7.11Natural Selection

Variations in Populations — AP BIOLOGY Revision Notes & Topical Questions

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7.11 Variations in Populations — Notes — AP Biology notes: Explain how the genetic diversity of a species or population affects its ability to withstand environmental pressures.Genetic diversity — the variety of alleles in the gene pool, the raw material for natural selection.Sur…

Monoculture
Bottleneck effect
Continuous variation
Heritability
Polymorphism
Crossing over

Section 1 of 10Diversity and resilience

Genetic diversity — the variety of alleles in the gene pool, the raw material for natural selection.

Survival — a diverse population is more likely to include individuals able to survive disease, drought or a new predator,

whereas a uniform one may be wiped out.

Low diversity — species with little diversity are at risk of decline or extinction.

Context — alleles adaptive in one environment may be harmful in another because selective pressures differ.

Conservation — this is why biologists maintain diversity and why monoculture crops are at risk.

Section 2 of 10Sources of genetic variation

Mutation — the origin of all new alleles (point mutations, duplications, chromosome changes).

Crossing over — in prophase I exchanges segments between homologues, giving new allele combinations.

Independent assortment — homologous pairs at metaphase I give 2ⁿ chromosome combinations (over 8 million for the 23 human pairs, ignoring crossing over).

Random fertilisation — any sperm can fuse with any egg, so one couple can make over 70 trillion zygotes.

Gene transfer — gene flow between populations and horizontal transfer in prokaryotes by conjugation, transformation and transduction.

Asexual reproduction — makes clones,

so variation comes only from mutation, yet bacteria are diverse through large numbers, fast generations and gene transfer.

Sexual reproduction reshuffles alleles but only mutation creates new ones.

Section 3 of 10Types of variation

Feature	Discontinuous	Continuous
Phenotype classes	Distinct categories	Range of values, bell-shaped
Genetic control	One or few genes	Many genes (polygenic)
Environmental effect	Little	Large
Examples	ABO blood group, pea seed shape	Height, mass, skin colour, milk yield

Causes — variation arises from genotype, environment and their interaction; identical twins differ through environment and epigenetic differences.

Separating components — compare clones raised in different environments, grow organisms from different populations in a common environment,

or use pedigrees and twin studies.

Analysis — continuous variation is described by the mean and standard deviation, with about 68 per cent of values within one standard deviation and about 95 per cent within two.

t-test — compares the means of two groups to decide whether differences are likely real.

Section 4 of 10Low-diversity populations

Examples — clonal crops such as the potatoes of the Irish famine (potato blight, Phytophthora infestans, spreads by spores and destroys uniform crops), Cavendish bananas,

and cheetahs.

Cheetahs passed through a severe bottleneck, so nearly all share the same immune alleles and are vulnerable to disease.

Endangered species — the black-footed ferret (depends on declining prairie dogs) and the greater prairie chicken (habitat lost to agriculture) have small, low-diversity populations.

Risk — greater vulnerability to disease or environmental change.

Antibiotic resistance — a diverse bacterial population is more likely to include survivors of a new antibiotic, which then multiply.

Section 5 of 10Sustaining diversity

Measures — comparing DNA sequences, counting alleles (allelic richness) and the proportion of heterozygotes.

Maintained polymorphism — heterozygote advantage and frequency-dependent selection keep variation, e.g. the many alleles of MHC genes,

where rare alleles protect against pathogens.

Outbreeding mechanisms — separate sexes, self-incompatibility in plants and dispersal of young avoid inbreeding.

Other forces — drift (bottleneck and founder effects) lowers diversity, while gene flow moves alleles between populations and makes them more similar.

Hardy–Weinberg — p + q = 1 and p² + 2pq + q² = 1; equilibrium needs a large population, no mutation, no migration, random mating and no selection,

and deviation from expected genotype frequencies shows the population is evolving.

Section 6 of 10Variation and the environment

Variation and the environment

Environmental variation — also causes phenotypic variation (hydrangea flower colour depends on soil pH; shade-grown plants are taller and paler than identical plants in sunlight; muscle gained by exercise is not passed on).

Selection — acts on the heritable component.

Heritability = the proportion of variation in a trait in a population due to genetic differences; high heritability allows rapid response to selection.

Section 7 of 10Why variation matters compared

Population	Diversity	Threat	Outcome	Example
High diversity	Many alleles	New disease	Some survive	Wild wheat
Low diversity	Few alleles	New disease	Mass death	Irish potatoes 1845
Cheetah	Very low (bottleneck)	Disease	High risk	Conservation
Bananas (Cavendish)	Clones	Panama disease	Threatened	Monoculture
Bacteria	High mutation	Antibiotics	Resistance evolves	—

Section 8 of 10More from the specification

Hardy-Weinberg equation

P² + 2pq + q² = 1, relating allele and genotype frequencies.

Recessive

Expressed only when no dominant allele is present.

Dominant

Expressed when present in the genotype.

Homozygous

Having two identical alleles.

Homozygous dominant

Two copies of the dominant allele.

p2 + 2pq + q2 = 1

What the symbols mean: p = frequency of the dominant allele · q = frequency of the recessive allele · p² = homozygous dominant · 2pq = heterozygous · q² = homozygous recessive

Section 9 of 10Key terms

Term	Meaning
Genetic diversity	variety of alleles in a population
Monoculture	genetically identical crop
Bottleneck effect	loss of diversity after a population crash
Continuous variation	variation showing a range, usually polygenic
Discontinuous variation	variation in distinct categories
Heritability	proportion of variation due to genetic differences
Polymorphism	existence of several forms in a population
Crossing over	exchange of segments between homologous chromosomes

Section 10 of 10Exam technique

Saying diversity is a disadvantage — it lets populations adapt.

Calling a monoculture resilient — all individuals share the same vulnerability.

Saying variation from the environment is inherited — only heritable variation is selected.

Treating continuous variation as controlled by one gene.

Saying a small population is safe because it is well adapted — low diversity reduces its ability to respond.

Last updated: 14 October 2026