选择与进化

Variation 变异 means the differences between individuals. It has three possible causes:

• genetic factors only — set by the alleles 等位基因 you inherit (for example human blood group).
• environmental factors 环境因素 only — set by your surroundings (for example a scar, or the language you speak).
• a combination of both — most features, such as height and body mass, depend on genes and on diet and lifestyle.

There are two patterns of variation:

• discontinuous variation 不连续变异 — clear, separate groups with nothing in between (for example blood group A, B, AB or O). It is usually controlled by one or a few genes 基因, with little effect from the environment.
• continuous variation 连续变异 — a smooth range from one extreme to the other (for example height). It is controlled by many genes together, plus the environment.

To compare the means of two samples (for example the heights of plants in sun and in shade), you use the t-test, which tells you whether the difference is large enough to be real, or just due to chance.

A population 种群 produces far more offspring 后代 than can survive, so the offspring must compete 竞争 for resources such as food and space. This is the "struggle for existence". The individuals that are best adapted 适应 are most likely to survive, reproduce 繁殖, and pass on their alleles to the next generation. Over many generations, the helpful alleles become more common in the population. This is natural selection 自然选择.

Environmental conditions can push selection in three ways:

• stabilising selection 稳定选择 favours the average and removes the extremes, so the mean stays the same and the range of variation narrows.
• directional selection 定向选择 favours one extreme, so the mean shifts that way.
• disruptive selection 分裂选择 favours both extremes and removes the average.

Allele frequencies can also change in other ways:

• the founder effect 奠基者效应 — a few individuals start a new population, so they carry only some of the alleles of the original group.
• genetic drift 遗传漂变 — in a small population, allele frequencies change by chance from generation to generation.
• the bottleneck effect 瓶颈效应 — a sudden fall in population size leaves few survivors, reducing the variety of alleles.

Antibiotic resistance as natural selection

A chance mutation 突变 makes a few bacteria resistant to an antibiotic 抗生素. When the antibiotic is used, the non-resistant bacteria die, but the resistant ones survive and reproduce. Over time the resistance 耐药性 spreads through the population. This is natural selection in action.

The Hardy–Weinberg principle 哈迪-温伯格原理 lets you calculate the allele frequencies 等位基因频率 and genotype 基因型 frequencies in a population. It only holds true when there is a large population, mating is random, and there is no mutation, no migration and no natural selection.

Call the frequency of the dominant allele $p$ and the frequency of the recessive allele $q$. There are only two alleles, so:

The genotype frequencies then add up to 1, where $p^2$ is homozygous dominant, $2pq$ is heterozygous (the carriers), and $q^2$ is homozygous recessive:

Worked example. A recessive condition affects $1$ in every $100$ people. Find the frequency of carriers.

Only the homozygous recessive genotype ($q^2$) shows the condition, so $q^2 = \tfrac{1}{100} = 0.01$, giving $q = \sqrt{0.01} = 0.1$. Then $p = 1 - q = 0.9$. The carriers are the heterozygotes:

So about $18\%$ of the population are carriers — far more than the $1\%$ who show the condition.

In selective breeding 选择育种, also called artificial selection 人工选择, humans (not nature) choose which organisms breed, so that useful features are passed on. Examples:

• breeding disease resistance 抗病性 into varieties of wheat and rice.
• using inbreeding 近交 and hybridisation 杂交 (crossing different lines) to make vigorous, uniform maize.
• breeding dairy cattle to improve their milk yield 产量.

Evolution 进化 is the slow formation of new species 物种 from earlier ones, as the gene pool 基因库 (all the alleles in a population) changes from generation to generation.

DNA sequence 序列 data can show how closely related two species are: the more similar their DNA sequences, the more recently they shared a common ancestor.

Speciation 物种形成 happens when two populations become genetically separated, so they can no longer breed together. This genetic isolation 隔离 can come about in two ways:

• allopatric speciation 异域物种形成 — the populations are kept apart by a geographical separation 地理隔离, such as a sea or a mountain range.
• sympatric speciation 同域物种形成 — the populations live in the same area but are separated by differences in behaviour or way of life.

Common mistakes.

• Using the frequency of the dominant phenotype as $p^2$. People with the dominant phenotype are $p^2 + 2pq$; only the recessive phenotype gives one genotype directly, so always start from $q^2$.
• Forgetting the square root. If $q^2 = 0.01$ then $q = 0.1$, not $0.01$, and the carrier frequency is $2pq$, not $pq$.
• Saying a mutation happens because an organism needs it. Mutations are random; the selection pressure only decides which existing alleles become more common.
• Saying individuals evolve. A single organism cannot change its alleles; it is the allele frequency in the population that changes over generations.
• Mixing up genetic drift and natural selection. Drift is change by chance, strongest in small populations; selection is change because some alleles give a better chance of survival.
• Saying speciation has happened just because two groups look different. A new species forms only when the populations are reproductively isolated and can no longer breed to give fertile offspring.

• Explain natural selection as a sequence: variation → selection pressure → the better-adapted survive and reproduce → allele frequency changes.
• Use the Hardy–Weinberg equations ($p+q=1$, $p^2+2pq+q^2=1$); $q^2$ is the recessive-phenotype frequency — a common calculation.
• Distinguish stabilising, directional and disruptive selection with an example of each.
• Distinguish allopatric vs sympatric speciation: both need reproductive isolation, caused by geographical separation (allopatric) or by ecological or behavioural differences within one area (sympatric).