Natural Selection (College Board AP® Biology): Study Guide

The process of natural selection

• Natural selection is the mechanism that drives evolution

  • Charles Darwin developed the theory of evolution by natural selection after observations made during a research expedition

• Evolution by natural selection occurs as follows:

  1. phenotypic variation exists between individuals in a population

  2. competition for limited resources leads to differing rates of survival

     • Individuals with an advantageous phenotype will be more successful competitors

  3. selective pressures in the environment mean that individuals with favorable phenotypes are more likely to survive and reproduce

     • An individual with high reproductive success is said to have high reproductive fitness

     • Fitness reflects an organism's ability to survive and pass on its alleles

  4. the alleles that code for favorable phenotypes are passed on to the next generation

  5. the alleles that code for beneficial traits become more frequent in the population over time

• Factors in the environment, both biotic (e.g. predators, competition) and abiotic (e.g. temperature, water availability), influence the rate and direction of evolution

  • Environmental change means that these factors may act differently on populations from one generation to the next, meaning that different genetic variations can be selected in each generation

• The peppered moth is an example of natural selection leading to evolution over time

The importance of variation

• Phenotypic variation arises from genetic differences

  • E.g. mutations can introduce new alleles into the gene pool

• Populations with greater variation can better adapt to changing environments

• Natural selection acts on variation:

  • Some traits significantly increase fitness

  • Others decrease fitness in specific environments

Examples of traits that increase fitness

DDT resistance in insects

• DDT (Dichlorodiphenyltrichloroethane) was a widely used insecticide for control of mosquito populations and other pests

• DDT resistance increases fitness; the allele for resistance increases in frequency in insect populations when DDT is present in the environment

  • Application of DDT kills insects without resistance and allows those with mutations for resistance to survive

  • Resistant individuals reproduce, passing on the resistance alleles

  • Over time the population shifts to predominantly DDT-resistant insects

• In this example DDT is a selective pressure, driving the evolution of DDT resistance in insect populations

Sickle cell anemia and malaria

• Sickle cell anemia is a genetic condition in which altered hemoglobin protein causes red blood cells to take on a sickled shape

• Sickle cell trait increases evolutionary fitness as it provides resistance to malaria, a significant cause of death in some parts of the world

  • In malaria-prevalent areas individuals carrying one sickle cell allele (heterozygotes) resist malaria more effectively

  • These individuals survive and reproduce

  • This reproductive success increases the frequency of the sickle cell allele in the population

Examples of traits that decrease fitness

Flowering time and global climate change

• Shifting flowering times due to global warming results in a decrease in evolutionary fitness due to disruption of crucial plant-pollinator synchronization

  • Earlier or later blooming may cause plants to bloom at a time of year when their pollinators are not present

  • Plants are less likely to be pollinated

  • Reduced reproductive success leads to lower seed production and population decline

• Some plant species may face increased extinction risk if they cannot adapt quickly enough to changing environmental conditions