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Sympatric speciation
 Unlike the previous modes, sympatric speciation does not require large-scale geographic distance to reduce
gene flow between parts of a population. How could a randomly mating population reduce gene flow and speciate?
Merely exploiting a new niche may automatically reduce gene flow with individuals exploiting the other niche.
This may occasionally happen when, for example, herbivorous insects try out a new host plant.
For example, 200 years ago, the ancestors of apple maggot flies laid their eggs only on hawthorns — but today,
these flies lay eggs on hawthorns (which are native to America) and domestic apples (which were introduced to
America by immigrants and bred). Females generally choose to lay their eggs on the type of fruit they grew up
in, and males tend to look for mates on the type of fruit they grew up in. So hawthorn flies generally end up
mating with other hawthorn flies and apple flies generally end up mating with other apple flies. This means that
gene flow between parts of the population that mate on different types of fruit is reduced. This host shift from
hawthorns to apples may be the first step toward sympatric speciation — in fewer than 200 years, some genetic
differences between these two groups of flies have evolved.
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| apple maggot flies |
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apples |
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hawthorns |
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Gene flow has been reduced between flies that feed on different food
varieties, even though they both live in the same geographic area. |
However, biologists question whether this type of speciation happens very often. In general, selection
for specialization would have to be extremely strong in order to cause the population to diverge. This
is because the gene flow operating in a randomly-mating population would tend to break down differences
between the incipient species.
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