Natural selection and speciationEdexcel A-Level Biology B: Revision notes
Section 1
Variation and selection pressures
Members of a population differ from one another. This variation can be genetic (different alleles, produced originally by mutation and shuffled by meiosis and sexual reproduction) or environmental. Only genetic variation can be inherited.
Individuals that happen to have alleles that suit their environment are more likely to survive and reproduce. Factors that reduce survival or reproduction, such as predation, disease, competition and climate, are selection pressures.
Section 2
Natural selection
Natural selection brings about evolution, a change in allele frequencies in a population over generations:
- Variation exists in the population because of different alleles.
- A selection pressure acts, so individuals with advantageous alleles are more likely to survive.
- These survivors reproduce more successfully.
- They pass the advantageous alleles to their offspring.
- The frequency of these alleles increases in the next generation, and over many generations the population becomes better adapted.
A feature that increases the chance of survival and reproduction is an adaptation.
Selection acts on alleles that are already present. Organisms do not develop new alleles because they need them, and individuals do not evolve; populations do.
Section 3
Adaptations and niches
A niche is the role of a species in its ecosystem: where it lives, what it feeds on, when it is active and how it interacts with other species. Each species occupies a niche according to its adaptations:
- Anatomical adaptations: structural features, e.g. long hind legs, thick fur, sharp claws.
- Physiological adaptations: processes inside the body, e.g. producing concentrated urine, venom production, antibiotic resistance in bacteria.
- Behavioural adaptations: the way an organism acts, e.g. nocturnal feeding, courtship displays, hibernation.
Habitat is where an organism lives; niche is its whole way of life. Use these terms separately.
Section 4
Reproductive isolation and speciation
A species is a group of organisms that can interbreed to produce fertile offspring. Speciation is the formation of new species. It needs reproductive isolation, so that gene flow between populations stops and their allele frequencies diverge under different selection pressures and mutations.
Allopatric speciation: a geographical barrier (river, mountain, sea) separates populations. Each experiences different selection pressures, so they evolve differently until they can no longer interbreed.
Sympatric speciation: new species arise in the same area without a physical barrier. Reproductive isolation arises through, e.g., different breeding seasons (temporal), different courtship behaviour (behavioural), or changes in chromosome number (polyploidy, common in plants).
Section 5
The evolutionary race between pathogens and medicines
Pathogens such as bacteria, viruses and Plasmodium evolve quickly because they have short generation times and large populations. When a medicine is used it is a strong selection pressure, so resistant pathogens survive and reproduce. For example, MRSA is resistant to methicillin, and influenza viruses change so that vaccines must be updated.
New medicines are developed, resistance evolves again, and so on: an evolutionary race. Resistance spreads vertically (to offspring) and, in bacteria, horizontally on plasmids.
Resistance can be slowed by prescribing antibiotics only when necessary, completing courses, good hygiene and isolating infected patients, and using combinations of drugs.
Bacteria do not become resistant because of the antibiotic. Resistance alleles arise by random mutation, and the antibiotic selects for them.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Natural selection and speciation
- Rock pocket mice live in the Arizona desert. On pale sandy ground most of the mice have pale fur, but on areas of dark volcanic rock most have dark fur. Owls hunt the mice by sight at night. Fur colour is controlled by a single gene with a dark allele and a pale allele, and mice of both fur colours are born in both types of area.Explain how natural selection has led to a higher proportion of dark-furred mice on the dark rock.2 marks
- The kangaroo rat lives in North American deserts. It has long, powerful hind legs and a long tail, spends the hot day in a cool burrow, forages for seeds at night, and produces very small volumes of highly concentrated urine.Explain how the kangaroo rat's behaviour of spending the day in a burrow increases its chance of survival in the desert.2 marks
- About 10 000 years ago a river changed course and became a wide, fast-flowing barrier that ground squirrels cannot cross, splitting one population into two. The environment differs on the two sides: one side is dry scrubland and the other is woodland. The two populations now differ in body size and breeding season, and in captivity their members do not produce fertile offspring.Explain how the change in the course of the river could have led to the formation of two species of ground squirrel.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).