Selection and Evolution

Evolution is a change in the allele frequency (the frequency of alleles) within a population over time / over many generations.

The only source of completely new alleles is mutation — a random change in the base sequence of DNA. (Meiosis and random fertilisation produce new combinations of existing alleles, but do not create new alleles.)

Genetic variation is generated by:

• Mutation — a random change in the DNA base sequence, producing new alleles.
• Meiosis — crossing over (exchange of alleles between homologous chromosomes) and independent assortment of chromosomes create new combinations of alleles in gametes.
• Random fertilisation — any genetically different gamete can fuse with any other, combining alleles in new ways.

This variation is important because natural selection can only act if individuals differ in their phenotypes: without genetic variation, no individuals would be better adapted than others, so allele frequencies could not change and evolution could not occur.

Directional selection occurs when a new or changed selection pressure favours one extreme of the range of a characteristic. Individuals with that extreme phenotype are more likely to survive and reproduce, so over generations the mean of the characteristic shifts towards that extreme (the allele frequency for that phenotype increases).

A named example is antibiotic resistance in bacteria (or the increase in the dark/melanic form of the peppered moth during the Industrial Revolution): the antibiotic selects for resistant bacteria, so the resistance allele increases in frequency and the population becomes predominantly resistant.

There was genetic variation in the moth population: a chance mutation produced an allele for the dark (melanic) colour, so both pale and dark phenotypes were present before the bark changed.

Originally the pale form was camouflaged on the pale lichen-covered bark and the dark form was conspicuous, so birds (the selection pressure, acting by predation) ate more dark moths; the pale allele was therefore at high frequency.

When pollution blackened the bark (a changed environment), the selection pressure changed: now the dark form was camouflaged against the dark bark while the pale form was conspicuous and easily seen by birds.

As a result, more pale moths were eaten and fewer survived to reproduce, while more dark moths survived, reproduced and passed on the allele for dark colour to their offspring.

Over many generations the frequency of the dark (melanic) allele increased in the population, so the dark form became much more common. This is directional selection — a shift in the population towards one extreme phenotype.

This is an example of evolution, defined as a change in allele frequency over time, driven by a change in the environment.

The Hardy–Weinberg principle states that, in an idealised population, the allele frequencies and genotype frequencies remain constant from one generation to the next — that is, the population is not evolving. The relationships are described by the equations $p + q = 1$ and $p^2 + 2pq + q^2 = 1$.

For a population to stay in Hardy–Weinberg equilibrium, the following conditions must be met:

• the population is very large (so chance/genetic drift has a negligible effect);
• mating is random;
• there is no selection (all genotypes are equally likely to survive and reproduce);
• there is no migration (no immigration or emigration of alleles into or out of the population);
• there is no (net) mutation producing new alleles.

If any of these conditions is broken, allele frequencies will change and the population will evolve.

Let $p$ = frequency of the normal (dominant) allele and $q$ = frequency of the cystic fibrosis (recessive) allele. Affected babies are homozygous recessive, so their frequency = $q^2$.

Step 1 — find $q^2$: affected frequency = 1 in 1600, so

$q^2 = \frac{1}{1600} = 0.000625$

Step 2 — find $q$: take the square root:

$q = \sqrt{0.000625} = 0.025$

Step 3 — find $p$: using $p + q = 1$,

$p = 1 - 0.025 = 0.975$

Step 4 — find the carrier frequency, $2pq$:

$2pq = 2 \times 0.975 \times 0.025 = 0.04875$

So the frequency of carriers is about 0.049, i.e. 4.9% of the population (roughly 1 in 20). Notice that carriers are far more common than affected individuals (1 in 1600), because a heterozygote needs only one copy of the recessive allele.

Speciation begins when two parts of a population become reproductively isolated, so that they can no longer interbreed and gene flow between them stops.

Each isolated population contains genetic variation and acquires its own new mutations. The two populations are exposed to different selection pressures (or experience different genetic drift), so natural selection acts differently on each: different alleles are advantageous in each group.

Over many generations the allele frequencies of the two populations diverge, making them increasingly genetically and phenotypically different. Eventually the populations become so different that, even if brought back together, they can no longer interbreed to produce fertile offspring — by definition they are now two separate species.

The two types differ in how the isolation arises:

• Allopatric speciation ("different homeland") — the populations are separated by a geographical barrier such as a sea, river, mountain range or new island (e.g. Darwin's finches on separate Galápagos islands).
• Sympatric speciation ("same homeland") — the populations live in the same area but are isolated by behavioural, temporal or reproductive mechanisms, such as breeding at different times of year, different courtship behaviours, or a chromosome change (polyploidy) in plants.

(a) Describe the change. The percentage of resistant bacteria increased over the 20 generations of antibiotic treatment, rising from 2% to 96% — an increase of 94 percentage points. The frequency of the resistance allele therefore rose steeply as treatment continued.

(b) Explain the advice.

Resistance arises by random mutation, and the antibiotic acts as a selection pressure that kills non-resistant bacteria while resistant bacteria survive and reproduce, passing on the resistance allele.

Completing the full course ensures that the antibiotic is taken for long enough to kill all the bacteria, including any that are only partially resistant or slower-growing. If the course is stopped early, these less-susceptible bacteria survive and reproduce, so the surviving population is enriched for resistance alleles, accelerating the spread of resistance.

Avoiding overuse (for example, not using antibiotics for viral infections, and limiting their use in agriculture) reduces the frequency and intensity of the selection pressure. With less selection favouring resistant bacteria, the resistance allele frequency rises more slowly in the population, slowing the emergence of resistant strains such as MRSA.

White-flowered plants are homozygous recessive (rr), so their frequency = $q^2$.

Step 1 — frequency of white plants ($q^2$):

$q^2 = \frac{320}{5000} = 0.064$

Step 2 — recessive allele frequency ($q$):

$q = \sqrt{0.064} = 0.253\;(\text{3 s.f.})$

Step 3 — dominant allele frequency ($p$):

$p = 1 - q = 1 - 0.253 = 0.747$

So the frequency of the R allele is about 0.75.

Step 4 — expected heterozygotes ($2pq$):

$2pq = 2 \times 0.747 \times 0.253 = 0.378$

Number of heterozygous plants $= 0.378 \times 5000 \approx \mathbf{1890}$ plants.

Using Hardy–Weinberg to detect selection. Hardy–Weinberg predicts the genotype frequencies expected if the population is NOT evolving (no selection, random mating, large population, no migration, no mutation). The biologist could count the actual numbers of each genotype (e.g. by test crosses or molecular markers) over successive generations and compare the observed genotype/allele frequencies with those predicted by $p^2 + 2pq + q^2 = 1$. If the observed frequencies differ significantly from the predicted values (and other conditions such as random mating and no migration are met), then a Hardy–Weinberg condition is being broken — natural selection is acting and the population is evolving. A constant match across generations indicates equilibrium (no selection).

The statement contains two key errors. First, it implies that the insects deliberately adapt in order to survive — that the pesticide causes the resistance. In fact, resistance is not produced in response to the pesticide. Second, it implies the change happens within an individual's lifetime; in reality natural selection acts on variation already present in the population.

A scientifically accurate account is as follows.

There is genetic variation in the insect population. By random mutation, an allele for pesticide resistance arises in one or a few individuals before the pesticide is ever applied. The mutation is a chance event, not a response to need.

When the pesticide is applied (the selection pressure), the non-resistant insects are killed, but the resistant insects survive because they are better adapted to the new environment.

The surviving resistant insects reproduce and pass on the resistance allele to their offspring.

Over successive generations this is repeated, so the frequency of the resistance allele increases in the population until most insects are resistant. This is directional selection and is an example of evolution (a change in allele frequency over time).

The crucial correction is that the variation arises randomly and beforehand; the pesticide merely selects which individuals survive — it does not create the resistance.