Theories of evolution

MDCAT Biology · Biology · Evolution. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Evolution is the change in the inherited features of a population over many generations. Lamarck said that organs grow with use and shrink with disuse, and that these acquired characters are inherited. Weismann showed this is wrong: changes to body cells are not passed on.

Darwin's natural selection starts from variation that already exists. More young are produced than can survive, so there is a struggle for existence. Individuals with helpful variations survive, breed and pass these on.

Neo-Darwinism (the modern synthesis) joins Darwin's idea with Mendelian genetics. Variation comes from mutation and from the shuffling of alleles in meiosis and fertilisation.

Rules to remember

Lamarck: use and disuse of organs, and inheritance of acquired characters.
Darwin: overproduction → struggle for existence → variation → survival of the fittest → inheritance of the favoured variations.
Darwin published 'On the Origin of Species' in 1859; Malthus gave him the idea that populations outgrow their food supply.
Fitness means leaving the most offspring that survive to breed, not being the strongest.
Mutation is the only source of new alleles; crossing over and independent assortment only make new combinations.
Only mutations in germ cells (which form gametes) can be inherited.
Hardy-Weinberg: allele frequencies stay constant if the population is large, mates at random and has no mutation, migration or selection.

Common mistake

Do not write that organisms change because they need to. In Darwin's theory the variation is there first, by chance; the environment only selects among it.

Worked example

A field is sprayed with an insecticide every year. At first almost all the insects die, but after ten years most survive the spray. Explain this as a neo-Darwinist would.

Before any spraying, a few insects already carried a chance mutation that made them resistant.
The spray killed the non-resistant insects; the resistant ones survived and bred.
They passed the resistance allele to their offspring, so its frequency rose in each generation.

Answer: The insecticide did not cause the resistance. It selected insects that were already resistant, so the population changed by natural selection.

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