17.3.3

Speciation

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Overview of Speciation

Speciation is the where two new species arise from a single species. This happens when two populations are prevented from interbreeding.

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Reproductive isolation

  • If two populations are prevented from interbreeding, differences begin to accumulate in the two gene pools.
  • Gene flow is the movement of alleles between a population. When two populations are reproductively isolated, the gene flow of the populations is restricted.
  • The two populations are said to be reproductively isolated.
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Accumulation of differences

  • If the gene pools are reproductively isolated for an extended period of time, they will eventually accumulate enough genetic differences that the two populations can no longer interbreed.
  • If individuals from the two populations can no longer interbreed to produce fertile offspring, the two populations are considered separate species.
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Speciation

  • Speciation is when two (or more) populations have been reproductively isolated to produce two (or more) separate species.
  • This is how the thousands of species that exist today have been produced.

Allopatric Speciation

Allopatric speciation is a type of speciation where two (or more) populations are geographically isolated from each other.

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Geographical isolation

  • Populations of a species can split into two or more populations.
  • This may be caused by environmental changes (e.g. emergence of a river) or random events (e.g. a storm causing a population of birds to be carried to another island).
  • When two populations are physically separated, they are geographically isolated.
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Selection pressures

  • Populations in different environments will be exposed to different environmental conditions (e.g. climate, food availability).
  • The different environments will place different selection pressures on the populations.
  • The different selection pressures will drive natural selection in different directions.
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Allele frequencies

  • The different selection pressures on the populations cause the allele frequencies in the two populations to change.
  • If the populations continue to be geographically isolated for a long time, eventually the allele frequencies will change so much that if the populations are brought back together they can no longer interbreed.
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Speciation

  • Species are defined as a group of actually or potentially interbreeding individuals.
  • If two populations that have been geographically isolated can no longer interbreed when they are brought back together, they are considered to be two species.
  • This is allopatric speciation.

Sympatric Speciation

Sympatric speciation is a type of speciation where two (or more) populations are not geographically isolated from each other and involves reproductive isolation.

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Reproductive isolation

  • Scientists organise reproductive isolation into two groups: prezygotic and postzygotic.
    • Recall that a zygote is a fertilised egg: the first cell of the development of an organism that reproduces sexually.
  • So, a prezygotic barrier blocks reproduction from taking place such as barriers that prevent fertilisation when organisms attempt reproduction.
  • A postzygotic barrier occurs after zygote formation, such as organisms that don’t survive the embryonic stage and those born sterile.
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Reproductive isolation mutations

  • Sympatric speciation could be caused by mutations that influence many different factors:
    • Polyploidy - where a cell or organism has an extra set, or sets, of chromosomes. Polyploidy organisms are reproductively isolated from diploid organisms.
    • Mutations may influence the flowering times or mating times of individuals.
    • Mutations may alter the reproductive organs.
    • Mutations may alter mating behaviours.
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Prezygotic barriers

  • Differences in breeding schedules, called temporal isolation, can act as a form of reproductive isolation.
    • For example, two species of frogs inhabit the same area, but one reproduces from January to March, whereas the other reproduces from March to May.
  • In some cases, populations of a species move or are moved to a new habitat and take up residence in a place that no longer overlaps with the other populations of the same species.
    • This situation is called habitat isolation.
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Prezygotic barriers - 2

  • Behavioural isolation occurs when a specific behaviour prevents reproduction from taking place.
    • For example, male fireflies use specific light patterns to attract females. If a male of one species tried to attract the female of another, she would not recognize the light pattern.
  • Other prezygotic barriers work when differences in their gamete cells (eggs and sperm) prevent fertilization from taking place; this is called a gametic barrier.
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Postzygotic barriers

  • When fertilisation takes place and a zygote forms, postzygotic barriers can prevent reproduction.
  • Hybrid individuals in many cases cannot form normally in the womb and simply do not survive past the embryonic stages.
    • This is called hybrid inviability because the hybrid organisms simply are not viable.
  • In another postzygotic situation, reproduction leads to the birth and growth of a hybrid that is sterile and unable to reproduce offspring of their own.
    • This is called hybrid sterility.
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Speciation

  • The biological definition of a species is a group of individuals that can actually or potentially interbreed.
  • The emergence of a mutation that prevents two (or more) populations from interbreeding is called sympatric speciation.

Jump to other topics

1Cell Structure

2Biological Molecules

3Enzymes

4Cell Membranes & Transport

5The Mitotic Cell Cycle

6Nucleic Acids & Protein Synthesis

7Transport in Plants

8Transport in Mammals

9Gas Exchange

10Infectious Diseases

11Immunity

12Energy & Respiration (A2 Only)

13Photosynthesis (A2 Only)

14Homeostasis (A2 Only)

15Control & Coordination (A2 Only)

16Inherited Change (A2 Only)

17Selection & Evolution (A2 Only)

18Classification & Conservation (A2 Only)

19Genetic Technology (A2 Only)

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