D4.1 Natural selection

Natural selection explains how heritable variation, selection pressures, differential survival, reproduction and allele-frequency changes drive evolutionary adaptation in populations in evolving populations.

Learning objectives

D4.1.1Natural selection as mechanism• Natural selection is the mechanism driving evolutionary change• It acts on heritable variation and can produce adaptation, speciation, and biodiversity D4.1.2Roles of mutation and sexual reproduction• Mutation creates new alleles, especially when germ-line mutations are inherited• Meiosis and random fertilization create new combinations of existing alleles D4.1.3Overproduction and competition• Overproduction of offspring leads to high mortality in limited environments• Competition for food, space, mates, and other resources promotes selection D4.1.4Abiotic factors as selection pressures• Abiotic factors can act as density-independent selection pressures• Temperature, drought, light, salinity, and pH can favour different variants D4.1.5Differences in adaptation, survival, reproduction• Individuals vary in adaptation, survival, and reproductive success• Fitness means passing alleles to offspring in a particular environment D4.1.6Traits must be heritable• Natural selection causes evolution only if traits are heritable• Acquired characteristics are not inherited through DNA base sequences D4.1.7Sexual selection• Sexual selection favours traits that increase mate choice or mating competition success• Displays, ornaments, and behaviours such as birds-of-paradise plumage can be selected D4.1.8Modelling selection• Selection can be modelled by experimentally controlling selection pressures• Endler's guppy experiments test predation pressure, colour pattern, and mating success

Natural selection is differential survival and reproduction caused by heritable differences among individuals. Across generations, alleles associated with greater reproductive contribution can become more frequent.

Individuals in a population vary.
Some variation is heritable.
The environment creates selection pressures.
Heritable variants cause differences in survival or mating success.
Individuals leave different numbers of surviving offspring.
Allele frequencies change across generations.

Individuals are selected; populations evolve. An individual does not genetically adapt because it needs to—the frequency of inherited variants changes in descendants.

Mutation location	What can inherit it?	Evolutionary consequence
germ-line cell or its precursor	gametes and potentially offspring	can add a new allele to the population gene pool
somatic cell	descendant cells within that individual	normally ends when the individual dies

Mutation is random with respect to what the organism needs. Selection is non-random in the sense that existing variants leave different numbers of offspring in a particular environment.

A new allele may be harmful, neutral or advantageous depending on phenotype and environment; mutation does not mean automatically beneficial or harmful.

Process	Source of new combination
crossing over	homologous chromosomes exchange DNA between non-sister chromatids
independent assortment	maternal and paternal homologs enter gametes in varied combinations
random fertilization	one of many possible sperm fuses with one of many possible eggs

Sexual reproduction usually reshuffles existing alleles; mutation is the ultimate source of a completely new allele.

Recombination does not guarantee a better offspring. It creates varied genotypes on which environmental selection pressures can act.

potential offspring production exceeds the number the environment can support → food, space, light, territories or mates become limiting → many individuals die or fail to reproduce → variants differ in representation among the offspring that breed

Competition type	Who competes?	Why it matters for selection
intraspecific	members of the same species	they share a niche and compete for very similar resources and mates
interspecific	members of different species	overlapping resource use can also change survival and reproduction

A roughly stable population size does not mean every offspring survives. High offspring production can be balanced by high pre-reproductive mortality.

Abiotic factor	Variant that could be favoured in one context
drought	lower water loss or deeper roots
salinity	ion exclusion or salt tolerance
temperature	enzymes and membranes that function at local extremes
light	leaf form or pigment investment suited to the light regime
pH	proteins and transport systems stable at local pH

A density-independent pressure can affect survival regardless of how crowded the population is, although its total impact may still differ among places and times.

The environment does not cause organisms to develop the needed inherited trait. It changes which already varying individuals contribute most to the next generation.

Individual outcome	Contribution to evolutionary fitness
survives but produces no surviving offspring	little or none
reproduces early and offspring later breed	high contribution
wins mates but offspring have low survival	mating success may not become high lifetime fitness

Fitness is relative to a particular environment. A phenotype advantageous during drought may be neutral or costly when water is abundant.

Better adapted does not mean perfect. Selection compares variants currently present and is constrained by trade-offs, ancestry and chance.

Feature	Acquired change	Heritable variant
origin	arises during one individual’s lifetime through use, injury or environment	encoded in inherited genetic or transmissible biological information
inheritance	usually not copied into gamete DNA	can be passed through gametes
population effect	ends with the individual	can change frequency across generations

Selection can act on a phenotype, but evolutionary change follows only when differences affecting reproductive success are heritable. Ordinary muscle gain or a scar is not inherited as a new DNA sequence.

Parallel giraffe sequences contrast Lamarckian individual neck lengthening with Darwinian inherited variation and differential reproduction across generations.

The population already contained pale and melanic heritable forms.
Soot darkened resting surfaces and reduced pale lichen cover in industrial areas.
Birds more often detected the less-camouflaged form.
Better-camouflaged moths survived and reproduced at a higher rate.
The allele associated with that form increased locally across generations.

When air pollution later declined and lichens returned, the direction of the camouflage advantage could reverse. Fitness belongs to a phenotype–environment relationship, not permanently to one colour.

Pale and melanic peppered moth forms are shown side by side, with pale associated with non-polluted habitats and melanic with industrially polluted habitats.

Route	Selection event	Example trait
mate choice	one sex preferentially mates with particular partners	bright plumage or complex courtship display
mating competition	individuals compete directly or indirectly for access	body size, weapons, territories or dominance behavior

heritable variation in display or competition trait → unequal mating success → unequal offspring contribution → associated alleles increase

Persistent differences in selected traits between sexes can produce sexual dimorphism, such as bright displaying males and more cryptic females.

Effect of an exaggerated ornament	Possible fitness consequence
more visible or energetically costly	lower survival through predation or resource cost
attractive display or honest condition signal	greater chance of mating
species-specific courtship signal	mating isolation from other populations

The trait can spread when its gain in reproductive success exceeds its survival cost. Fitness integrates the whole route to descendants, not survival alone.

Sexual selection names the pressure—competition for mates or mate choice—rather than competition for food or escape from predators.

Design element	Selection experiment question
independent variable	which pressure is deliberately changed?
dependent variable	which phenotype, survival or mating outcome is measured?
controls	which habitat, starting population and observation conditions stay comparable?
replication	are multiple populations exposed to each treatment?
time	are enough generations observed for frequency change?

Random assignment and controlled treatments make a causal claim stronger; repeated measurements reveal whether a pattern persists rather than reflecting one generation of chance.

A simulation or enclosure simplifies nature. Its value depends on whether the manipulated pressure and measured fitness components represent the biological system.

Treatment	Predator pressure	Prediction for male spots
K	none	female choice can favour conspicuous males
R	weak, Rivulus hartii	sexual selection remains influential
C	dangerous, Crenicichla alta	conspicuous males suffer stronger predation

Fine and coarse gravel were crossed with predator treatments. Under predation, spot size matching the substrate can improve camouflage; without strong predation, contrast can improve visibility to females.

Guppies were allowed to breed, predator treatments were imposed, and spot number and size were measured over later generations in replicated ponds.

Evidence	Interpretation
spots increased before strong predators were added	female choice favored conspicuous males
mean spot number fell with C. alta	dangerous predators selected against conspicuous males
weak-predator field transplants became more colorful over 15 generations	reduced predation allowed sexual selection to dominate
spot size tracked gravel under predation	background matching improved camouflage

Natural and sexual selection acted on the same heritable color traits in opposing directions. The observed phenotype reflected the balance of those pressures, not one universal best color.

Guppy treatments K, R and C show spot number rising with no or weak predation and falling under dangerous C. alta predation.

mutation creates alleles + sexual reproduction reshuffles them → offspring vary → limited resources and abiotic or biotic pressures produce unequal survival and mating → heritable variants reach offspring unequally → population trait and allele frequencies change

Claim to test	Evidence needed
adaptation occurred by selection	heritable variation plus unequal reproductive contribution across generations
sexual selection acted	trait differences predict mating success
environmental pressure caused change	controlled manipulation or strong comparative evidence links pressure to fitness
population evolved	heritable trait or allele frequency changed, not merely one individual

Avoid purpose-driven language. Populations do not create useful mutations on demand; selection filters inherited variation already produced by mutation and recombination.

8 marks

Explain how evolution may happen in response to environmental change with evidence from examples.

8 marks

Explain how sexual reproduction can eventually lead to evolution in offspring.

4 marks

Outline how overpopulation of a species in a given environment may lead to evolution.

2 marks

Explain how natural selection is influenced by changes in the environment.

1 mark

In a natural population, what is a feature of individuals that are better adapted?

1 mark

What is required for natural selection to occur?

I. Acquired characteristics
II. Advantageous characteristics
III. Genetic variation

6 marks

Explain what is meant by exaggerated traits and how they may develop in males of a species.

1 mark

John Endler experimented on populations of guppies (Poecilia reticulata) with different colouration. A male guppy fish is shown with large spots, which makes the fish more attractive to females, but more visible to predators.

The table shows the male colouration of guppy fish and number of predators in three different ponds.

	Predator X	Predator Y	Male guppy colouration
Pond 1	12	0	Large colourful spots
Pond 2	15	0	Medium colourful spots
Pond 3	5	17	None/very small drab spots

What can be concluded from the data?

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—D4.2 Stability and changeRead

D4.1 Natural selection

Learning objectives

Natural Selection Changes Populations Through Unequal Reproductive Success

Mutation Creates New Alleles; Germ-Line Location Makes Them Heritable

Sexual Reproduction Rearranges Existing Alleles into New Genotypes

Overproduction Makes Reproductive Success Unequal When Resources Are Limited

Abiotic Conditions Select Existing Variants Without Regard to Population Density

Fitness Means Alleles Reaching Future Generations, Not Strength or Longevity Alone

Only Heritable Differences Can Accumulate Across Generations

Peppered-Moth Frequencies Changed When Camouflage Changed Survival

Sexual Selection Rewards Traits That Increase Access to Mates

A Mating Advantage Can Persist Despite a Survival Cost

A Selection Experiment Must Manipulate Pressure and Measure Reproductive Consequences

Endler Separated Predation Strength from Gravel Background

Endler’s Guppies Reveal a Trade-off Between Predation and Mate Choice

Natural Selection Connects Variation, Environment and Reproduction Across Generations

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