Natural Selection: Study Guide (Biology 7.2)

Variation is the raw material

Topic 7.1 showed that natural selection needs three things: variation, inheritance and differences in reproductive success. This topic looks at the first of these closely, and asks what decides which variants win.

Selection acts on phenotypes

A phenotype is an organism’s observable or measurable characteristics: its colour, size, behaviour, blood proteins or enzyme activity. A genotype is the set of alleles it carries.

Natural selection acts on phenotypes. A hawk sees a mouse’s coat, not its alleles. A drought kills plants whose roots are too shallow, not plants with a particular DNA sequence. Genotypes are affected only because they help produce the phenotype. So:

• two genotypes that give the same phenotype are selected in the same way;
• a harmful allele can stay hidden in a population if it has no effect on the phenotype of carriers (for example, a recessive allele in a heterozygote).

Heritable and non-heritable variation

Variation in a population has two sources.

Source	Examples	Passed on?
Genetic	new alleles from mutation (Topic 6.7); new combinations of alleles from meiosis and fertilisation (Unit 5)	yes
Environmental	a plant grown in shade is taller and thinner; an animal that eats well is larger; a scar	no

Most traits are affected by both. Only the genetic part changes the next generation. If tall plants survive better, but they are tall only because they grew in better soil, their offspring will not be taller. This is why you must show a trait is heritable before you can claim it is evolving.

Environments change and apply selective pressures

A selective pressure is any feature of the environment that causes some phenotypes to survive or reproduce better than others. Environments do not stay the same, so selective pressures appear, disappear and change strength.

Selective pressure	Biotic or abiotic?	Phenotype likely to be favoured
A new predator arrives	biotic	camouflage, speed, defensive structures
A disease-causing organism spreads	biotic	resistance to infection
Spring arrives earlier as the climate warms	abiotic	earlier flowering or breeding
Long dry period	abiotic	deeper roots, thicker leaf cuticles, drought-tolerant seeds
An insecticide is sprayed	abiotic (a chemical)	resistance to the insecticide
Winters become colder	abiotic	cold tolerance, such as membranes that stay fluid

Flowering time and climate change. In many plant species, the date of first flowering is partly heritable. As springs warm, pollinating insects become active earlier and summer droughts can start earlier. Plants that flower late may miss their pollinators or run out of water before their seeds ripen, so earlier-flowering variants can leave more seeds. Be careful when you interpret records of earlier flowering: part of any shift is each plant responding directly to warmth, which is not evolution. Only the part caused by a change in heritable flowering time counts.

The same trait can help or harm

Fitness is not a fixed property of a phenotype. A variant that raises fitness in one environment can lower it in another, or make no difference.

Sickle-cell haemoglobin. The Hb S allele differs from the usual Hb A allele by a single base. This changes one amino acid in the β-globin chain of haemoglobin (the kind of mutation you met in Topic 6.7).

• People with two Hb S alleles have sickle cell disease. Their red blood cells can become rigid and sickle-shaped when oxygen is low, which blocks small blood vessels and damages organs.
• People with one Hb S allele and one Hb A allele have sickle-cell trait. They usually have few or no symptoms, and they are more resistant to severe malaria, a disease caused by a parasite that lives inside red blood cells.
• Where malaria is common, the carrier phenotype therefore has the highest fitness. Where malaria is absent, it gives no advantage.

This explains why the Hb S allele is common in some regions where malaria has long been widespread, even though two copies cause serious disease. Worked example 1 puts numbers on the idea.

Insecticide resistance. DDT, an insecticide widely used from the 1940s, kills insects by acting on a protein in their nerve cell membranes. In many insect populations, a few individuals carried variants that let them survive: some had a slightly different version of the target protein; others made more of an enzyme that breaks DDT down. Repeated spraying killed the susceptible insects and left the resistant ones to breed, and resistance became common. Worked example 2 models this.

Variation in molecules affects fitness

Differences between individuals often come down to differences in the types or numbers of molecules inside their cells. A population that contains a range of molecular variants has a better chance that some individuals will survive and reproduce when conditions change.

Molecular variation	What differs	Environment where it helps
Membrane phospholipids	more unsaturated fatty acid tails (with kinks) or more saturated ones	unsaturated tails keep membranes fluid in the cold; saturated tails keep them stable in heat
Enzyme variants	two forms of the same enzyme with different temperature optima	each form works best in a different temperature range
Photosynthetic pigments	chlorophyll a and b plus accessory pigments that absorb different wavelengths	a wider range of absorbed light helps in shade or changing light
Antifreeze proteins	some polar fish make proteins that stop ice crystals growing in their blood	water below 0 °C
Number of copies of a gene	extra copies of a gene for a detoxifying enzyme mean more enzyme	habitats exposed to a toxin, such as an insecticide

The link to fitness always runs through the same chain: molecule → cell function → phenotype → survival and reproduction in this environment. When you explain an example, state each link. Note that an individual with two different alleles for an enzyme can make both forms, which may let it work across a wider range of conditions.

Worked example 1: sickle-cell trait in two environments

Question. The table gives fictional model values for the fraction of children of each phenotype who survive to adulthood in two regions without modern medical care. Assume survivors of all phenotypes have the same number of children, so survival can stand in for fitness. (Real figures vary widely, and modern treatment greatly improves survival with sickle cell disease.)

Phenotype (genotype)	Region with malaria	Region without malaria
Unaffected, no trait (Hb A Hb A)	0.80	0.98
Sickle-cell trait (Hb A Hb S)	0.95	0.98
Sickle cell disease (Hb S Hb S)	0.20	0.20

(a) Calculate the relative fitness of each phenotype in each region. (b) Explain the difference between the regions.

(a) Region with malaria. The most successful phenotype is sickle-cell trait (0.95).

1. w(trait) = 0.95 ÷ 0.95 = 1.00
2. w(unaffected) = 0.80 ÷ 0.95 = 0.842, so 0.84
3. w(disease) = 0.20 ÷ 0.95 = 0.211, so 0.21

Region without malaria. Unaffected and trait both have 0.98.

1. w(unaffected) = w(trait) = 0.98 ÷ 0.98 = 1.00
2. w(disease) = 0.20 ÷ 0.98 = 0.204, so 0.20

(b) Explanation. In the malaria region, carriers survive malaria better than unaffected people, so the trait phenotype has the highest fitness (1.00 against 0.84). Each surviving carrier can pass Hb S to children, which keeps the allele common even though people with two copies have low fitness. Without malaria, carriers gain no advantage (both 1.00), and only the cost of the disease phenotype remains (0.20). In that region, selection steadily removes Hb S alleles, slowly because most of them are carried by people with the trait, who are not selected against. The same allele is favoured in one environment and disfavoured in the other.

Worked example 2: insecticide resistance

Question. A fictional mosquito population of 10 000 contains 2.0% resistant insects. One spraying kills 95% of susceptible and 10% of resistant mosquitoes. Survivors breed, the population grows back to 10 000, and offspring share their parent’s phenotype.

(a) Calculate the percentage of resistant mosquitoes after one spraying and after a second spraying. (b) Spraying then stops. Resistant insects make extra detoxifying enzyme, which costs energy: without insecticide, their relative fitness is 0.70. Calculate the percentage resistant one generation later. (c) Describe and explain the pattern in Figure 2.

(a) First spraying.

1. Start: resistant = 0.020 × 10 000 = 200; susceptible = 9800.
2. Survivors: resistant = 200 × 0.90 = 180; susceptible = 9800 × 0.05 = 490.
3. Fraction resistant = 180 ÷ (180 + 490) = 180 ÷ 670 = 0.269, so 26.9%.

Second spraying.

1. Start (10 000): resistant = 2687; susceptible = 7313.
2. Survivors: resistant = 2687 × 0.90 = 2418; susceptible = 7313 × 0.05 = 366.
3. Fraction resistant = 2418 ÷ 2784 = 0.869, so 86.9%.

(b) Per 1000: 869 resistant and 131 susceptible. Offspring: resistant = 869 × 0.70 = 608; susceptible = 131 × 1.00 = 131. Fraction resistant = 608 ÷ 739 = 0.823, so about 82%.

(c) Pattern and explanation. While spraying continues, the percentage resistant rises very steeply (2.0% to 86.9% in two generations), because resistant insects were 18 times as likely to survive each spraying (0.90 ÷ 0.05). After spraying stops, the percentage falls, but much more slowly (to 43.8% after six more generations), because the fitness difference is smaller (0.70 against 1.00). The selective pressure reversed when the environment changed, and so did the direction of evolution. The insecticide did not cause the resistance: the resistant variants were present before the first spray.

Common misconceptions

• “Natural selection acts directly on genes.” It acts on phenotypes. Alleles are affected through the phenotypes they help produce.
• “The insecticide caused the insects to become resistant.” The variation was already present. The insecticide acted as a selective pressure that removed susceptible insects.
• “A favourable trait is favourable everywhere.” Fitness depends on the environment. Sickle-cell trait helps only where malaria is common.
• “Any difference between individuals can be selected.” Only heritable differences change the next generation. Variation caused by the environment alone is not passed on.
• “Resistance, once it appears, is permanent.” If the variant has a cost, it can decline when the selective pressure is removed.
• “An allele that causes disease should disappear quickly.” It can persist if carriers have higher fitness in their environment, or if it is hidden in heterozygotes.
• “Molecular differences are too small to matter.” One changed amino acid, or one extra gene copy, can change a cell’s function and the organism’s survival.

Where this leads

Topic 7.3 looks at what happens when humans, not the environment, decide which individuals reproduce. Go to Artificial Selection next. First, test yourself with the practice questions, then use the revision notes and the checklist. If any of the basics feel shaky, go back to Introduction to Natural Selection.