Variations in Populations: Study Guide (Biology 7.11)

Why variation is a population’s insurance

Natural selection can only work with the variation a population already has. When the environment changes, selection “chooses” among the individuals that exist at that moment. It cannot wait for a helpful mutation to appear. So the amount of genetic variation in a population decides how well it can respond to a new pressure, such as a disease, a drought, a new predator or a warmer climate.

This topic is about that link: genetic diversity → ability to withstand environmental pressures. You need to explain it in words, support it with data, and connect the data to the theory of natural selection.

What genetic diversity means

Genetic diversity is the variety of alleles and genotypes in a population or species. Three simple ways to describe it:

• Number of alleles per gene. A gene with six alleles in a population is more diverse than one with a single allele.
• Heterozygosity. The proportion of individuals that are heterozygous at a gene. For a gene with two alleles at frequencies p and q, the expected value under Hardy–Weinberg conditions is 2pq. It is highest (0.5) when p = q = 0.5 and falls towards 0 as one allele takes over.
• Variation in a trait. A wide spread of a heritable trait, such as heat tolerance or beak depth, usually reflects more genetic variation behind it.

Diversity comes from mutation, from the reshuffling of alleles in meiosis and fertilisation, and from gene flow when individuals move in from other populations. It is lost by selection, by chance in small populations (genetic drift) and by inbreeding, where relatives mate and offspring become homozygous at more genes.

Low diversity: a population at risk

If almost every individual carries the same alleles, almost every individual responds to a pressure in the same way. One disease, or one change in conditions, can then harm the whole population at once. Species and populations with little genetic diversity are therefore at greater risk of decline or extinction.

Real cases show the pattern:

• Potato blight in Ireland. In the 1840s much of Ireland relied on one potato variety, the Lumper. Potatoes are grown from tubers, so the plants in a field were genetically almost identical. When the blight organism Phytophthora infestans arrived, nearly every plant was susceptible. The crop failed year after year, and the famine that followed killed about a million people.
• Rusts and blights of crops. Modern farms often plant huge areas with one variety. If a new strain of a fungal pathogen, such as a corn rust, can infect that variety, it can spread through field after field, because there are no resistant plants in the way.
• California condors. By 1987 only 27 birds were left, and the last wild ones were taken into captivity for breeding. Every condor alive today descends from that small group. Inbreeding may be behind cases of a lethal inherited form of dwarfism in condor chicks.
• Black-footed ferrets. Only 18 ferrets were caught for a captive breeding programme in the 1980s, and just 7 of them have living descendants. The whole species now carries the alleles of those few founders. Disease, such as sylvatic plague, is a major threat to it.
• Prairie chickens. In Illinois, greater prairie chickens fell from millions in the 1800s to a few dozen birds by the late 1990s. In the isolated, inbred population, fewer eggs hatched. Conservationists brought in birds from larger populations in other states. This genetic rescue adds new alleles through gene flow.

Notice the common thread: fewer individuals → fewer alleles → fewer ways to cope with the next change. A small population also tends to lose still more diversity, so the risk can grow over time.

High diversity: a resilient population

A genetically diverse population is more resilient to environmental disturbance. The reason is simple probability: with more variation, it is more likely that some individuals already have alleles that let them survive the new pressure. Those survivors reproduce, and the population can recover.

Antibiotic resistance in bacteria is a clear example. When an antibiotic is used, not every bacterial cell in a varied population is equally susceptible. A few may carry an allele that lets them survive. They multiply, and the population bounces back, now made up mostly of resistant cells. The same logic explains why not every individual in a varied population of animals or plants catches, or dies from, a disease outbreak.

Reading Figure 1. Both populations have the same average, so a comparison of means would miss the difference. What matters is the spread. Only the diverse population has a tail of individuals beyond 41 °C. Those survivors pass on their heat-tolerance alleles, so the population can persist and, over generations, shift towards higher tolerance. The uniform population has almost no one to pass anything on.

The same allele can help or harm

An allele is not “good” or “bad” on its own. Its effect on survival and reproduction depends on the selective pressures in the environment. An allele that is adaptive in one setting may be deleterious (harmful) in another.

Look at this fictional data for a meadow plant. Allele F makes antifreeze proteins, so F plants survive frost better, but making those proteins costs energy, so F plants make fewer seeds.

Winter	Genotype	Survival	Seeds per survivor	Seeds per plant sown
Cold	F_ (FF or Ff)	0.90	60	0.90 × 60 = 54
Cold	ff	0.30	100	0.30 × 100 = 30
Mild	F_	0.95	60	0.95 × 60 = 57
Mild	ff	0.97	100	0.97 × 100 = 97

In cold winters, F plants leave more offspring (54 against 30), so selection favours F. In mild winters the frost protection is no longer needed, but the seed cost remains, so ff plants leave more offspring (97 against 57) and selection works against F. If winters alternate, keeping both alleles in the population is what protects it. Antibiotic resistance works the same way: a resistance allele saves a bacterium when the drug is present, but it often slows growth when the drug is absent.

Worked example 1: who survives a disease outbreak?

Question. A new bark-killing disease reaches two isolated woodlands, A and B, each with 2000 trees of one fictional species. A dominant allele R gives resistance; rr trees die. The frequency of R is 0.30 in population A and 0.02 in population B. Assume both populations are in Hardy–Weinberg equilibrium for this gene. (a) Predict how many trees survive in each population. (b) Use the result to explain the link between genetic diversity and the risk of extinction.

1. Find q, the frequency of r. A: q = 1 − 0.30 = 0.70. B: q = 1 − 0.02 = 0.98.
2. Susceptible trees are rr, with frequency q². A: 0.70² = 0.49. B: 0.98² = 0.9604.
3. Resistant trees (RR or Rr) = 1 − q². A: 1 − 0.49 = 0.51. B: 1 − 0.9604 = 0.0396.
4. Survivors = 2000 × fraction resistant. A: 2000 × 0.51 = 1020 trees. B: 2000 × 0.0396 = 79.2, so about 79 trees.

Check. The fractions are between 0 and 1, and B, with the rarer resistance allele, has far fewer survivors (1020 ÷ 79.2 ≈ 13 times fewer).

(b) Interpretation. Population A had more variation at this gene, so about half of it already carried resistance. It survives the outbreak with a large breeding population. Population B was nearly uniform for the susceptible allele. Fewer than 80 trees remain. That small group will probably lose more alleles by chance and through inbreeding, which makes the next pressure even harder to survive. This is how low diversity raises the risk of decline or extinction. Note what selection did: it did not create R; it acted on R that was already present.

Assumptions. Hardy–Weinberg conditions (random mating, no recent selection at this gene) and complete dominance of R. In reality the disease might not kill every rr tree, and other genes may affect survival too.

Worked example 2: building an argument from crop data

Question. A fictional field trial grew a cereal crop in eight plots exposed to a rust fungus. Four plots were sown with a single variety. Four were sown with a mixture of four varieties that differ in their rust-resistance alleles. Results (percentage yield lost to rust):

Plot type	Plot 1	Plot 2	Plot 3	Plot 4	Mean
Single variety	41	35	44	32	38
Mixture of four	14	9	12	13	12

Make a claim, support it with evidence from the table, and give reasoning that connects the evidence to biological theory.

1. Claim. Genetically diverse plantings are more resilient to this rust than genetically uniform plantings.
2. Evidence. Mean yield loss was 38% in single-variety plots and 12% in mixed plots, a difference of 26 percentage points. The ranges do not overlap (32–44% against 9–14%), so the difference is unlikely to be chance alone.
3. Reasoning. In a single-variety plot every plant has the same resistance alleles, so if the rust can infect one plant it can infect them all and spread from neighbour to neighbour. In a mixture, some plants carry alleles that the rust cannot overcome. They survive, and they also slow the spread between susceptible plants. This matches the principle that diverse populations are more likely to contain individuals that withstand a pressure.
4. Limits. Four plots of each type is a small sample, and one rust strain was tested. A new strain that can infect all four varieties would remove the advantage, which is why breeders keep many varieties in seed banks.

Common misconceptions

• “The pressure causes the helpful allele to appear.” Mutations are random. Selection acts on alleles that are already there when the pressure arrives.
• “A population with a high average value of a trait is the best prepared.” Spread matters as much as the mean. Without variation, everyone fails together (Figure 1).
• “A large population is always genetically diverse.” A field of clones, or a large population descended from a few founders, can be huge yet uniform.
• “An adaptive allele is adaptive everywhere.” Its value depends on the environment. Allele F above helps in cold winters and harms in mild ones.
• “Genetic diversity means many different species.” Here it means variation within one species or population. Variety among species is a different measure, covered in ecology.
• “Resistant bacteria became resistant because they were exposed to the antibiotic.” The drug killed the susceptible cells; resistant ones were already present, or arose by random mutation, and then multiplied.
• “A bottleneck only matters at the moment of the crash.” The few survivors carry only some of the original alleles, so the population stays less diverse for many generations, unless gene flow adds new alleles.

Where this leads

Topic 7.12 steps back to ask how life on Earth began at all: read Origins of Life on Earth next. Genetic diversity returns in Unit 8, where the diversity of species and of ecosystems affects how well communities withstand disruption. If you need to revisit how new species form, go back to Speciation. Test yourself with the practice questions, then use the revision notes and the checklist.