D4.1 Natural selection | IB DP Biology

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Natural selection is the mechanism that drives evolution, and its logic fits in a few lines: individuals vary, much of the variation is inherited, more offspring are produced than can survive, and those best suited to their environment leave more offspring. The difficulty is in writing it precisely — without implying that organisms change because they need to. At HL the topic becomes quantitative, measuring evolution as changing allele frequencies with the Hardy–Weinberg equation.

🎯What you need to be able to do

Explain natural selection as the mechanism of evolution, and the paradigm shift from Lamarckism to Darwinism.
Explain the roles of mutation and sexual reproduction in generating variation.
Explain how overproduction of offspring and competition for resources promote natural selection.
Explain abiotic factors as selection pressures, and differences in adaptation, survival and reproduction, including fitness.
Explain why traits must be heritable for evolution to occur.
Explain sexual selection, and interpret data from Endler’s experiments with guppies.
AHL Explain the gene pool, allele frequencies in isolated populations, and change in allele frequency by natural selection.
AHL Distinguish directional, disruptive and stabilizing selection.
AHL Calculate allele and genotype frequencies with the Hardy–Weinberg equation, and state its conditions; distinguish artificial from natural selection.

📚The biology

Natural selection drives evolution

Natural selection is the mechanism that causes evolutionary change. It operates continuously, in every population, and has done so for billions of years. Acting over that timescale, together with speciation, it has produced the whole biodiversity of life on Earth (A4.1).

In Darwin’s time it was already widely accepted that species change over time, but there was no convincing mechanism. The leading idea, Lamarckism, proposed that organisms change in response to need and pass on acquired characteristics. Darwin’s theory of natural selection provided a mechanism that fitted the evidence far better, and it replaced Lamarckism. This was a paradigm shift: a fundamental change in the framework of ideas within which a science operates, changing what questions are asked and how evidence is interpreted.

Sources of variation

Natural selection can only act if individuals differ. Two processes produce heritable variation:

Mutation
Generates new alleles. It is the original source of all genetic variation (D1.3).

Sexual reproduction
Generates new combinations of alleles, through crossing over and random orientation in meiosis, and random fertilization (D2.1).

Overproduction and competition

Organisms produce more offspring than the environment can support: a cod lays millions of eggs, an oak drops thousands of acorns. Resources are limited — food, water, space, light, nesting sites, mates — so the environment has a carrying capacity (C4.1). There is therefore a struggle for existence: individuals compete for resources, and most offspring die before reproducing. Which ones survive is not purely random, and that is what allows selection.

Abiotic selection pressures

A selection pressure is any factor that affects the survival or reproduction of individuals differently according to their traits. As well as competition, predators and disease, abiotic factors act as selection pressures. Density-independent factors such as unusually high or low temperatures, drought or floods may kill individuals regardless of population size — but not all individuals equally. In a severe cold spell, individuals with thicker insulation or better cold tolerance are more likely to survive.

Adaptation, survival and reproduction

Individuals in a population differ in how well they are adapted to their environment. Those with traits better suited to the conditions are more likely to survive and to reproduce, leaving more offspring, which inherit those traits. This happens most intensely through intraspecific competition, because members of the same species need exactly the same resources.

Fitness is the relative ability of an individual (or a genotype) to survive and pass its alleles on to the next generation: its survival value and reproductive potential combined. An individual that lives a long time but produces no offspring has a fitness of zero. Over generations, the alleles of fitter genotypes increase in frequency.

Traits must be heritable

Only heritable traits can evolve. Characteristics acquired during an individual’s life because of the environment — muscles built by exercise, a scar, a language learned — are not encoded in the base sequence of genes, so they are not passed on, however much they helped the individual survive. Natural selection changes a population only when the traits it favours have a genetic basis.

Sexual selection

Sexual selection is selection for traits that increase success in attracting or winning a mate. Differences in physical traits (bright colours, large antlers, long tails) and behavioural traits (songs, displays, dances) can act as signs of overall fitness: a male that can grow elaborate plumage and still survive is probably healthy, well fed and free of parasites. Females that choose such males leave fitter offspring. Over many generations, sexual selection drives the evolution of these traits, sometimes to extremes.

The birds of paradise of New Guinea are a striking example. Males have evolved spectacular plumes, colours and elaborate courtship dances, even though these make them more conspicuous to predators and cost energy to produce. Females are dull. Female choice has driven the evolution of male plumage because the males with the most impressive displays mate most often.

Endler’s guppies

John Endler studied guppies, small freshwater fish in streams in Trinidad. Males have coloured spots; in some streams they are brightly coloured, in others dull. Endler observed that bright males were found where predators were few, and dull males where dangerous predators (such as the pike cichlid) were common. Two selection pressures act in opposite directions:

sexual selection — females prefer brighter males, favouring more and brighter spots;
natural selection by predation — bright males are more easily seen and eaten, favouring fewer and duller spots.

He tested this experimentally, controlling the selection pressure. Guppies from one population were placed in large artificial ponds with gravel beds, some with no predators, some with a weak predator and some with a strong predator. Within roughly ten generations, males in ponds without dangerous predators had more, and more colourful, spots, while males in ponds with the strong predator had fewer, duller spots. He then moved guppies from a high-predation stream to a stream without dangerous predators, and within a couple of years the males had become brighter. The results showed that the balance between sexual selection and predation determines male colour, and that evolution by selection can be rapid.

The gene pool AHL

A gene pool consists of all the genes and their different alleles present in a population. Evolution can be measured as a change in the allele frequency — the proportion of all copies of a gene in the gene pool that are a particular allele.

Allele frequencies in isolated populations AHL

Populations that are geographically isolated from one another have separate gene pools, and their allele frequencies often differ, because of different selection pressures, chance, and the alleles their founders happened to carry. Databases of human allele frequencies show this clearly. For example:

the allele allowing lactase persistence (digesting milk in adulthood) is at high frequency in populations with a long history of dairy farming, such as those of northern Europe, and at low frequency in most East Asian populations;
the sickle cell allele is most frequent in regions where malaria has long been common, because heterozygotes are more resistant to malaria.

Allele frequency and natural selection AHL

When individuals differ in their heritable traits, those with alleles that increase fitness survive and reproduce more, so those alleles become more frequent in the next generation’s gene pool, while alleles that reduce fitness become less frequent. Evolution by natural selection, in modern terms, is this change in allele frequency over generations. Darwin developed the theory without knowing how inheritance worked; later biologists integrated Mendelian genetics with natural selection in what is known as neo-Darwinism (the modern synthesis).

Types of selection AHL

Directional selection
Favours individuals at one extreme of the range of variation. The mean shifts in that direction. Example: antibiotic resistance in bacteria; longer necks in giraffes.

Disruptive selection
Favours both extremes and selects against intermediates. The population may split into two groups. Example: seed-cracking finches where only very small or very large seeds are available, favouring small and large beaks.

Stabilizing selection
Favours intermediate phenotypes and selects against both extremes. The mean stays the same but the variation narrows. Example: human birth mass, where very small and very large babies have lower survival.

All three types result in a change in allele frequency: even stabilizing selection reduces the frequency of alleles contributing to extreme phenotypes.

The Hardy–Weinberg equation AHL

For a gene with two alleles, let \( p \) be the frequency of the dominant allele and \( q \) the frequency of the recessive allele. Since these are the only alleles:

\[ p + q = 1 \]

If mating is random, the genotype frequencies in the next generation are predicted by:

\[ p^{2} + 2pq + q^{2} = 1 \]

where \( p^{2} \) is the frequency of homozygous dominant individuals, \( 2pq \) of heterozygotes, and \( q^{2} \) of homozygous recessive individuals. If one genotype frequency is known — usually \( q^{2} \), since homozygous recessives can be recognized by their phenotype — the allele frequencies can be calculated: \( q = \sqrt{q^{2}} \), then \( p = 1 - q \).

Hardy–Weinberg conditions AHL

A population is in genetic equilibrium — its allele frequencies do not change — only if all these conditions hold:

random mating (no mate choice based on genotype);
no natural selection (all genotypes survive and reproduce equally);
no mutation creating new alleles;
no migration into or out of the population (no gene flow);
a very large population, so chance changes in allele frequency (genetic drift) are negligible.

If observed genotype frequencies do not fit the Hardy–Weinberg predictions, one or more conditions is not being met — for example mating is non-random, or survival rates differ between genotypes. The equation is therefore used as a baseline to detect evolution.

Artificial selection AHL

Artificial selection is the deliberate choice by humans of individuals with desirable traits for breeding, in crop plants and domesticated animals — for example high milk yield in cattle, or large grains in wheat. It changes allele frequencies just as natural selection does, but the selection pressure is human choice.

Unintended consequences of human actions are not artificial selection. When antibiotics are used and resistant bacteria increase, no one has chosen to breed resistant bacteria: the antibiotic is simply an environmental factor that kills susceptible individuals. The evolution of resistance is therefore natural selection.

✏️Worked example

AHL In a population of 5000 plants, flower colour is controlled by one gene with two alleles: purple (R) is dominant to white (r). 200 plants have white flowers. Assume the population is in Hardy–Weinberg equilibrium.
(a) Calculate the frequencies of alleles R and r.
(b) Calculate the expected number of heterozygous plants and homozygous purple plants.
(c) A survey of a different population of 1000 plants found 600 RR, 200 Rr and 200 rr. Determine whether it is in Hardy–Weinberg equilibrium, and suggest a reason for any difference.

(a) White plants are homozygous recessive, so

\[ q^{2} = \frac{200}{5000} = 0.04 \qquad q = \sqrt{0.04} = 0.2 \qquad p = 1 - 0.2 = 0.8 \]

Frequency of R = 0.8; frequency of r = 0.2.

(b)

\[ 2pq = 2 \times 0.8 \times 0.2 = 0.32 \quad \Rightarrow \quad 0.32 \times 5000 = 1600\ \text{heterozygous plants} \]

\[ p^{2} = 0.8^{2} = 0.64 \quad \Rightarrow \quad 0.64 \times 5000 = 3200\ \text{homozygous purple plants} \]

(c) First find the allele frequency directly from the genotype counts. There are 2000 alleles in total; R alleles = 2 × 600 + 200 = 1400.

\[ p = \frac{1400}{2000} = 0.7 \qquad q = 0.3 \]

Expected numbers if in equilibrium: RR = 0.49 × 1000 = 490; Rr = 0.42 × 1000 = 420; rr = 0.09 × 1000 = 90. The observed numbers (600, 200, 200) have far fewer heterozygotes and more of both homozygotes than expected, so the population is not in equilibrium. A likely reason is non-random mating: for example if plants mostly self-pollinate, or if pollinators visit plants of the same colour, heterozygotes become less common. Selection against heterozygotes, or immigration from populations with different frequencies, could also cause it.

Check it. In (b), the three genotype numbers must add up to the population: 3200 + 1600 + 200 = 5000. In (c), p + q = 1 and the expected numbers must also sum to 1000. The three expected frequencies, 0.49 + 0.42 + 0.09, sum to 1.00.

Taking \( q \) as 0.04 instead of \( q^{2} \). The proportion of individuals showing the recessive phenotype is \( q^{2} \), the genotype frequency, not \( q \), the allele frequency. Take the square root first. Using 0.04 as \( q \) gives p = 0.96 and far too few heterozygotes. And in (c), never calculate \( q \) from the rr count when you have been given all three genotypes and are testing for equilibrium: that assumes the very equilibrium you are testing.

📝Practise

Work through these on paper, then reveal the answer. Questions 5 and 6 are AHL.

1. Explain how natural selection could lead to an increase in the proportion of rabbits with thick fur in a population during a series of cold winters.

There is variation in fur thickness among the rabbits, and some of it is heritable, arising from mutation and sexual reproduction. More rabbits are born than can survive, and cold winters act as a selection pressure. Rabbits with thicker fur lose less heat, so they are more likely to survive the cold and to reproduce, while those with thin fur are more likely to die. Survivors pass the alleles for thick fur to their offspring. Over several generations, the frequency of alleles for thick fur increases, so a greater proportion of the population has thick fur.

2. Explain why the ability of a bodybuilder to lift heavy weights will not be inherited by their children, and why this matters for evolution.

The bodybuilder’s strength is largely an acquired characteristic, developed during their life by training. It does not change the base sequence of their genes, and in particular not of the DNA in their gametes, so it is not heritable. Evolution by natural selection requires that favourable traits be heritable, so that they are passed to offspring and increase in frequency in the population. Acquired characteristics, however beneficial, cannot contribute to evolution (the error of Lamarckism).

3. Define fitness, and explain why a long-lived individual that never reproduces has a fitness of zero.

Fitness is the relative ability of an individual or genotype to survive and reproduce, passing its alleles to the next generation. It combines survival value and reproductive potential. An individual that never reproduces passes none of its alleles on, however long it lives, so it contributes nothing to the next generation’s gene pool — its fitness is zero. Survival matters only insofar as it leads to reproduction.

4. In Endler’s experiments, male guppies in ponds without predators became more colourful over many generations. Explain this result.

Male colour is heritable and variable. Two selection pressures act on it: sexual selection, because females prefer brighter males, and natural selection by predation, because bright males are more easily seen and eaten. In ponds without predators, predation no longer selects against bright colours, so sexual selection dominates: brighter males mate more often and father more offspring, so alleles for more and brighter spots increase in frequency over the generations.

5. AHL In a human population, 1 in 2500 people has cystic fibrosis, a recessive condition. Calculate the percentage of the population who are carriers.

Affected people are homozygous recessive: \( q^{2} = \dfrac{1}{2500} = 0.0004 \), so \( q = \sqrt{0.0004} = 0.02 \) and \( p = 1 - 0.02 = 0.98 \). Carriers are heterozygous: \( 2pq = 2 \times 0.98 \times 0.02 = 0.0392 \), which is 3.9% of the population (about 1 in 25).

6. AHL Distinguish between directional, disruptive and stabilizing selection, and explain why all three are forms of evolution.

Directional selection favours individuals at one extreme of the range, so the mean phenotype shifts in that direction. Disruptive selection favours both extremes and selects against intermediates, so the population may split into two phenotypic groups. Stabilizing selection favours intermediate phenotypes and selects against both extremes, so the mean is unchanged but variation decreases. All three change the allele frequencies in the gene pool (even stabilizing selection reduces the frequency of alleles contributing to extremes), and evolution is a change in the heritable characteristics, and allele frequencies, of a population.

🔗Go deeper — other people’s work

These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.

HHMI BioInteractive — the Natural Selection and Adaptation films and data activities, including Endler-style guppy data.
ALFRED (Allele Frequency Database) and gnomAD — real allele frequencies across human populations, for the database skill.
Understanding Evolution (UC Museum of Paleontology) — clear explanations of natural and sexual selection and common misconceptions.