Selection and evolution

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.

Banded snails (Cepaea nemoralis) show striking variation in shell colour and banding

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.

Discontinuous variation falls into separate groups; continuous variation is a smooth range

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.

The peppered moth shows natural selection: the dark form hides on dark, sooty bark, while the pale speckled form hides on pale lichen — birds eat whichever stands out

Environmental conditions can push selection in three ways:

• stabilising selection 稳定选择 favours the average and removes the extremes (the population stays the same).
• 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.

Stabilising narrows the range, directional shifts the mean, disruptive splits it in two

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.

A bottleneck: only a few survive a disaster, so the recovered population has less genetic variety

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.

Selective breeding: choosing only the best to breed each generation makes the wanted feature more and more common

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.

Allopatric speciation needs a physical barrier; sympatric speciation happens in the same area

• 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 (geographic vs reproductive isolation).