Natural selection and its types – Variation and Selection (Senior 6 Biology) | Ganzaa

Variation and Selection

Lessons

Senior 6 · Biology

Variation and Selection

5 lessons

Lesson 3 of 5

Natural selection and its types

30 min

A drought on a small island

Daphne Major is a tiny volcanic island in the Galápagos. In 1977 almost no rain fell there. Plants produced few seeds, and the small, soft seeds were eaten first, leaving mainly large, hard ones. Peter and Rosemary Grant and their team had measured and ringed nearly every medium ground finch on the island. They found that most of the finches died, but birds with deeper beaks, able to crack the hard seeds, survived better. Because beak depth is largely inherited, the next generation had deeper beaks on average. Natural selection had been seen happening, measured in millimetres, in a single year. This lesson sets out the theory and the different ways selection can act on a population.

Definition

Natural selection: the process by which individuals with characteristics that suit their environment survive and reproduce more successfully, so that the alleles for those characteristics become more common in later generations.

Selection pressure: an environmental factor, such as a predator, disease, competition or climate, that affects which individuals survive and reproduce.

Fitness: an organism's ability to survive and pass on its alleles to the next generation, relative to others in the population.

Adaptation: an inherited feature that increases an organism's fitness in its environment.

Stabilising, directional and disruptive selection: selection that favours, respectively, the intermediate phenotypes, one extreme, or both extremes of a range.

Balanced polymorphism: two or more alleles kept in a population by selection, for example because heterozygotes have the highest fitness.

The theory of natural selection

Charles Darwin and Alfred Russel Wallace proposed the theory together in 1858. In modern terms it rests on a few observations and the conclusions drawn from them:

Overproduction. Organisms produce more offspring than the environment can support, so populations do not grow without limit.
Competition. There is therefore a struggle for existence: competition for food, space, light and mates, and exposure to predators and disease.
Variation. Individuals vary, and much of the variation is genetic (lesson 1).
Selection. Individuals whose variations suit the current selection pressures are more likely to survive and to reproduce.
Inheritance. They pass on the alleles responsible, so the frequency of advantageous alleles increases from generation to generation.

Selection acts on phenotypes, but what changes over the generations is the frequency of alleles in the population. Individuals do not evolve: populations do.

Three types of selection

Stabilising selection favours the average and acts against both extremes. The mean stays the same and the variation narrows. It is the commonest type when the environment is stable. Human birth mass is a classic example: very small and very large babies have historically had lower survival.
Directional selection favours one extreme, so the mean shifts in that direction over the generations. It happens when the environment changes or a new selection pressure appears: the deeper beaks of the Daphne Major finches after the drought; bacteria resistant to an antibiotic; and the insecticide resistance that has evolved in many mosquito populations exposed to the insecticides used on bed nets and indoor sprays.
Disruptive selection favours both extremes and acts against intermediates, which can split a population into two forms and may lead to new species (Unit 12). In the black-bellied seedcracker of West and Central Africa, studies in Cameroon found two bill sizes: large-billed birds crack the seeds of a hard-seeded sedge, small-billed birds feed efficiently on a soft-seeded one, and young birds with intermediate bills survived less well than either.

Worked example 1: how fast can resistance spread?

Illustrative values. A person's infection contains $1 \times 10^{9}$ bacteria, of which 100 carry a mutation that makes them resistant to an antibiotic. The antibiotic kills 99.9% of the susceptible bacteria and none of the resistant ones. (a) What fraction of the population is resistant before and after treatment? (b) If the resistant bacteria then divide every 30 minutes with no losses, how many are there after 10 hours?

(a) Before: $\frac{100}{1 \times 10^{9}} = 1 \times 10^{-7}$ , one in ten million.
After: susceptible survivors $= 1 \times 10^{9} \times 0.001 = 1 \times 10^{6}$ . Resistant fraction $= \frac{100}{1 \times 10^{6} + 100} \approx 1 \times 10^{-4}$ , one in ten thousand: a thousandfold increase in one treatment.
(b) 10 hours is 20 generations: $100 \times 2^{20} = 1.0 \times 10^{8}$ resistant bacteria (2 s.f.).

Interpretation. This is directional selection: the antibiotic is the selection pressure, and the resistance allele, which was there before the antibiotic was used, becomes common. Stopping a course of antibiotics early leaves the most resistant bacteria alive to multiply, which is why patients are told to finish the course as prescribed.

Selection that keeps two alleles: sickle-cell and malaria

The allele Hb^S for sickle-cell haemoglobin is harmful when homozygous: people with Hb^SHb^S have sickle-cell disease. Without other effects, selection would make the allele rarer and rarer. Yet in regions where falciparum malaria is, or was, common, Hb^S is found at much higher frequencies. The reason is that heterozygotes are protected:

Genotype	Effect where malaria is common	Relative fitness
Hb^AHb^A	Normal haemoglobin, but more likely to die of severe malaria	Reduced
Hb^AHb^S	Sickle-cell trait: usually healthy, and much less likely to develop severe malaria	Highest
Hb^SHb^S	Sickle-cell disease, with anaemia and painful crises	Greatly reduced

Because the heterozygote has the highest fitness, selection removes both alleles in homozygotes and keeps both in the population: a balanced polymorphism maintained by heterozygote advantage. Where malaria is absent, the advantage disappears and selection against Hb^SHb^S slowly lowers the allele's frequency. The distribution of the allele follows the history of malaria, not any other grouping of people.

Worked example 2: identifying the type of selection

Illustrative data, in the pattern of classic hospital studies before modern neonatal care. The table shows the percentage of babies in each birth-mass class who died in the first month.

Birth mass / kg	below 1.5	1.5 to 2.5	2.5 to 3.5	3.5 to 4.5	above 4.5
Deaths / %	50	15	2	1.5	6

(a) Which type of selection do the data show? (b) Predict its effect on the distribution of birth mass. (c) Suggest why the pattern is weaker today in places with good neonatal care.

(a) Mortality is lowest for intermediate masses (about 2.5 to 4.5 kg) and highest at both extremes: stabilising selection.
(b) The mean birth mass stays about the same and the variation (standard deviation) becomes smaller, because both tails are selected against.
(c) Incubators, feeding support and caesarean sections allow many very small or very large babies to survive, which reduces the selection pressure against the extremes.

Warning

Common mistakes

"The bacteria became resistant because they needed to." Resistance arises by random mutation, before the antibiotic is used; the antibiotic only selects it. Organisms do not change their genes in response to need.
"Individuals evolve." Individuals survive or die; it is the allele frequencies of the population that change.
"Survival of the fittest means the strongest." Fitness means reproductive success in a particular environment; a small, well-hidden or disease-resistant individual may be the fittest.
Confusing the graphs. Stabilising narrows the curve without moving it; directional moves it; disruptive splits it.

OtterBioTutor: "Natural Selection: Stabilizing, Directional, Disruptive" (Cambridge AS and A Level Biology). Compare the graphs in the video with Figure 1.

✦

Practice: natural selection

List the observations and conclusions on which the theory of natural selection is based.
Define selection pressure and give three examples.
Name the type of selection in each case: (a) birth mass in humans, (b) antibiotic resistance, (c) bill size in black-bellied seedcrackers.
Sketch and label graphs showing the effect of directional selection on a population.
Explain why the Daphne Major drought led to deeper beaks in the next generation, and state which condition must be true for this to happen.
In Worked example 1, how many resistant bacteria would there be after 8 hours?
Explain how heterozygote advantage maintains the Hb^S allele where malaria is common.
Mosquito populations that were sprayed with an insecticide for several years have become resistant. Explain this using the theory of natural selection, and suggest one way to slow it.

Show answersHide answers

Overproduction of offspring; competition (struggle for existence); variation, much of it inherited; those best suited survive and reproduce (selection); they pass on their alleles, so advantageous alleles increase in frequency.
An environmental factor that affects which individuals survive and reproduce. Examples: predation, disease, competition for food, climate, antibiotics, insecticides.
(a) Stabilising. (b) Directional. (c) Disruptive.
A dashed bell curve (before) and a solid bell curve of similar shape shifted towards one extreme (after), with the opposite extreme marked as selected against.
Only hard seeds remained, so finches with deeper beaks could feed and survived better; they bred and passed on alleles for deep beaks, raising the mean in the next generation. Beak depth must be largely inherited (genetic).
8 hours = 16 generations: $100 \times 2^{16} = 6\,553\,600 \approx 6.6 \times 10^{6}$ .
Heterozygotes (Hb^AHb^S) are protected from severe malaria and do not have sickle-cell disease, so they have the highest fitness. Both Hb^AHb^A (malaria) and Hb^SHb^S (sickle-cell disease) have lower fitness, so both alleles are kept.
Mosquitoes varied; a few carried resistance alleles from random mutation. Spraying killed susceptible mosquitoes, so resistant ones survived and reproduced, passing on the alleles; over generations the resistance allele became common (directional selection). Slow it by rotating or combining insecticides with different modes of action, or by using other control methods as well.

Key points

Natural selection: overproduction, competition, variation, differential survival and reproduction, inheritance → change in allele frequencies.
Stabilising: favours intermediates; same mean, less variation (birth mass).
Directional: favours one extreme; the mean shifts (finch beaks after drought, antibiotic and insecticide resistance).
Disruptive: favours both extremes; can produce two forms (seedcracker bills).
Heterozygote advantage keeps Hb^S where malaria is common: a balanced polymorphism.

Sources and credits

19.3 Adaptive Evolution and 18.1 Understanding Evolution, from Biology 2e by Mary Ann Clark, Matthew Douglas and Jung Choi, OpenStax (Rice University), licensed under CC BY 4.0. Used for the theory of natural selection, the Galápagos finches and the types of selection. Adapted by Ganzaa.
Photo: Black-bellied Seedcracker, Kakum, Ghana, by Francesco Veronesi, via Wikimedia Commons, licensed under CC BY-SA 2.0.
Video: "Natural Selection: Stabilizing, Directional, Disruptive", OtterBioTutor, embedded from YouTube.

Figure 1, the table, the worked examples (with illustrative data) and the practice questions were written by Ganzaa.

This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1A drought on a small island

2The theory of natural selection

3Three types of selection

4Selection that keeps two alleles: sickle-cell and malaria

A drought on a small island

The theory of natural selection

Three types of selection

Selection that keeps two alleles: sickle-cell and malaria