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5.4.1Topic 5 · On the Wild Side

Evolution by Natural Selection — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions

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📚 Edexcel A Level Biology A (SNAB) 9BI0 Hub / Topical Revision
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5.4.1 Evolution by Natural Selection — Edexcel A Level Biology A (SNAB) 9BI0 notes: Explain how variation, differential survival and inheritance lead to evolution by natural selection, and describe the types of selection and their effect on allele frequency.Evolution is a change …

  • struggle for existence
  • differential survival and reproduction
  • Antibiotic resistance
  • Peppered moth
  • adaptation
  • Exam-style question 1

Section 1 of 9Evolution and the theory of natural selection

Evolution is a change in the heritable characteristics, and so in allele frequencies, of a population over successive generations. The theory of evolution by natural selection was proposed independently by Charles Darwin and Alfred Russel Wallace in the 1850s. Its logic can be set out as a series of observations and deductions.

Evolution and the theory of natural selection
Variation
Individuals in a population show variation in their phenotypes.
Much of this variation is inherited, because it is caused by differences in alleles.
Struggle for existence
Organisms produce more offspring than the environment can support,
  • leading to a struggle for existence for food, space, mates and other resources, and to the action of predators, disease and the abiotic environment.
Differential survival and reproduction
Individuals with characteristics that make them better adapted are more likely to survive and reproduce: differential survival and reproduction.
They pass on the alleles for those characteristics to more offspring, so the frequency of the advantageous alleles increases in the next generation.

Section 2 of 9The source of variation

New alleles arise by mutation, which is random and not directed by need.

1
Mutations are rare and most have no effect or are harmful, but a few provide an advantage under particular conditions.
2
In sexually reproducing species, variation is increased by independent assortment and crossing over in meiosis, and random fertilisation,
  • which create new combinations of existing alleles.
3
Natural selection does not create variation; it acts on the variation that is present.
4
Environmental factors can also cause phenotypic variation, but only differences caused by alleles are inherited, so only these respond to selection.

Selection pressures can be biotic (predation, competition, disease) or abiotic (temperature, drought, salinity).

Section 3 of 9Types of selection

TypeEffect on the populationWhen it occursExample
StabilisingExtremes are selected against; the mean is kept; variation fallsStable environmentHuman birth mass: very small and very large babies have lower survival
DirectionalOne extreme is favoured; the mean shiftsEnvironment changesAntibiotic resistance in bacteria; peppered moth colour in polluted areas
DisruptiveBoth extremes are favoured over the mean; may produce two groupsEnvironment has two distinct nichesBeak size in some finches when only large or small seeds are available

On a normal distribution graph, stabilising selection narrows the curve, directional selection moves the peak to one side and disruptive selection produces two peaks.

Section 4 of 9Worked examples

Antibiotic resistance. In a population of bacteria a random mutation gives a few cells an allele that allows them to survive an antibiotic.

Worked examples
When the antibiotic is used, susceptible bacteria die and the resistant ones survive and reproduce with less competition.
Resistance can be passed to offspring by vertical transmission and between bacteria by horizontal transfer of plasmids.
Over time the population is mostly resistant.

Peppered moth. Before the Industrial Revolution most Biston betularia were pale and camouflaged against lichen-covered bark. Soot darkened trees and killed lichens, so pale moths were more visible to birds. Dark moths survived and reproduced, and the frequency of the dark allele rose. After air quality improved, pale moths became common again.

Section 5 of 9Allele frequency and adaptation

Evolution can be described in numerical terms.

If an allele provides an advantage, the proportion of that allele in the gene pool rises with each generation.
Adaptation
An adaptation is an inherited feature that increases an organism's chance of survival and reproduction in its environment; it may be anatomical, physiological or behavioural.
Selection acts on phenotype, but it changes the frequency of alleles.
Populations evolve, individuals do not: an individual cannot change its genes in response to its environment.

The term fitness refers to reproductive success, meaning the number of fertile offspring produced, not strength. Related lessons deal with how isolated populations diverge (speciation) and the evidence that supports evolution.

Section 6 of 9Data and graph skills

Questions often show a bar chart or histogram of a phenotype before and after a change in the environment.

1
Describe the shift in the mean, any change in range, and name the type of selection.
2
Then explain, using the sequence: variation, selection pressure, differential survival, reproduction, increase in allele frequency.
3
If asked to calculate a selection effect, work with percentages or proportions and quote the values.
4
A chi-squared or t-test can be used to test whether a difference between two samples is significant.

Section 7 of 9Exam-style questions

  1. Explain how natural selection could lead to an increase in the proportion of antibiotic-resistant bacteria in a population. [4]4 marks

  2. A population of beetles has a mean body length of 12 mm. Over many generations the mean rises to 15 mm after a predator that targets small beetles arrives. Name the type of selection and explain. [3]3 marks

  3. In a sample of 400 moths, 36 are pale. After a period of pollution, 400 are sampled and 12 are pale. Calculate the percentage of pale moths in each sample and the percentage change. [3]3 marks

Section 8 of 9Key terms

TermMeaning
EvolutionChange in the heritable characteristics of a population over generations.
Natural selectionDifferential survival and reproduction of individuals due to their phenotypes.
AdaptationAn inherited feature that increases fitness in a given environment.
Selection pressureAn environmental factor that affects survival or reproduction.
FitnessReproductive success of an individual.
Allele frequencyThe proportion of a particular allele in a gene pool.
Directional selectionSelection favouring one extreme phenotype.
Stabilising selectionSelection favouring the mean and removing extremes.

Section 9 of 9Exam technique

1
Saying organisms change because they need to. Mutations are random; the environment selects among the existing variation.
2
Writing that individuals adapt or evolve. It is populations that evolve over generations.
3
Using 'survival of the fittest' to mean strongest. Fitness is reproductive success.
4
Forgetting to link survival to passing on alleles; survival without reproduction has no evolutionary effect.

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