traits.
• Environmental Factors: Filter heritable variations, favoring some over others.

Key Principles of Natural Selection

• A population is the smallest unit that can evolve.
• Natural selection acts only on individuals and heritable traits.
• It functions as a remodeling tool rather than a creative one.
• Environmental context determines trait utility; a beneficial trait in one environment may be useless in another.

Types of Selection

• Directional Selection: Favors extreme phenotypes; shifts the average value of a trait.
• Stabilizing Selection: Favors intermediate phenotypes; reduces variation.
• Disruptive Selection: Favors multiple phenotypes; increases trait variation.

Sexual Selection

acts on males more strongly than on females

Asymmetry of sex—“eggs are expensive but sperm are cheap”

Genetic drift: alleles can fluctuate randomly from one gen to another

Bottleneck effect: reduction in the size of a population due to environmental events

Founder effect: new population established by a small number of individuals from a large population

Geneflow: when a population gain or losses alleles due to immigration or emigration of a fertile individuals