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SPECIATION:

Unlike the previous modes, sympatric speciation doesnot require large-scale geographic distance to reducegene flow between parts of a population. How could a

randomly mating population reduce gene flow andspeciate? Merely exploiting a new niche mayautomatically reduce gene flow with individuals exploiting the other niche. This mayoccasionally 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 onlyon hawthornsbut today, these flies lay eggs on hawthorns (which are native toAmerica) and domestic apples (which were introduced to America by immigrants andbred). Females generally choose to lay their eggs on the type of fruit they grew upin, and males tend to look for mates on the type of fruit they grew up in. Sohawthorn flies generally end up mating with other hawthorn flies and apple fliesgenerally end up mating with other apple flies. This means that gene flow betweenparts of the population that mate on different types of fruit is reduced. This host shiftfrom hawthorns to apples may be the first step toward sympatric speciationinfewer than 200 years, some genetic differences betweenthese two groups of flies have evolved.

apple maggot flies apples hawthorns

Gene flow has been reduced between flies that feed on different food varieties,even though they both live in the same geographic area.

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However, biologists question whether this type of speciation happens very often. Ingeneral, selection for specialization would have to be extremely strong in order tocause the population to diverge. This is because the gene flow operating in arandomly-mating population would tend to break down differences between theincipient species.

Allopatric speciation is just a fancy name for speciation by geographic isolation,discussed earlier. In this mode of speciation, something extrinsic to the organismsprevents two or more groups from mating with each other regularly, eventuallycausing that lineage to speciate. Isolation might occur because of great distance or aphysical barrier, such as a desert or river, as shown below.

Allopatric speciation can occur even if the barrier is a little porous, that is, even if afew individuals can cross the barrier to mate with members of the other group. Inorder for a speciation even to be considered allopatric,gene flow between thesoon-to-be species must be greatly reducedbut it doesnt have to be reducedcompletely to zero.

Speciation in Plants

In terms of reproduction, plants have a lot more options than animals do. Manyplants can reproduce sexually, by fertilizing other individuals or themselves, andasexually, by creating clones of themselves through vegetative reproduction, whilemost animals only reproduce sexually.

Similarly, in terms of speciation, plants have more options than animals do. Twomodes of speciation are particularly common in plants:

Speciation by hybridization: For example, when Loren Rieseberg andcoworkers reconstructed the phylogeny of several sunflower species, they

found that several species had been formed by fertilizations between otherspecies. Often the hybrid offspring of such fertilizations are sterile, butoccasionally they are fertile and are reproductively isolated from theirparent species. In the latter case, a new species is formed.

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The sunflower speciesHelianthus anomalus was produced by the hybridization oftwo other sunflower species.

Speciation by ploidy changes: In terms of plant speciation, a ploidy changegenerally means multiplying the number of chromosomes the species has bysome number. So a species that normally has 18 chromosomes mightproduce a lineage that has 36 or 54 chromosomes. Ploidy changes are

common in plants and often produce a species that is reproductively isolatedand distinct from the parent species. For example, speciation in theseanemones involved a ploidy change.

Here we see two species of anemone flower and theirchromosomes. Changes in number of chromosomes, as hasoccurred in this genus, can cause speciation.