Sympatric speciation

Sympatric speciation is the formation of a new species from a population that stays in the same place. No river, mountain range or stretch of sea splits the population. Instead, something else stops two groups within it from interbreeding, such as a change in chromosome number (polyploidy) or a difference in breeding behaviour. Once gene flow stops, the two gene pools change separately until the groups can no longer produce fertile offspring together.

What is sympatric speciation?

In sympatric speciation the new species and the one it came from live side by side. OpenStax defines it as speciation occurring within a parent species that remains in one location. The alternative is allopatric speciation, where a physical barrier separates populations first.

Both end the same way. AQA's A-level specification (3.7.3) puts it like this: reproductive separation of two populations lets differences build up in their gene pools, and a new species arises when those differences stop members of the two populations interbreeding to produce fertile offspring. AQA lists allopatric and sympatric speciation by name.

How sympatric speciation happens

1. Variation. Mutation creates new alleles, and meiosis and random fertilisation shuffle them, so individuals in the population differ.
2. Reproductive isolation in one place. A group within the population stops breeding with the rest, through polyploidy, a change in courtship, a different breeding time or a different part of the habitat.
3. No gene flow. Alleles no longer pass between the two groups.
4. The gene pools diverge. Different mutations arise in each group, natural selection may favour different alleles, and genetic drift changes allele frequencies by chance.
5. A new species. The groups become so different genetically that they can't interbreed to produce fertile offspring, even though they still share the same area.

Step 2 is the hard part. Without a barrier, interbreeding tends to mix the gene pools back together, so sympatric speciation needs a mechanism that isolates a group quickly or keeps it isolated while differences build up. The Natural Selection Simulator shows how selection shifts allele frequencies in a population over generations.

Polyploidy: speciation in one generation

The clearest route to sympatric speciation is polyploidy: having more than two complete sets of chromosomes. It is common in plants and rare in animals. OpenStax reports that more than half of all plant species studied trace back to a species that evolved through polyploidy.

Autopolyploidy

Normally meiosis halves the chromosome number, so a diploid (2n) plant makes haploid (n) gametes. If the chromosomes fail to separate, every chromosome ends up in one cell and the gamete is diploid. Two diploid gametes fusing, for example when the plant self-pollinates, give a tetraploid (4n) plant. A tetraploid makes diploid gametes and breeds successfully with other tetraploids, but not with its diploid parents.

A triploid has three copies of each chromosome, so they can't all line up in pairs during meiosis. A 2014 study in New Phytologist notes that triploids often have strongly reduced fitness because of these meiotic disturbances and low fertility. So the tetraploids are cut off from the diploids straight away: in one generation, in the same field, a new species has started.

Allopolyploidy

Allopolyploidy starts with two different species. Their hybrid is usually sterile, because chromosomes from different species can't pair in meiosis. If a hybrid's chromosome sets double, every chromosome has a partner again, and the plant becomes fertile, but only with others like it. OpenStax names cultivated wheat, cotton and tobacco as allopolyploids.

Worked example: counting chromosomes

A diploid plant species has 2n = 16. A tetraploid arises from it. How many chromosomes are in the tetraploid's body cells and gametes, and in offspring from a cross with the diploid?

1. One set (n) is 16 ÷ 2 = 8 chromosomes.
2. The tetraploid has four sets: 4 × 8 = 32 chromosomes in each body cell.
3. Meiosis halves that, so its gametes have 32 ÷ 2 = 16.
4. A diploid gamete has 8. Crossing the two gives 16 + 8 = 24 chromosomes: three sets, a triploid, which is usually sterile.

Other ways to isolate a group in one place

In animals, sympatric speciation relies on barriers that stop mating even though the animals live together. OpenStax describes these prezygotic barriers:

These are examples of the barriers, not of speciation in progress. The best known animal case is the cichlid fish of Lake Victoria in Africa, where OpenStax reports researchers found hundreds of sympatric speciation events over a short period of time. AQA also lists disruptive selection, which favours both extremes of a trait: if individuals at each extreme also mate mostly with each other, the population can split.

Allopatric vs sympatric speciation

Writing a speciation answer

A strong answer is a chain of linked ideas. Use the specification wording and keep the order:

• Name the isolating mechanism and say there is no gene flow between the groups, even though they share one area.
• Say there is variation because of mutation.
• Say different alleles are selected in each group, or change by genetic drift, so allele frequencies change.
• Say the gene pools become so different that the groups can't interbreed to produce fertile offspring.

Common mistakes

• Saying individuals change. Populations evolve; write about allele frequencies in gene pools, not an animal adapting.
• Missing "fertile". Two species can sometimes produce offspring. The test is whether those offspring are fertile.
• Calling any isolation sympatric. If a barrier such as a river or mountains splits the population, it is allopatric.
• Saying polyploids can't reproduce. Tetraploids breed well with each other. It is the cross with the diploid parent that fails.
• Forgetting the time scale. Polyploidy can isolate a group in one generation; other routes take many.

Evolution and speciation sit in the second year of A-level Biology, so practise them on today's A-level Biology: Year 2 short timed paper. For the cell division behind polyploidy, see the genetics and DNA course, which compares mitosis and meiosis.

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