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Speciation

Speciation is the evolution of a new species. Some members of a population within a single species may evolve divergently (i.e. with different characteristics) leading to the emergence of a new species.

Speciation requires genetic divergence within a species. Genetic divergence occurs when the gene pool within a single species becomes segregated.

Segregation occurs when gene flow within a species is disrupted. This means that members of different subgroups of a population do not interbreed (i.e. reproductive isolation).

Reproductive isolation results in alleles (different versions of a gene) no longer being exchanged through the entire population. As a result, the frequencies of certain alleles are not the same across subgroups.

Some of these subgroups develop characteristics that are so distinct from the original species that they are unable to produce fertile offspring if they interbreed. These subgroups are therefore considered to be separate species.

Reproductive isolation is vital for speciation. It leads to genetic divergence by disrupting gene flow.

Reproductive isolation can be divided into two broad categories: Pre-zygotic (before fertilisation) and post-zygotic isolation (after fertilisation) isolation.

Pre-zygotic isolation can be further divided into:

  • Pre-mating isolation is the prevention of mating. This can be due to:

    • Geographic isolation (physical barriers)
    • Temporal isolation (populations breed at different times)
    • Behavioural isolation (populations may have different mating rituals)
  • Post-mating isolation occurs after mating but before gamete fusion. It is due to:

    • Mechanical/physiological isolation (physiology of the organisms prevents gamete transfer)
    • Gametic isolation (gametes fail to fuse)

Post-zygotic isolation isolation is due to:

  • Hybrid inviability (embryos or hybrids die before they can reproduce)
  • Hybrid sterility (hybrids are infertile)
  • Hybrid breakdown (first-generation hybrids are fertile but subsequent hybrids generations are either inviable or infertile

Allopatric speciation occurs when two populations of a single species become geographically isolated.

The drying of river systems between mountain lakes can lead to geographic isolation between the fish in the lakes.

As the two populations are spatially separated, the two populations evolve independently.

Over time these differences may become so great that if members of the two populations meet again they are unable to interbreed.

Geographic isolation as a mechanism for speciation was studied in fruit flies.

Fruit flies from a single (interbreeding) population were taken and divided into two separate populations. One population was fed a maltase-based diet, while the other was fed a starch-based diet.

The different diets created different selection pressures in the two geographically isolated populations.

After several generations the two populations were given the opportunity to interbreed. The fruit flies clearly preferred breeding with flies from their own population.

Allopatric speciation
Allopatric speciation

Sympatric speciation is the evolution of new species from an ancestor without geographic isolation.

Most speciation is allopatric. However, sympatric speciation is possible when genetic differences directly affect the probability of mating.

Changes that could lead to sympatric speciation include:

  • The time when organisms are fertile (temporal separation)
  • Changes that make organisms of the same population unable to mate with each other (physiological separation)

    Small changes in the structure of a flower could prevent it from being pollinated by certain insects. This isolates the flower from the rest of the population.

  • Mating signals (behavioural separation):

    If a few members of a songbird species evolve a slightly different birdsong, it may isolate them from the rest of the population.

Sympatric speciation may explain why there are so many species of song-bird even though song-birds can easily pass most geographic obstacles.

A polyploid organism has more than two sets of chromosomes. Many plant species contain additional sets of chromosomes.

Animal polyploids are inviable. Polyploid plants can usually grow and survive but they cannot reproduce with their parent population. As a result, they have to either self-fertilise or reproduce with other polyploids.

Polyploidy results in instant speciation as polyploid offspring are unable to produce viable offspring with their parent population.

There are over ten species of wheat in cultivation. These have diverged through polyploid speciation. Bread wheat is hexaploid.
There are over ten species of wheat in cultivation. These have diverged through polyploid speciation. Bread wheat is hexaploid.

Eukaryotic speciation always involves two distinct but related phenomena: genetic divergence and reproductive isolation.

Genetic divergence leads to reproductive isolation. Furthermore, reproductive isolation reinforces genetic divergence.

A small degree of reproductive isolation is sufficient to disrupt or at least reduce gene flow, which leads to some genetic divergence.

This leads to more reproductive isolation and even more genetic divergence, eventually resulting in speciation.

Speciation is a self-reinforcing circular process based on the interaction of reproductive isolation and genetic divergence.

If we use the biological species concept to define a species, speciation can be seen as the evolution of reproductive isolation.