Cambridge International A Level Biology Revision Guide

Cambridge International A Level Biology
Reproductive isolation can take very different
forms. Prezygotic (before a zygote is formed) isolating
mechanisms include:
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individuals not recognising one another as potential
mates or not responding to mating behaviour
animals being physically unable to mate
incompatibility of pollen and stigma in plants
inability of a male gamete to fuse with a female gamete.
Postzygotic isolating mechanisms include:
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failure of cell division in the zygote
non-viable offspring (offspring that soon die)
viable, but sterile offspring.
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Obviously, the last is the most wasteful of energy and
resources.
Investigating how reproductive isolation can arise is
not easy. The main difficulty is that this process takes
time. A speciation experiment in a laboratory would
have to run for many years. The evidence that we have
for the ways in which speciation can occur is almost all
circumstantial evidence. We can look at populations
of organisms at one moment in time, that is now, and
use the patterns we can see to suggest what might have
happened, and might still be happening, over long periods
of time.
Allopatric speciation
One picture that emerges from this kind of observation is
that geographical isolation has played a major role in the
evolution of many species. This is suggested by the fact
that many islands have their own unique groups of species.
The Hawaiian and Galapagos islands, for example, are
famous for their spectacular arrays of species of all kinds
of animals and plants found nowhere else in the world
(Figure 17.23).
Geographical isolation requires a barrier of some
kind to arise between two populations of the same
species, preventing them from mixing. This barrier
might be a stretch of water. We can imagine that a group
of organisms, perhaps a population of a species of bird,
somehow arrived on one of the Hawaiian islands from
mainland America; the birds might have been blown
off course by a storm. Here, separated by hundreds of
miles of ocean from the rest of their species on mainland
America, the group interbred. The selection pressures on
the island were very different from those on the mainland,
resulting in different alleles being selected for. Over time,
the morphological, physiological and behavioural features
of the island population became so different from the
Figure 17.23 Hibiscus clayi is found only on Hawaii, where it is
in danger of extinction.
mainland population that the two populations could no
longer interbreed. A new species had evolved.
You can probably think of many other ways in which
two populations of a species could be physically separated.
A species living in dense forest, for example, could become
split if large areas of forest are cut down, leaving ‘islands’ of
forest in a ‘sea’ of agricultural land. Very small or immobile
organisms can be isolated by smaller-scale barriers.
Speciation which happens like this, when two
populations are separated from each other geographically,
is called allopatric speciation. ‘Allopatric’ means ‘in
different places’.
However, it is also possible for new species to arise
without the original populations being separated by
a geographical barrier. This is known as sympatric
speciation.
Sympatric speciation
Perhaps the commonest way in which sympatric speciation
can occur is through polyploidy.
A polyploid organism is one with more than two
complete sets of chromosomes in its cells. This can happen
if, for example, meiosis goes wrong when gametes are