How do new species arise, and what is the difference between allopatric and sympatric speciation?
Explain the concept of a species and the mechanisms of allopatric and sympatric speciation
A focused answer to the H2 Biology Inheritance and Evolution outcome on speciation. The biological species concept, reproductive isolation, allopatric speciation by geographical separation and sympatric speciation without it, and how isolation plus selection produce new species.
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What this dot point is asking
SEAB wants you to explain what a species is (the biological species concept), the central role of reproductive isolation, and the two main mechanisms of speciation: allopatric (with geographical separation) and sympatric (without it). It builds directly on natural selection and the sources of variation.
The answer
What is a species?
Under the biological species concept, a species is a group of organisms that can interbreed to produce fertile offspring and that is reproductively isolated from other such groups. Two populations are different species once they can no longer interbreed successfully.
Reproductive isolation
Speciation requires reproductive isolation: something that stops gene flow between two groups so they can diverge. Isolating mechanisms include geographical barriers, different breeding seasons or behaviours, and incompatibilities that prevent fertile offspring.
Allopatric speciation
In allopatric speciation a population is divided by a geographical barrier (a mountain range, river or sea). Gene flow stops; the two groups face different environments, accumulate different mutations, and are shaped by different selection pressures. Over many generations they diverge so far that they can no longer interbreed even if reunited: they are now separate species.
Sympatric speciation
In sympatric speciation new species arise without geographical separation, while the groups still share an area. Reproductive isolation develops by other means, such as differences in breeding time or behaviour, or chromosome changes (for example polyploidy in plants) that prevent successful breeding between the groups.
Examples in context
Example 1. Island finches. Finches colonising separate islands diverged in beak shape as each population adapted to different food sources, and eventually became distinct species. This is a classic case of allopatric speciation driven by isolation and different selection pressures.
Example 2. Polyploidy in plants. A doubling of chromosome number can instantly prevent a plant from breeding with its parent population while still allowing it to breed with others like itself, producing a new species in the same location. This is a route to sympatric speciation unique to plants.
Try this
Q1. State the biological definition of a species. [1 mark]
- Cue. A group of organisms that can interbreed to produce fertile offspring and that is reproductively isolated from other such groups.
Q2. Explain why reproductive isolation is necessary for speciation. [2 marks]
- Cue. It stops gene flow between the two groups, allowing them to accumulate different genetic changes independently until they can no longer interbreed.
Q3. State the key difference between allopatric and sympatric speciation. [1 mark]
- Cue. Allopatric speciation involves a geographical barrier separating the populations, whereas sympatric speciation occurs without geographical separation.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original6 marksExplain how a single population can give rise to two separate species by allopatric speciation.Show worked answer →
Examiners want geographical isolation, divergence and reproductive isolation.
Allopatric speciation begins when a population is divided by a geographical barrier, such as a mountain range, a river or the sea, so that the two groups can no longer interbreed. Gene flow between them stops.
The two groups now experience different environments and selection pressures, and they accumulate different mutations. Over many generations, natural selection and chance change the allele frequencies of each group in different directions, so the two groups diverge genetically.
Eventually the two groups become so genetically different that even if they were brought back together they could no longer interbreed to produce fertile offspring. They are then reproductively isolated and are classified as two separate species.
Markers reward the geographical barrier preventing gene flow, divergence through different selection pressures and mutations, and the development of reproductive isolation defining the two as separate species.
Original4 marksDistinguish between allopatric and sympatric speciation, and explain what they have in common.Show worked answer →
The answer should contrast the two and identify the shared requirement.
Allopatric speciation occurs when populations are separated by a geographical barrier, so they are physically prevented from interbreeding while they diverge.
Sympatric speciation occurs without geographical separation: the populations live in the same area but become reproductively isolated by other means, such as differences in breeding season, behaviour, or chromosome changes that prevent successful breeding.
What they have in common is that both require reproductive isolation between the diverging groups, so that gene flow is prevented and the groups can accumulate enough genetic differences to become separate species.
Markers reward the geographical barrier in allopatric, the absence of one in sympatric with an alternative isolating mechanism, and the shared requirement of reproductive isolation preventing gene flow.
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