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Natural selection

Natural selection is the theory proposed by Charles Darwin to explain how evolution occurs.

Darwin proposed that animals that were well-adapted to their environment would survive and reproduce more than others. Genes from these animals would be passed onto to the next generation, whereas genes from weaker animals would be lost.

Natural selection relies on three conditions:

  • Genetic variation : Genetic differences between individuals lead to differences in phenotypes (physical characteristics).
  • Heredity : Genetic traits in parents are inherited by their offspring.
  • Fitness differences : Fitness is a measure of an organism’s ability to survive and reproduce. Variation means that individuals have different fitness.

Important! Variation can be caused by genes and the environment. However, only variation (and fitness differences) caused by genes can be inherited. Environmental variation does NOT influence natural selection.

The peacock’s impressive wing display is a result of natural selection.
The peacock’s impressive wing display is a result of natural selection.

Natural selection is driven by environmental factors that put pressure on the fitness of an organismaffect the ability of an organism to survive and reproduce.

An organism's fitness is relative to the environment it lives in.

Cheetahs require strong, muscular legs to catch fast prey. These would be less useful for a spider, that which uses a web to catch its food!

Environmental factors are anything around an organism that eaffects the organism’s fitness. These include: There are some common eenvironmental factors that affect fitness:

  • Food sources: sheep are well-adapted for eating grass.
  • Predators: antelope can run very fast to escape from lions.
  • Pathogens: resistance to infections increases survival.

For trees, the ability to absorb light drives natural selection. Taller trees get more light and can cast their seeds more widely. This trait will be selected for.

If the a tree is large simply because it grew in an area with more nutrients, the trait will not be selected for as it is has no genetic basis.

Trees in the amazon rainforest grow over 30 meters high to compete for light!
Trees in the amazon rainforest grow over 30 meters high to compete for light!

Natural selection alters the frequency of alleles in three ways:

  • Stabilising selection favours intermediate phenotypes.

    The frequency of alleles coding for intermediate characteristics will increase over those that code for extremes.

    Selection that favours organisms that are of average height within a population will stabilise the average height of the population.

  • Directional selection favours one extreme.

    Frequency of alleles that code for one extreme will be increasingly expressed over the rest.

    If taller organisms produce most offspring, the population will become taller over time.

  • Disruptive selection favours both extremes.

    The frequency of alleles coding for extremes will increase over the intermediates.

    Selection that favours the tallest and the shortest organisms over the intermediate.

Adaptive radiation is the rapid evolution of new species from a single lineage to fill a variety of ecological niches.

The new species have phenotypic differences that make them more adapted to the new niches, such as new types of food sources.

Adaptive radiation usually occurs under specific circumstances:

  • After a mass extinction, certain niches become unoccupied. Other organisms speciate to fill those niches.

    The extinction of dinosaurs approximately 65 million years ago opened up new niches. Mammals evolved quickly to fill those niches.

  • Organisms might colonise territories that are uninhabited such as recently emerged islands.

    Darwin's finches on the Galapagos Islands are an example of a species that colonised a new territory and then speciated rapidly.

Darwin's finches are a group of about 15 species inhabiting the Galapagos Islands.

The Galapagos are a group of isolated volcanic islands in the Pacific.

Darwin's finches probably descended from a single flock of ground finches which flew over from the South American continent and colonised the islands.

At that time, most niches were unoccupied. The ground finches evolved to fill the various niches.

Darwin's finches share a similar appearance but differ substantially in beak size and shape. These are adaptations to allow them to access different food sources.

The woodpecker finch uses a twig to extract insects from under bark.

Darwin's finches.
Darwin's finches.

The researchers Peter and Rosemary Grant observed natural selection acting on Darwin's finches during the 1970s.

In 1977 there was a severe drought affecting one of the Galapagos Islands. As a result, the number of seeds available for food fell and they became larger and harder.

The drought caused an 85% decline in the number of medium ground finches on the islands.

The average body size of finches on the island was larger in 1978 than in 1976. The beak size was also larger after the drought.

Only the largest finches with the biggest beaks could eat the harder, larger seeds, so only these finches survived to reproduce. The frequency of alleles coding for beak and body size had changed drastically.

The drought had exerted a strong environmental selection pressure on the finch population.

Evolution had occurred, driven by natural selection.

Medium ground finch.
Medium ground finch.

Natural selection can favour the maintenance of genetic variation in cases where the favourable phenotype arises through a heterozygous genotype.

Humans with heterozygous genotypes with one allele for sickle-cell anaemia have a certain degree of immunity against malaria. As a result, sickle-cell anaemia is much more common in regions with high malaria prevalence.

Mutations are the only way genetic variation can be increased.

The historic distribution of malaria is shown on the left. The distribution of the sickle cell allele is shown on the right.
The historic distribution of malaria is shown on the left. The distribution of the sickle cell allele is shown on the right.

Selective breeding (or artificial selection) is the process of humans breeding animals and plants for desirable traits.

Humans have known for a long time that offspring can inherit certain traits from their parents (heredity). This has been used to breed improved offspring.

For example, to breed better crops:

  1. Parent plants with the largest corn grains are used for breeding.
  2. The F1 offspring with the largest corn grains are used for breeding.
  3. The F2 offspring with the largest corn grains are used for breeding.

This process generates an extreme artificial selection pressure for large corn grains. Over many generations, the corn grains will get progressively larger.

Selective breeding is a slow process, although it is much faster than evolution!

There are problems with selective breeding. Artificial selection tends to reduces the variation of alleles genetic variation within a population. This increases the susceptibility to disease.

Dogs have been selectively breed for different traits. These dogs are now almost unrecognisable as the same species!
Dogs have been selectively breed for different traits. These dogs are now almost unrecognisable as the same species!