Chapter 17: Selection and Evolution


The characteristics of an organism, which result from the interaction of the genes of the organism with the environment in which it lives is phenotype.

Phenotypic variation can also be explained by a combination of genetic and environmental factors

  • For example, the recessive allele that causes sickle cell anemia has a high frequency in populations where malaria is prevalent due to heterozygous individuals being resistant to malaria.

Phenotype = Genotype + Environment

Phenotypic variation may be continuous (as in the height or mass of an organism) or discontinuous (as in the human ABO blood groups). The genotype of an organism gives it the potential to show a particular characteristic. In many cases, the degree to which this characteristic is shown is also influenced by the organism’s environment. Genetic variation within a population is the raw material on which natural selection can act.

Types of Variation

  • Continuous variation: It is variation within a range and it includes mass and height.
  • Discontinuous variation: It can only take particular values such as gender or shoe size.


Genetic Variation

Genetic variation is caused by the following processes as they result in a new combination of alleles in a gamete or individual:

  • Independent assortment of homologous chromosomes during metaphase
  • Crossing over of non-sister chromatids during prophase
  • Random fusion of gametes during fertilization

Environmental Factors

  • Variation in phenotype caused solely by environmental pressures or factors cannot be inherited by an organism’s offspring
    • Only alterations to the genetic component of gametes will ever be inherited

Natural Selection

Natural selection is the process in which fitter individuals who are better adapted to the environment survive and pass on the advantageous genes to future generations. Four conditions are needed for natural selection to occur: reproduction, heredity, variation in fitness or organisms, variation in individual characters among members of the population. If they are met, natural selection automatically results.

  • There are a variety of phenotypes within a population
  • An environment change occurs and as a result of that the selection pressure changes
  • Some individuals possess advantageous alleles which gives them a selective advantage and allows them to survive and reproduce
  • The advantageous alleles are passed on to their offspring
  • Over time, the frequency of alleles in a population change and this leads to evolution


Evolution is the process by which the frequency of alleles in a gene pool changes over time as a result of natural selection.

Factors that can affect the evolution of a species

  • Gene drift: It is a phenomenon where there is a small change in allele frequency which occurs as a result if the fact that not all the individuals in a population reproduce. This effect is amplified in very small group, isolated from the rest of the population.
  • Genetic bottleneck: Rapid reduction in population size which has an effect on the population size and genetic variation in future generations
  • Founder effect: Decrease in genetic diversity which occurs when the population descends from a small number of ancestors.

Evolution is the formation of new species from pre-existing species over time, as a result of changes to gene pools from generation to generation.

Hardy-Weinberg Principle

The Hardy-Weinberg equilibrium is a principle stating that the genetic variation in a population will remain constant from one generation to the next in the absence of disturbing factors. Because all of these disruptive forces commonly occur in nature, the Hardy-Weinberg equilibrium rarely applies in reality.

The Hardy-Weinberg equation used to determine genotype frequencies is: p2 + 2pq + q2 = 1. Where ‘p2‘ represents the frequency of the homozygous dominant genotype (AA), ‘2pq’ the frequency of the heterozygous genotype (Aa) and ‘q2‘ the frequency of the homozygous recessive genotype (aa).

Conditions for the Hardy-Weinberg principle

  • Organisms are diploid
  • Organisms reproduce by sexual reproduction only
  • There is no overlap between generations
  • Mating is random
  • The population is infinitely large
  • There is no migration, mutation or selection
  • Allele frequencies are equal in both sexes

Artificial Selection

Artificial selection is the process where selection pressures are artificially created by human thus allowing the breeding of the desired characteristics. Artificial selection, also called “selective breeding”, is where humans select for desirable traits in agricultural products or animals, rather than leaving the species to evolve and change gradually without human interference, like in natural selection.

  • Examples of artificial selection include:
    • Increased milk yield from cattle
    • Faster racehorses
    • Disease-resistant crops
  • Some examples of selective breeding in agriculture and livestock include:
    • Disease-resistance in wheat and rice varieties
    • Hybridization in maize
    • Milk yield in cattle

Extinction of organisms

Species can become extinct for a variety of reasons:

  • Killing by humans: some endangered species are hunted for food, medicine and sport. Species such as the quagga and Tasmanian tiger have become extinction as a result of this.
  • Competition: Organisms compete for food, space, and mates. Competition is one of the main contributors to the evolution of canines, causing the extinction of 40 of their species as the family evolved.
  • Habitat loss and climate change: Climate change can cause habitats to be loss as temperatures rise and natural disasters become more common. Humans can also damage habitats.