• Natural selection

• explains how populations change (evolve) over time.
• Individuals that have inherited traits suited to their environment are more likely to survive and reproduce than individuals without those traits.
• These useful traits are passed to the offspring and become more common in each generation.
• Important points:

• The environment does not give the organism new traits. It selects, among the traits that individuals already have, those that help survival.
• Better-adapted individuals live longer and leave more offspring.
• Less adapted individuals are more likely to die before reproducing or reproduce less.
• Over many generations the proportion of useful traits rises, while the proportion of less useful traits falls.

• Before the Industrial Revolution in Europe, tree trunks were light-coloured and covered with lichens.
• Light-coloured moths were hard for predators (birds) to see on them, so they survived more often.
• After the Industrial Revolution, air pollution killed the lichens and darkened the trunks.
• Now light moths were easy to see and were eaten, while dark moths were hidden.
• Over time dark moths became common and light moths became rare.
• Explanation using natural selection (1) the moths were already varied in colour (variation); (2) the changed environment made the dark colour an advantage; (3) dark moths survived and reproduced more; (4) they passed the dark-colour allele to their offspring; (5) the proportion of dark moths rose over generations.

• Lichens as indicators. Lichens are formed by a fungus living with an alga or cyanobacterium.
• They are sensitive to air pollution, so their presence or absence shows the quality of the air.

Worked example 2: a mathematical model of selection

Worked example 3: adaptation to the cold

• In a cold region, animals with thick fur lose less heat.
• Suppose in a population of 200 animals, 120 have thick fur and 80 thin fur.
• A very cold winter kills 10% of the thick-fur animals and 50% of the thin-fur animals.
• Survivors: thick: 120 − 12 = 108; thin: 80 − 40 = 40.
• Total survivors: 148.
• Thick fur is now 108/148 = 73% (up from 60%).
• The trait that helps survival has become more common.