IMAT Biology · Evolution and Ecology

Natural Selection, Drift and Hardy-Weinberg

Evolution is a change in allele frequencies in a population over generations, driven by natural selection, genetic drift, gene flow and mutation.

All Evolution and Ecology notes

Why this matters

Most of the chapter's questions sit here. The papers asked which organisms natural selection acts on, why antibiotics lose their power, what can make a phenotype commoner, which processes are random, what directional selection can look like, and (in 2023) which mutations can take part in evolution.

Concept 1 of 4: Darwin's natural selection, and why Lamarck's idea fails

Individuals differ, and many of the differences are inherited. More young are born than can survive, so the ones whose traits suit the environment leave more offspring. Over generations those traits become common. Nothing in the animal changes on purpose: the population changes because some variants out-breed others.

Definition

Natural selection (Darwin and Wallace, 1858; Darwin's book On the Origin of Species, 1859): heritable variants that raise survival and reproduction become more common in a population.

Fitness means reproductive success, not strength.
Selection acts on phenotypes of individuals, but it is populations that evolve.
It acts on every population with heritable variation: sexual or asexual, in a changing or a stable environment.
Lamarck proposed that organs grow with use and shrink with disuse, and that these acquired characteristics are inherited. This fails: changes to body cells during a lifetime do not alter the DNA of the gametes, so they are not passed on.
The modern synthesis (1930s to 1940s) joined Darwin's theory with Mendelian genetics: evolution is a change in allele frequencies in a population's gene pool.

Step	What happens	Bacteria and an antibiotic
Variation	Individuals differ; much of it is genetic	A few cells already carry a resistance allele (from a random mutation)
Overproduction	More offspring are made than can survive	Bacteria multiply very fast
Selection	Some variants survive and reproduce better	The antibiotic kills sensitive cells; resistant cells survive
Inheritance	Survivors pass their alleles on	Resistant cells divide and their offspring are resistant
Change over generations	Favoured alleles become more frequent	The infection becomes resistant; overuse and unfinished courses speed this up

Practice this conceptself-check · 4 quick reps

Bacteria become resistant, not the patient

Antibiotic resistance is a property of the bacteria, selected when the drug kills their sensitive neighbours. People do not become resistant or immune to an antibiotic. Overuse and not finishing a course both help resistant bacteria take over.

Selection works in stable environments and on asexual organisms

In a stable environment, selection keeps removing the extremes (stabilising selection). Asexual organisms such as bacteria vary through mutation, so they are selected too. Any population with heritable variation is subject to natural selection.

Concept 2 of 4: Sources of variation and the forces that change allele frequencies

Mutation writes new alleles. Meiosis and fertilisation shuffle them into new combinations. Then several forces change how common each allele is: selection does it by fitness, drift by pure luck, gene flow by migration, and humans by choosing which animals and plants breed.

Definition

Variation comes from:

Mutation: the only source of new alleles. It is random with respect to what the organism needs. In animals only mutations in the germ line (cells that make gametes) are inherited; a mutation in a body cell dies with the individual. Whether the allele is dominant or recessive does not decide this.
Recombination in sexual reproduction: crossing over, independent assortment and random fertilisation.

Allele frequencies are then changed by:

Genetic drift: random change by chance, strongest in small populations. A bottleneck (a population crashes, as in cheetahs) and the founder effect (a few individuals start a new population, often on an island) are extreme cases.
Gene flow: alleles move between populations through migration, making them more alike.
Artificial selection: humans choose which individuals breed (dog breeds, crops, dairy cattle). Darwin used it as a model for natural selection.

Force	Random?	Main effect
Mutation	Yes: not directed by need	Adds new alleles
Genetic drift	Yes: chance decides who breeds	Can fix or lose alleles, even useful ones, in small populations
Gene flow	Not driven by fitness; depends on who migrates	Brings in alleles; makes populations more similar
Natural selection	No: better-suited variants reproduce more	Adapts the population to its environment
Artificial selection	No: humans pick the parents	Exaggerates traits humans want

Practice this conceptself-check · 4 quick reps

Artificial selection is not a random process

Mutation and genetic drift are random. Artificial selection is the opposite: a breeder deliberately chooses which individuals reproduce. Natural selection is not random either, although the variation it acts on arises at random.

Only germ-line mutations matter to evolution

For a mutation to change the next generation it must be in a cell that makes gametes. A mutation in a body cell, dominant or recessive, however it was caused, is not passed on in animals.

Concept 3 of 4: Directional, stabilising and disruptive selection

Picture a bell-shaped curve of a trait, such as body size. Selection can push the whole curve one way, squeeze it towards the middle, or split it into two peaks. Which one happens depends on which part of the curve survives and breeds best.

Definition

Three patterns of selection on a continuous trait:

Directional: one extreme is favoured, so the mean shifts. The favoured phenotype may start rare, and its allele may be recessive or dominant. Any agent can cause it: a predator, a drug, a change of climate.
Stabilising: the middle is favoured, so variation shrinks. Typical of a stable environment.
Disruptive: both extremes beat the middle, which can split a population and lead to new species.
Sexual selection: traits that win mates spread, even at a cost to survival.

Type	Favoured	Effect on the curve	Example
Directional	One extreme	Mean shifts that way	Dark peppered moths on sooty trees in industrial England
Stabilising	The middle	Narrower, same mean	Human birth weight: very light and very heavy babies fared worst
Disruptive	Both extremes	Splits into two peaks	Finches with large or small beaks where only large or small seeds exist
Sexual	Traits that attract mates	Showy traits spread	The peacock's tail

Practice this conceptself-check · 3 quick reps

Directional selection can favour a rare or a recessive phenotype

At the start, the favoured phenotype is often rare (dark moths were once uncommon). The allele behind it may be recessive, and its frequency still rises. Directional selection is defined by which end of the range is favoured, not by how common or how dominant it is.

Concept 4 of 4: The Hardy-Weinberg principle for allele and genotype frequencies

If nothing is changing a population, its allele frequencies stay the same generation after generation, and the genotype frequencies follow from them by simple probability. You can only see the recessive homozygotes, so start from them and work back to the alleles.

Definition

For a gene with two alleles, dominant frequency $p$ and recessive frequency $q$ :

$p + q = 1$ , and the genotype frequencies are $p^2$ (AA), $2pq$ (Aa) and $q^2$ (aa).
The frequencies stay constant only if: the population is large, mating is random, there is no mutation, no migration and no selection.
So a population in Hardy-Weinberg equilibrium is one that is not evolving. A mismatch with the predicted frequencies is evidence that something is acting on it.

Hardy-Weinberg equations

p + q = 1 \qquad p^2 + 2pq + q^2 = 1

$p$ frequency of the dominant allele
$q$ frequency of the recessive allele
$q^2$ frequency of homozygous recessive individuals (the visible recessive phenotype)
$2pq$ frequency of heterozygotes (carriers)

Worked example

Albinism is recessive. In one population, 1 person in 10,000 is albino. Find the frequency of the albino allele and the share of people who are carriers.

Practice this conceptself-check · 4 quick reps

The recessive phenotype gives q squared, not q

Individuals showing a recessive trait are aa, so their share is

q^2

. Take the square root to get

q

. Using the phenotype percentage directly as the allele frequency is the commonest mistake, and IMAT includes that number among the options.

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

The Hardy-Weinberg principle for allele and genotype frequencies
Hardy-Weinberg equations
$p + q = 1 \qquad p^2 + 2pq + q^2 = 1$

Reference tables (3)

Darwin's natural selection, and why Lamarck's idea fails5 rows

Step	What happens	Bacteria and an antibiotic
Variation	Individuals differ; much of it is genetic	A few cells already carry a resistance allele (from a random mutation)
Overproduction	More offspring are made than can survive	Bacteria multiply very fast
Selection	Some variants survive and reproduce better	The antibiotic kills sensitive cells; resistant cells survive
Inheritance	Survivors pass their alleles on	Resistant cells divide and their offspring are resistant
Change over generations	Favoured alleles become more frequent	The infection becomes resistant; overuse and unfinished courses speed this up

Sources of variation and the forces that change allele frequencies5 rows

Force	Random?	Main effect
Mutation	Yes: not directed by need	Adds new alleles
Genetic drift	Yes: chance decides who breeds	Can fix or lose alleles, even useful ones, in small populations
Gene flow	Not driven by fitness; depends on who migrates	Brings in alleles; makes populations more similar
Natural selection	No: better-suited variants reproduce more	Adapts the population to its environment
Artificial selection	No: humans pick the parents	Exaggerates traits humans want

Directional, stabilising and disruptive selection4 rows

Type	Favoured	Effect on the curve	Example
Directional	One extreme	Mean shifts that way	Dark peppered moths on sooty trees in industrial England
Stabilising	The middle	Narrower, same mean	Human birth weight: very light and very heavy babies fared worst
Disruptive	Both extremes	Splits into two peaks	Finches with large or small beaks where only large or small seeds exist
Sexual	Traits that attract mates	Showy traits spread	The peacock's tail

Watch out for (6)

Bacteria become resistant, not the patient
→ Darwin's natural selection, and why Lamarck's idea fails
Selection works in stable environments and on asexual organisms
→ Darwin's natural selection, and why Lamarck's idea fails
Artificial selection is not a random process
→ Sources of variation and the forces that change allele frequencies
Only germ-line mutations matter to evolution
→ Sources of variation and the forces that change allele frequencies
Directional selection can favour a rare or a recessive phenotype
→ Directional, stabilising and disruptive selection
The recessive phenotype gives q squared, not q
→ The Hardy-Weinberg principle for allele and genotype frequencies

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