Rucete ✏ Campbell Biology In a Nutshell
Unit 4 MECHANISMS OF EVOLUTION — Concept 23.1 Genetic Variation Makes Evolution Possible
Evolution occurs at the level of populations, not individuals. Populations evolve through changes in genetic makeup over generations—a process known as microevolution. Genetic variation within populations provides the essential raw material for evolution.
Natural Selection Acts on Populations, Not Individuals
- Individuals do not evolve; natural selection acts on individual differences, but evolutionary change occurs at the population level.
- Example: Medium ground finches during drought—birds with larger beaks survived, and average beak size increased in later generations.
Genetic Variation: Foundation for Evolution
- Evolution depends on genetic variation—differences in genes or DNA sequences among individuals.
- Variation can be visible (height, facial features) or molecular (blood types, enzymes).
- Phenotypic traits may be discrete (either-or) or continuous, depending on the number of genes involved.
Sources of Genetic Variation
- Mutations: Changes in DNA that may create new alleles.
- Gene Duplication: Adds extra copies of genes, allowing new functions to evolve.
- Rapid Reproduction: In organisms with short life cycles, variation accumulates quickly (e.g., bacteria).
- Sexual Reproduction: Crossing over, independent assortment, and fertilization shuffle existing genes into new combinations.
Mutations and Genetic Diversity
- Most mutations are neutral or harmful; only a few are beneficial.
- Neutral variation: DNA changes that have no impact on survival or reproduction.
- Harmful mutations may persist if masked in heterozygotes ("heterozygote protection").
Sexual Reproduction and Genetic Variation
- Sexual reproduction increases genetic diversity without creating new alleles directly.
- Crossing over, independent assortment, and fertilization generate unique genetic combinations in offspring.
Measuring Genetic Variation
- Gene variability: Percentage of loci heterozygous in individuals.
- Nucleotide variability: Base pair differences in DNA, mostly found in noncoding regions with minimal phenotypic effects.
Nonheritable Variation
- Some phenotypic variation arises from environmental factors (e.g., caterpillars looking different based on their diet).
- Only genetic variation contributes to evolution.
In a Nutshell
Evolution depends on genetic variation within populations. Mutations, gene duplications, rapid reproduction, and sexual recombination generate this variation. Natural selection acts on individual differences, driving evolution in populations rather than individuals.