Natural selection practice questions.

State the source of the heritable variation on which natural selection acts.

• Random mutation and reshuffling of alleles during sexual reproduction
• The needs of the organism directing useful changes
• Use and disuse of structures during an organism's lifetime
• Changes acquired from the environment and passed to offspring

Answer: Random mutation and reshuffling of alleles during sexual reproduction

Heritable variation arises from mutation and, in sexually reproducing organisms, from meiosis and fertilisation; selection then acts on this pre-existing variation.

In evolutionary terms, the fitness of an organism is best defined as its:

• physical strength and speed relative to competitors
• ability to survive extreme environmental conditions
• relative contribution of its alleles to the next generation
• total lifespan compared with other members of the population

Answer: relative contribution of its alleles to the next generation

Fitness is a measure of reproductive success, the relative contribution an individual makes to the gene pool of the next generation, not physical strength or longevity.

A population of insects is treated repeatedly with an insecticide, and over several generations it becomes resistant. Which statement best explains this using natural selection?

• The insecticide caused the insects to develop resistance so they could survive
• Individuals that needed resistance mutated to acquire it
• Resistant alleles were already present, and survivors reproduced
• All insects became equally resistant through exposure to the chemical

Answer: Resistant alleles were already present, and survivors reproduced

Resistance alleles existed by chance before exposure; the insecticide selectively killed susceptible individuals, so resistant survivors reproduced and the allele frequency rose over generations.

Which of the following is an example of directional selection?

• Human birth mass, where very light and very heavy babies have lower survival
• The peppered moth population shifting towards darker forms in polluted areas
• Maintenance of two distinct beak sizes in a seed-eating bird population
• Stabilising the mean value of a trait around its current average

Answer: The peppered moth population shifting towards darker forms in polluted areas

Directional selection favours one extreme of a trait, shifting the population mean; the darkening of peppered moths in soot-polluted areas is the classic example. Human birth mass is stabilising selection.

Over many generations, rock-pocket mice living on dark lava have come to have a much higher frequency of the dark-fur allele than nearby populations on light soil. Which process explains this?

• Genetic drift acting on a small population
• Natural selection favouring camouflage against predators
• Directed mutation in response to the dark rock
• Inheritance of coat colour acquired during life

Answer: Natural selection favouring camouflage against predators

Dark fur on dark lava provides camouflage from predators, giving those mice a survival and reproductive advantage, so the dark allele increases in frequency by natural selection.

Which condition is NOT required for natural selection to bring about evolution in a population?

• More offspring are produced than can survive
• There is heritable variation among individuals
• Every individual in the population is genetically identical
• Some variants have greater reproductive success than others

Answer: Every individual in the population is genetically identical

Natural selection requires heritable variation, overproduction of offspring and differential reproductive success; if individuals were genetically identical there would be no variation for selection to act on.