Natural and artificial selection

1. Overview

Populations contain inherited variation caused by mutation, meiosis and random fertilisation. Because populations can produce far more offspring than the environment can support, individuals compete in a struggle for existence. Those whose alleles make them best adapted are most likely to survive and reproduce, so over generations the frequencies of advantageous alleles increase — this is natural selection. Selection can act in three ways (stabilising, directional and disruptive), and allele frequencies can also change by chance through genetic drift, the founder effect and the bottleneck effect. Humans apply the same logic deliberately in selective breeding (artificial selection) to improve crops and livestock. The Hardy-Weinberg principle provides a way to calculate allele and genotype frequencies in a population.

Key Definitions

• Natural selection: the process by which individuals with alleles that make them better adapted to their environment are more likely to survive and reproduce, passing those alleles on to the next generation.
• Struggle for existence: the competition between individuals for limited resources that arises because populations produce more offspring than the environment can support.
• Stabilising selection: selection that favours the intermediate phenotypes and acts against both extremes, reducing variation around the mean.
• Directional selection: selection that favours one extreme phenotype, shifting the mean of the population in that direction over time.
• Disruptive selection: selection that favours both extreme phenotypes against the intermediate, increasing variation and producing two distinct groups.
• Genetic drift: the random change in allele frequencies from one generation to the next, which has the greatest effect in small populations.
• Founder effect: the loss of genetic variation when a new population is started by a small number of individuals carrying only a fraction of the original population's alleles.
• Bottleneck effect: a sharp reduction in population size that removes many alleles by chance, leaving a less varied population even after numbers recover.
• Selective breeding: the deliberate choice by humans of individuals with desirable phenotypes to act as parents of the next generation, repeated over many generations.

Content

Overproduction and the struggle for existence

Living things have the capacity to overproduce: a pair of organisms can produce many more offspring than are needed to replace them. Resources such as food, water, space, light and mineral ions are limited, so not all of these offspring can survive. The resulting competition is the struggle for existence.

Within any population there is inherited variation, and some individuals happen to carry alleles that make them better adapted to the current environment. These individuals are more likely to survive long enough to reproduce and pass their alleles to the next generation. Because the better-adapted individuals leave more offspring, the frequency of the advantageous alleles increases over successive generations.

Key point: The mutations that produce variation are random. The environment does not create the variation it favours — it only acts as the selection pressure that decides which existing alleles give a survival advantage.

Three types of natural selection

Environmental factors act as selection pressures that can change the distribution of phenotypes in three distinct ways.

Type	Phenotype favoured	Effect on variation	Typical situation
Stabilising	the intermediate	reduced (narrows around the mean)	a stable, unchanging environment
Directional	one extreme	mean shifts towards that extreme	the environment changes or a new pressure appears
Disruptive	both extremes	increased (can split into two groups)	two different conditions favour two phenotypes

• Stabilising selection is the most common form. It selects against both extremes, so the population stays well matched to constant conditions. A classic illustration is human birth mass: very low mass is selected against because the infant is underdeveloped, while very high mass is selected against because it makes birth difficult and risky, so the intermediate mass is favoured.
• Directional selection shifts the population's mean towards one extreme. As the graph below shows, the original distribution (dashed) is gradually replaced by a new distribution (solid) whose mean has moved along the phenotype axis. This happens, for example, when an allele giving resistance to a pesticide spreads through a population once the pesticide is introduced.

• Disruptive selection favours both extremes against the intermediate, increasing variation and potentially splitting a population into two distinct groups, for example when two separate food sources favour two different beak sizes but penalise an intermediate size.

Genetic drift, the founder effect and the bottleneck effect

Allele frequencies do not change only because of selection; they can also change purely by chance, especially in small populations — this is genetic drift. In a small population, which individuals survive and reproduce is strongly influenced by random events, so some alleles may increase in frequency or even be lost while others become fixed, regardless of whether they are advantageous.

The founder effect occurs when a new population is established by a small number of individuals (for example a few organisms colonising an island). These founders carry only a small, non-representative sample of the alleles present in the original population, so the new population starts with low genetic variation — meaning a small number of different alleles at each gene locus, not simply alleles at low frequency.

The bottleneck effect occurs when a population is drastically reduced in size by an event such as disease, hunting or a natural disaster. Many alleles are lost by chance because the few survivors carry only some of the original alleles. Even when the population later grows back to its former size, its genetic variation remains low, leaving it less able to respond to future environmental change.

Antibiotic resistance as natural selection

The evolution of antibiotic-resistant bacteria is a clear, fast example of natural selection.

1. Within a large bacterial population there is genetic variation: a random mutation (or an allele acquired on a plasmid) gives a few cells resistance to an antibiotic. Crucially, this mutation arises before the antibiotic is present — the antibiotic does not cause it.
2. When the antibiotic is applied it acts as a selection pressure, killing the non-resistant cells.
3. The resistant cells survive, reproduce by binary fission, and pass on the resistance allele, so the proportion of resistant cells increases in the next generation.
4. Resistance can spread rapidly between bacteria — even between different species — because the resistance gene is often carried on a plasmid, which can be transferred from one cell to another. Overuse and incomplete courses of antibiotics increase the selection pressure and speed up this process.

The Hardy-Weinberg principle

The Hardy-Weinberg principle predicts the frequencies of alleles and genotypes in a population and shows that, in the absence of disturbing factors, these frequencies stay constant from generation to generation. For a gene with two alleles, the dominant allele frequency is $p$ and the recessive allele frequency is $q$.

The two equations (provided to you in the exam) are: $$p + q = 1$$ $$p^2 + 2pq + q^2 = 1$$

Here $p^2$ is the frequency of homozygous dominant individuals, $2pq$ the frequency of heterozygous individuals, and $q^2$ the frequency of homozygous recessive individuals. The principle only holds when the following conditions are met:

• the population is large;
• mating is random;
• there is no selection (all genotypes are equally fertile and survive equally well);
• there is no mutation producing new alleles;
• there is no migration (no movement of alleles into or out of the population by immigration or emigration).

Real populations rarely meet every condition, so if the observed genotype frequencies differ markedly from the expected $p^2 : 2pq : q^2$, that departure is evidence that an evolutionary force (selection, genetic drift or migration) is acting on the population.

Worked example

Exam-style question: In a large, randomly mating population of beetles, body colour is controlled by a single gene with two alleles. The dark allele is dominant to the pale allele. Pale beetles are homozygous recessive and make up 16% of the population. Assuming the population is in Hardy-Weinberg equilibrium, calculate the percentage of beetles that are heterozygous, and the percentage that are homozygous dominant. [3]

Model answer:

• The pale (homozygous recessive) frequency is $q^2 = 0.16$, so $q = \sqrt{0.16} = 0.4$.
• Using $p + q = 1$: $p = 1 - 0.4 = 0.6$.
• Heterozygous frequency $= 2pq = 2 \times 0.6 \times 0.4 = 0.48 = \mathbf{48\%}$; homozygous dominant frequency $= p^2 = 0.6^2 = 0.36 = \mathbf{36\%}$.

Worked example

Exam-style question: A genetic condition is caused by a recessive allele. In a large, randomly mating population of 5000 people, 80 individuals are affected (homozygous recessive). Assuming Hardy-Weinberg equilibrium, calculate the frequencies of the two alleles and estimate the number of people who are carriers (heterozygous). [4]

Model answer:

• Frequency of affected individuals $= q^2 = \dfrac{80}{5000} = 0.016$.
• Recessive allele frequency $q = \sqrt{0.016} = 0.1265$ (keep the full value in the calculator — do not round yet).
• Dominant allele frequency $p = 1 - q = 1 - 0.1265 = 0.8735$.
• Carrier frequency $= 2pq = 2 \times 0.8735 \times 0.1265 = 0.2210$.
• Number of carriers $= 0.2210 \times 5000 \approx \mathbf{1105\ people}$.

Each line earns an independent method mark: 1 for $q^2 = 0.016$, 1 for $q = 0.126$, 1 for $2pq = 0.221$, and 1 for $\times 5000 \approx 1105$ carriers. Write every step, because a later slip still leaves the earlier marks intact.

Principles of selective breeding (artificial selection)

In selective breeding humans, rather than the environment, decide which individuals reproduce. The general procedure is:

1. Identify the desirable characteristic(s) (e.g. high yield, disease resistance).
2. Select the parents showing those characteristics most strongly from the existing variation.
3. Breed these chosen parents together.
4. Select the best offspring from the next generation and breed them.
5. Repeat over many generations so that the alleles for the desirable features steadily increase in frequency.

This deliberate choice steadily shifts the population, much like directional selection, but the selection pressure is applied by people. A drawback is that repeated breeding from a small number of "best" individuals reduces genetic variation and can cause inbreeding depression, where harmful recessive alleles become more likely to appear in the homozygous state.

Examples of selective breeding

• Disease resistance in wheat and rice: varieties that carry alleles for resistance to fungal or other diseases are bred with high-yielding varieties. Offspring combining resistance and high yield are then selected and bred over several generations, producing crops that lose less of their harvest to disease.
• Inbreeding and hybridisation in maize: continued inbreeding produces lines that are uniform (homozygous for many genes) but often weaker. Crossing two different inbred lines then produces a hybrid offspring that is both uniform and vigorous. This increased vigour of the hybrid — its faster growth and higher yield compared with the inbred parents — is called hybrid vigour.
• Milk yield in dairy cattle: bulls whose female relatives produce high milk yields are selected, and their sperm is used to inseminate many cows by artificial insemination (AI). Because one outstanding bull can father very large numbers of calves, the alleles for high milk yield spread quickly through the herd and the average milk yield rises over successive generations.

Key Equations

The Hardy-Weinberg principle (provided in the exam): for a gene with a dominant allele frequency $p$ and recessive allele frequency $q$, $$p + q = 1$$ $$p^2 + 2pq + q^2 = 1$$ where $p^2$ = frequency of homozygous dominant, $2pq$ = frequency of heterozygous, and $q^2$ = frequency of homozygous recessive individuals.

Common Mistakes to Avoid

• Saying the environment "causes" the helpful mutations. Mutations occur randomly and independently of need. The selection pressure does not create new alleles — it only determines which of the existing alleles give a survival advantage.
• Calling two separated populations "different species" too soon. When a population is split, the two groups remain populations of the same species until they become reproductively isolated and can no longer interbreed to produce fertile offspring. Only then can you describe them as different species.
• Defining low genetic variation as an allele simply being "rare". Low genetic variation means there is a small number of different alleles at a gene locus in the population, not just one allele present at a low frequency.
• Confusing the three types of selection. Stabilising selection favours the intermediate and narrows variation; directional selection favours one extreme and shifts the mean; disruptive selection favours both extremes and can split the population.
• Treating antibiotic resistance as something the antibiotic "creates". The resistance allele is already present (by mutation or on a plasmid) before the antibiotic is used; the antibiotic merely selects for the cells that already carry it.
• Rounding part-way through Hardy-Weinberg calculations. Keep full values (e.g. $q$, $p$) in your calculator throughout and only round the final answer, otherwise small errors build up and the genotype frequencies will not add to 1.
• Forgetting the conditions for Hardy-Weinberg. The principle only applies to a large, randomly mating population with no selection, no mutation and no migration; do not assume it holds without stating these.
• Confusing hybrid vigour with inbreeding. Inbreeding produces uniform but often weaker lines; crossing two inbred lines then yields vigorous, uniform hybrids — vigour comes from the cross, not from the inbreeding itself.

Exam Tips

• Build natural-selection answers as a clear sequence: variation (from random mutation) → overproduction and competition → better-adapted individuals survive and reproduce → advantageous alleles increase in frequency. Each link in the chain needs to be stated.
• Always use the word allele, not "gene", when describing changes in frequency, and refer to populations of the same species rather than "new species" unless reproductive isolation has been established.
• In Hardy-Weinberg problems, start from the genotype you are given (usually the homozygous recessive, $q^2$), find $q$ first, then $p$, and finally the genotype frequencies; show every line of working so method marks are secured.
• To test whether a population is in equilibrium, work out the allele frequencies a second way: count the alleles directly from the genotype numbers (each homozygote contributes two of the same allele, each heterozygote contributes one of each), then compare the expected $p^2$, $2pq$ and $q^2$ counts with the observed counts. A clear mismatch means the population is not at equilibrium.
• Quote final percentages and frequencies clearly, and check your three genotype frequencies add up to 1 (or 100%) as a quick self-check.
• For selective breeding, make it explicit that humans apply the selection and that the process is repeated over many generations; name the technique (e.g. artificial insemination) when the example calls for it.
• When asked to compare natural and artificial selection, write a single comparative point — both increase the frequency of advantageous alleles, but the selecting agent differs (the environment versus humans).