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Human Biology (Year 12) - Evolutionary Mechanisms

Ben Whitten

What is speciation?

Speciation occurs when a single population becomes two separate, genetically non-identical populations that are unable to interbreed due to changes that produce physical, biological or behavioural barriers. There are three processes that work in conjunction in speciation.

  1. Microevolution: Natural selection favours phenotypes that are best suited to the environment, and as populations change over time, their gene pool accumulates small changes in response to natural selection; called microevolution

  2. Accumulation: Eventually a population accumulates so much change that a new species can be identified, which leads to speciation

  3. Macroevolution: A rapid series of speciation events leads to the development of a whole new collection of species, genera or higher classification; called macroevolution

Speciation follows the biological species concept, in which species are defined as a genetically isolated group that can only interbreed with itself.

The two forms of speciation are sympatric speciation and allopatric speciation, and for these to occur, different isolating mechanisms take place.

What are the different isolating mechanisms?

Reproductive Isolating Mechanisms

Isolating mechanisms separate two groups and prevent them from producing both fertile and viable offspring. Genetic isolation can occur either prior to or following physical isolation, however in both cases, once isolation has occurred, the two groups acquire different phenotypes. Natural selection then works on the new population members so they become adapted to their new environments.

Pre-productive Isolating Mechanisms

Some isolating mechanisms prevent organisms from being able to interact and reproduce, and these include;

  • Geographic mechanisms (separation via mountains, sea or landmasses)

  • Temporal mechanisms (breeding during different seasons or times of day)

  • Behavioural mechanisms (differing courtship patterns)

  • Morphological mechanisms (physically different reproductive mechanisms)

Post-productive Isolating Mechanisms

If a horse and a donkey for example mated, they would not be able to produce viable offspring as their chromosomes would not match up correctly in meiosis (64 and 62 respectively (diploid)). These mechanisms don't prevent mating but prevent the formation of viable, fertile offspring, including;

  • Gamete mortality (gametes do not survive)

  • Zygote mortality (zygote is formed but do not survive)

  • Hybrid sterility (adult offspring are formed but are infertile)

How does allopatric speciation work?

Allopatric speciation is speciation that occurs due to geographic isolation. The gene flow is disrupted as populations become physically separated. This may be due to selection pressures or genetic drift. Different geographic conditions leading to the separation of species includes;

  • Water (terrestrial organisms)

  • Land (aquatic organisms)

  • Mountains

  • Continental drift

  • Rising sea levels

  • Climate change

An acronym for remembering speciation, in general, is VISS, meaning variation, isolation, selection and speciation.

  1.  Variation: There is variation present in a population which has occurred due to processes in meiosis, such as independent assortment, crossing over and random fertilisation, as well as things such as mutation, errors in meiosis, errors in DNA replication etc.

  2. Isolation: A geographic change separates members of a population into more than one group; such changes could include the formation of a new mountain range or a new waterway. The geographic barrier is actively preventing gene flow from occurring.

  3. Selection: The selection pressures on either side of the barrier are different to one another. The populations on either side may become different due to the process of natural selection, or they may become different due to mutation/genetic drift. Different gene variations will accumulate over time.

  4. Speciation: The populations become so different that members of the different populations can no longer interbreed with one another to produce fertile offspring, and hence, speciation has occurred (leading to the creation of new species).

How does sympatric speciation occur?

Sympatric speciation is speciation that occurs without any physical or geographical isolation; the evolution of two or more new species from the same population within the same place.

This may occur due to individuals feeding off of different things or choosing specific mates, and may occur due to the post- or pre-productive isolating mechanisms.

This form of speciation is rare and lacks many clear examples.

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