Microevolution and Natural Selection

The Evolution of Populations

• A common misconception about evolution is that organisms evolve during their lifetimes, but it is the population that evolves, not the individual.
• Natural selection acts on individuals, affecting their survival and reproductive success relative to others in the population.

The Medium Ground Finch Example

• In 1977, a drought on the Galápagos Islands affected the medium ground finch (Geospiza fortis) population, with only 180 out of 1200 birds surviving.
• The surviving finches had larger, deeper beaks than those that died, allowing them to crack open large, hard seeds that were abundant during the drought.
• After the drought, the average beak size in the population was larger, demonstrating evolution by natural selection.

Microevolution and Mechanisms of Change

• Microevolution is defined as a change in allele frequencies in a population over time.
• Three mechanisms can cause allele frequencies to change: natural selection, genetic drift, and gene flow.
• Natural selection is the only mechanism of adaptive evolution, improving the match between organisms and their environment.

Genetic Variation and Evolution

• Mutation and sexual reproduction produce the genetic variation that makes evolution possible.
• Charles Darwin proposed a mechanism for change in species over time, but lacked an understanding of inheritance that could explain how chance variations arise in a population.
• Gregor Mendel's particulate hypothesis of inheritance supported Darwin's theory, stating that parents pass on discrete heritable units (genes) that retain their identities in offspring.

Mechanisms of Evolutionary Change

• New mutations can modify allele frequencies, but the change is small because mutations are rare.
• Nonrandom mating can affect the frequencies of homozygous and heterozygous genotypes, but has no effect on allele frequencies in the gene pool.
• Natural selection, genetic drift, and gene flow are the three mechanisms that directly alter allele frequencies, leading to evolutionary change.

Natural Selection

• Individuals in a population vary in their heritable traits.
• Individuals with variations better suited to the environment tend to produce more offspring than those with less suitable variations.
• As a result of selection, alleles are passed on to the next generation in frequencies different from their relative frequencies in the present population.
• For example, the fruit fly Drosophila melanogaster developed an allele conferring resistance to insecticides, which increased in frequency from 0% to 37% over 20 years of DDT use.