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 changes occur in populations.
Example: Medium ground finches observed by Peter and Rosemary Grant:
During a drought, birds with larger beaks survived because they could eat larger, tougher seeds.
Average beak size increased in subsequent generations, demonstrating evolution by natural selection at the population level.
Genetic Variation: Foundation for Evolution
Evolution depends on genetic variation—differences among individuals' genes or DNA sequences.
Variation is often visible (e.g., human height, facial features) but can also be molecular (e.g., blood types, enzymes).
Phenotypic variation may be discrete ("either-or") or continuous, influenced by single or multiple genes respectively.
Sources of Genetic Variation
Genetic variation originates through:
Mutations: Changes in DNA sequences, potentially forming new alleles.
Gene Duplication: Provides new genetic material, allowing evolution of novel functions (e.g., olfactory genes in mammals).
Rapid Reproduction: Short generation times (like bacteria and viruses) accelerate genetic variation.
Sexual Reproduction: Genetic recombination via crossing over, independent assortment, and fertilization.
Mutations and Genetic Diversity
Most mutations are harmful or neutral; few are immediately beneficial.
Neutral variation: DNA changes without selective advantage or disadvantage.
Harmful mutations can persist if recessive, hidden in heterozygotes ("heterozygote protection").
Sexual Reproduction and Genetic Variation
Sexual reproduction shuffles existing genetic variation:
Crossing over, independent assortment, and fertilization create unique allele combinations.
Provides extensive genetic variation without creating new alleles directly.
Measuring Genetic Variation
Genetic variation is measured as:
Gene variability: Average percentage of loci heterozygous within individuals.
Nucleotide variability: Differences at the DNA sequence level; most changes occur in noncoding regions and don't affect phenotype significantly.
Nonheritable Variation
Not all phenotypic differences result from genetic factors; environmental influences also cause variation (e.g., diet-induced differences in caterpillars).
Only genetically determined variation influences 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.