SpeciationAQA A-Level Biology: Revision notes
Section 1
Species and reproductive isolation
A species is a group of organisms that can interbreed to produce fertile offspring. A species exists as one or more populations. If two groups can mate but produce sterile offspring (as horses and donkeys do, producing mules), they are separate species, because genes cannot pass between them.
Two populations become reproductively isolated when there is no gene flow between them. Without gene flow their gene pools evolve independently. Reproductive separation allows differences in their gene pools to accumulate.
Two organisms that can mate are not necessarily the same species. Offspring must also be fertile.
Section 2
How new species arise
New species arise by a sequence of steps:
- Populations of one species become reproductively separated.
- Mutations occur independently in each population, producing new alleles.
- The populations experience different selection pressures, so different alleles are favoured.
- Allele frequencies and phenotype frequencies change differently in each population.
- Over many generations the differences accumulate in the gene pools.
- Eventually individuals from the two populations can no longer interbreed to produce fertile offspring. They are now separate species.
A full answer links isolation, no gene flow, different selection, changes in allele frequency, accumulation and inability to produce fertile offspring.
Section 3
Allopatric speciation
Allopatric speciation occurs when populations are separated by a geographical barrier, such as a sea, river, mountain range or desert. Examples include populations colonising separate islands.
The barrier prevents gene flow. If the environments on either side differ, different selection pressures act. Even if the environments are the same, mutation and chance will lead to differences. Over time the populations diverge until they cannot interbreed.
Section 4
Sympatric speciation
Sympatric speciation occurs when new species arise in the same area, with no geographical barrier. Isolation arises from other differences, for example:
- different breeding seasons or times of day
- different courtship behaviour or mating preferences
- different food sources or habitats within the area, so groups rarely meet
Once the groups stop interbreeding, there is no gene flow and the process continues as in allopatric speciation. Mutation and selection make the gene pools differ until fertile interbreeding is impossible.
Sympatric does not mean there is no isolation. There is reproductive isolation, but it is not caused by a physical barrier.
Section 5
Comparing the two
Similarities: both need isolation (no gene flow), both involve mutation and natural selection changing allele frequencies, both end when the populations cannot interbreed to produce fertile offspring.
Difference: in allopatric speciation the isolation is caused by a geographical barrier; in sympatric speciation the populations share the same area and isolation is caused by other differences such as breeding time or behaviour.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Speciation
- Two populations of ground squirrels were once part of one interbreeding population. A river then changed course and now separates them. The squirrels cannot cross the river. The two sides of the river have different vegetation and different climates.Explain how, over many generations, the two squirrel populations could become two separate species.2 marks
- In a meadow, an insect species feeds and mates on two plant species that grow in the same area. One plant species flowers in May and the other flowers in August. Insects that feed on the May-flowering plant breed in May and insects that feed on the August-flowering plant breed in August. Over many generations the two groups of insects have become genetically different.Explain how the different breeding times could lead to the formation of two species.2 marks
- Horses and donkeys can mate and produce offspring called mules. Mules are almost always sterile. Horses and donkeys are thought to have evolved from a common ancestral population whose descendants became separated and lived in different regions with different climates and vegetation.Explain why horses and donkeys are classified as separate species even though they can mate and produce offspring.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).