Natural selectionIB MYP Biology: Revision notes

Members of the same species are not identical. This is called variation. Some variation is inherited, because individuals have different alleles (versions of a gene) passed on by their parents. Some is caused by the environment, such as a plant growing taller in better light. Only inherited variation can be passed on and acted on by natural selection.

New alleles arise by mutation, a random change in a gene. Most mutations make no difference. A few are harmful, and a few give an advantage in a particular environment.

Organisms produce more offspring than the environment can support. Resources such as food, water, light, space and mates are limited, so individuals compete. Predators and disease also kill many.

Individuals with characteristics that suit the environment are more likely to survive and reproduce. This is survival of the fittest. 'Fittest' means best adapted, not strongest or fastest.

Natural selection is a process with clear steps:

1. There is variation in the population, caused by different alleles.
2. More offspring are born than can survive, so there is competition.
3. Individuals with advantageous alleles are more likely to survive.
4. They reproduce more and pass those alleles to their offspring.
5. Over many generations the advantageous alleles become more common, and the population changes.

Individuals do not change during their lives. It is the population that changes, because the proportion of alleles changes.

Charles Darwin collected evidence in South America and on the Galápagos Islands. Alfred Russel Wallace worked separately in the Amazon and in South-East Asia. Both came to the same idea: species change over time by natural selection.

In 1858 their ideas were presented together. In 1859 Darwin published On the Origin of Species, with a large amount of evidence. Neither man knew about genes or DNA, which were understood only later, but the idea of inherited variation is now explained by alleles.

Before industry, pale peppered moths were camouflaged against pale, lichen-covered trees. Dark moths were easily seen by birds and rare.

Soot from factories blackened the trunks. Now the dark moths were camouflaged and the pale ones were eaten. Dark moths survived, reproduced and passed on the dark allele, so by 1895 about 98% of the moths in Manchester were dark.

After clean-air laws, trunks became paler again, and the pale form increased. The environment changed, so the selection pressure changed.

Bacteria reproduce very quickly, so they can evolve in a short time. In a population, a random mutation may give a few bacteria resistance to an antibiotic.

When the antibiotic is used, non-resistant bacteria are killed. The resistant ones survive, reproduce and pass on the allele. After a few generations most bacteria in the population are resistant. An example is MRSA, which is resistant to many antibiotics.

To slow this down, doctors prescribe antibiotics only when needed, and patients must complete their course. New antibiotics are also needed.