变异与选择

Variation 变异 means the differences between individuals of the same species 物种. There are two types:

• continuous variation 连续变异 — a smooth range of phenotypes 表现型 between two extremes (for example body length and body mass).
• discontinuous variation 不连续变异 — a few separate phenotypes with nothing in between (for example ABO blood groups 血型, or seed shape in peas).

Discontinuous variation is usually caused by genes 基因 only. Continuous variation is caused by both genes and the environment 环境 — for example, your height depends on your genes and also on your diet.

Mutation

A mutation 突变 is a genetic change. Mutations are the way new alleles 等位基因 are made, so they are the original source of all variation. (Supplement) A gene mutation 基因突变 is a random change in the base sequence 碱基序列 of DNA.

Ionising radiation 电离辐射 (such as X-rays) and some chemicals raise the rate 速率 of mutation. (Supplement) Other sources of genetic variation are meiosis 减数分裂, random mating and random fertilisation 受精.

An adaptive feature 适应特征 is an inherited 遗传 feature that helps an organism to survive 生存 and reproduce 生殖 in its environment.

(Supplement)

• hydrophytes 水生植物 (water plants) have features such as air spaces to help them float, and stomata on the upper leaf surface.
• xerophytes 旱生植物 (desert plants) have features such as a thick waxy cuticle, few small stomata, and the ability to store water — all to reduce water loss.

Natural selection 自然选择 explains how a species becomes better suited to its environment:

1. there is genetic variation in a population (caused by mutation).
2. organisms produce many offspring 后代 — more than can survive.
3. there is a struggle to survive, with competition 竞争 for resources 资源 such as food and space.
4. the individuals that are better adapted 适应 are more likely to survive and reproduce.
5. they pass on their alleles, so the helpful alleles become more common in the next generation.

(Supplement) Over many generations this makes the population more suited to its environment; this process is called adaptation. A clear example is antibiotic 抗生素-resistant bacteria 细菌: a few bacteria carry a resistant 耐药 allele and survive the antibiotic, then multiply, until the whole population is resistant.

Worked example. A moth species is mostly pale. Soot darkens the tree trunks the moths rest on, and within a few decades most of the moths are dark. Explain by natural selection. Variation is already there: a few dark moths exist by chance, caused by mutation. On sooty bark the dark moths are better camouflaged, so birds eat fewer of them and they are more likely to survive. The survivors reproduce and pass the allele for dark colour to their offspring, so it becomes more common each generation until most moths are dark. Follow the chain: variation, survival, reproduction, the allele becomes more common. The moths did not turn dark in order to hide - the dark ones simply survived more often.

In selective breeding 选择育种 (also called artificial selection 人工选择), humans choose which organisms breed:

1. choose the individuals that have the features you want.
2. cross them to produce the next generation.
3. choose the offspring that show those features, and repeat over many generations.

It improves crops 农作物 (higher yield) and farm animals (cows giving more milk).

(Supplement) The key difference: in natural selection the environment decides which individuals survive; in artificial selection humans decide.

Common mistakes.

• Saying organisms change in order to survive, or "adapt because they need to". A single organism cannot choose its alleles. The variation is already there by chance (mutation); the environment then selects which individuals survive.
• Saying the antibiotic causes the resistance mutation. The resistant allele was already there by chance; the antibiotic kills the others, so the resistant bacteria are left to multiply.
• Stopping at "the best-adapted survive". The marks need the whole chain: they reproduce and pass on their alleles, so the allele becomes more common in the next generation.
• Saying "survival of the strongest". The organism that survives is the one best suited to its environment, such as the best-camouflaged moth, not the biggest or strongest.
• Calling height discontinuous because you can group it into bars. Height has a smooth range of values, so it is continuous; blood group has a few separate groups, so it is discontinuous.
• Saying the environment causes discontinuous variation. Discontinuous variation is usually caused by genes only.
• Mixing up natural and artificial selection. In natural selection the environment decides who breeds; in selective breeding humans decide.

• Continuous variation = a range (genes + environment); discontinuous = separate groups (genes only).
• Mutation makes new alleles — the source of all variation. Radiation and some chemicals raise the mutation rate.
• Natural selection: variation → too many offspring → struggle and competition → the best-adapted survive and reproduce → they pass on their alleles.
• Antibiotic resistance in bacteria is natural selection in action.
• Selective breeding: humans, not the environment, pick which parents breed.