D.2b: Species and Speciation: Allopatric Speciation

02/09/2013 § Leave a comment

First alleles, now species. What is a species? I have two definitions for you, buddy, so listen up. The morphological definition of a species is a type of living organism with fixed characteristics, which distinguish it from other species. However, there were problems with this definition (as there are problems in many definitions). For example, members of one species have common features because they interbreed with each other. Also, when one species’ characteristics are different from those of another, it is because the two species are evolving separately. Interbreeding is than an important part of species and so the other definition – the biological definition – of a species is a group of actually or potentially interbreeding populations, with a common gene pool, which are reproductively isolated from other such groups.

There are some special cases when it comes to the biological definition of a species, such as:

  • some species don’t interbreed because they only reproduce asexually
  • some species hybridise; pairs of species that are definitely different will interbreed
  • some species are so similar in physical appearance that they can’t be told apart and are called sibling species.

When a new species is formed, that’s speciation – basically when two different species arise from a pre-existing species. They will then split up and isolate themselves from the other species and continue to evolve separately so each species has a different environment natural selection can act on. There is allopatric and sympatric speciation. Allopatric speciation occurs because of a geographic separation of species. For example, lava lizards in the Galápagos were physical isolated from each other (geographical isolation) and therefore couldn’t interbreed which resulted in the formation of new species. Speciation, if you will. Sympatric speciation is when two different species live in the same geographical area but simply don’t interbreed, like the apple maggot fly. The fly, which previously laid its eggs on hawthorn fruit, began to lay them on apples too. Because the fruits ripen at different times in the year, two different strains of adult flies emerged.

And finally, polyploidy is a cause of hybrid infertility. If a species has a certain number of chromosomes (an odd number typically) and is a triploid, meiosis would fail, therefore that species or that individual would be infertile and sterile. Meiosis only works with diploids and tetraploids (even numbered chromosomes).


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a) Identify three morphological differences between the two species of treecreeper shown in Figure 8.

C. familiaris has a white stripe on the side of its head while C. brachydactyla does not. C. brachydactyla’s beak is narrower and longer than C. familiaris. C. familiaris back claw is longer than C. brachydactyla’s back claw.

b) Discuss what makes these two different species.

C. brachydactyla and C. familiaris are examples of sympatric speciation. This means that though they live in the same geographic location, they simply don’t interbreed with each other because they have different behavioural patterns and most likely live in different niches in their environment. In this case, the two species don’t normally hybridise and continue to evolve separately.

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