Three types of natural selection
Natural selection is not always a mechanism for change. There three different types: stabilising selection, directional selection, and disruptive selection. These are three different ways in which natural selection acts on the phenotypes in a population (the observable characteristics such as colour or height). Typically, the frequency in the population of each phenotype has a normal distribution, described by a bell-shaped curve .
Stabilising selectionhappens in an unchanging environment. Extremes of the phenotype range are selected against, leading to a reduction in variation (more individuals tend to conform to the mean). Stabilising selection occurs in the natural selection of birth mass in humans.
Directional selectionfavours one extreme of the phenotype range and results in a shift of the mean either to the right or to the left. This type of selection usually follows some kind of environmental change. The long neck of the giraffe is thought to have evolved in this way. Probably, when food was in short supply, only the tallest individuals could reach enough food to survive. They passed on their genes to the next generation.
Disruptive selectionselects against intermediate phenotypes and favours those at the extremes. This leads to a bimodal distribution(the distribution curve has two peaks or modes) and two overlapping groups of phenotypes. If the two groups become unable to interbreed, then each population may give rise to a new species. Disruptive selection may have contributed to the evolution of Darwin’s finches. Because there were few other birds to compete, finches with short strong beaks had exclusive use of nuts as a food source, while those with long slender beaks had almost exclusive use of insects. Those finches with an average, unspecialised beak were more likely to have been in completion with other species of bird and would have reproduced less successfully.
■ Glossary of essential terms for you to know
■ Your Essential Assignments
I. Quick check
1. What is meant by fitness in evolutionary terms?
2. Some individuals of the European swallowtail butterfly (Papilio machaon) pupate on brown stems or leaves; others pupate on green stems or leaves. Two distinct colour forms of the pupae are found, namely brown and green, with very few intermediates.
a. What type of natural selection does this example show?
b. Explain why the intermediate colour forms would be at a selective disadvantage?
II. Fill in the missing words:
III. Use monolingual English dictionary and write down what could the words given below mean:
species, existence, selection, environment, to occur, gene.
IV. Match these words with their definitions:
V. Find English equivalents to the following word combinations:
VI. Give Russian equivalents to the following English terms:
VII. Find synonyms among the pool of words:
VIII. Answer the following questions. Use all information given before:
1. What might lead to famine, pestilence and war?
2. What is called selection pressures?
3. What environmental factors do selection pressures include?
4. What organisms will have the best chance of surviving and producing their offspring?
5. Why does the difference in natality and mortality result in natural selection?
6. What is meant by “survival of the fittest”?
7. How is fitness defined in evolution?
8. What are three types of natural selection?
9. What is the difference between them?
IX. Match the sentence halves. Make complete sentences: