Sympatric speciation and polyploidy

Sympatric speciation is speciation that occurs without any geographical barrier — the diverging populations live in the same place.

• Sym- means "together", -patric means "homeland".

It is harder than allopatric speciation, and understanding why is the point of this page.

The difficulty is gene flow. Individuals in the same area can meet and interbreed, so gene flow continually mixes any differences back together. For sympatric speciation to occur, something must reduce gene flow while the populations remain in contact.

• Disruptive selection favours both extremes of a trait and selects against the intermediate.
• If intermediates are at a disadvantage, selection also favours assortative mating — individuals preferring to mate with others like themselves — because a cross between the two extremes produces disadvantaged intermediate offspring.
• Assortative mating reduces gene flow between the two forms, allowing them to diverge further.
• Both conditions are needed. Disruptive selection alone does not cause speciation, because random mating keeps producing intermediates.

• Part of a population begins using a different habitat, food plant or host within the same area.
• If individuals also mate on or near that resource, those using different resources rarely meet, so gene flow falls without any geographical barrier.

Polyploidy is a condition in which an organism has more than two complete sets of chromosomes.

It arises when meiosis fails and chromosomes do not separate, producing gametes with the full diploid number rather than half.

Autopolyploidy — the extra sets come from the same species.

• A diploid (2n) individual produces an unreduced 2n gamete; fusion with another produces a tetraploid (4n) offspring.

Allopolyploidy — chromosome sets come from two different species.

• Two species hybridise, producing a sterile hybrid whose chromosomes cannot pair at meiosis.
• The chromosome number then doubles, so every chromosome suddenly has a partner. Meiosis works, and the plant becomes fertile.
• Allopolyploidy is important because it converts a sterile hybrid into a fertile new species.

This is the key reasoning, and exam questions turn on it. Consider a tetraploid (4n) plant arising in a diploid (2n) population:

• The tetraploid can breed with other tetraploids and with itself, producing fertile 4n offspring.
• If it crosses with a normal 2n plant, the offspring is triploid (3n).
• In a triploid, chromosomes exist in threes. At meiosis they cannot pair evenly into two equal sets, so gametes receive unbalanced chromosome numbers and are non-viable.
• The triploid is therefore sterile.

So the tetraploid is reproductively isolated from its own parent population in a single generation — postzygotic isolation exists immediately.

This is the only form of instant speciation, and the only common route to sympatric speciation. Everything else requires many generations.

Two further points:

• Polyploid plants can often self-pollinate or reproduce vegetatively, so a single individual can found a population without needing a mate of the same ploidy.
• Polyploids are frequently larger with bigger cells, flowers and fruit — which is why many crop plants are polyploid.

Polyploidy is common in plants but rare in animals, because most animals cannot self-fertilise, and because sex determination in many animals depends on chromosome ratios that polyploidy disrupts.

New Zealand's flora has an unusually high proportion of polyploid species, consistent with repeated hybridisation and polyploidy following colonisation of a geologically young, disturbed landscape.

• A polyploid often has larger cells and organs, and carries duplicate copies of every gene. Duplicates are advantageous because one copy can accumulate mutations and take on a new function while the other continues the original job.
• Allopolyploids combine the characteristics of two species, which may let them occupy conditions neither parent could — a new niche, and therefore reduced competition with both parents.
• Against this, a new polyploid is initially rare and cannot breed with the surrounding diploids, so it will die out unless it can self-pollinate or reproduce vegetatively. That is precisely why polyploidy succeeds in plants and rarely in animals.