10.3 Gene pools and speciation
Reproductive isolation of gene pools can come about in various ways.
Lakes can dry up and split in smaller lakes so aquatic life forms become seperated and subject to different environmental and selection pressures. Mountain ranges can form, glaciers and rivers can change their courses. Whole continents can drift apart. This means that isolated populations can no longer breed with each other.
These can be caused by preferences for a particular type of food, a change in the timing of breeding, different courtship, mate recognition and other mating rituals.
An example of how feeding behaviour has led to gene pools splitting apart is the so called Hawthorn fly Rhagoletis pomonella. Some of these flies started to infest the newly introduced apple in North America and have begun to form a new gene pool with distinct chromosomal changes being observed.
This development is described in detail by Feder et al (2001)
Evidence suggests that the apple maggot, Rhagoletis pomonella (Diptera: Tephritidae) is undergoing sympatric speciation (i.e., divergence without geographic isolation) in the process of shifting and adapting to a new host plant. Prior to the introduction of cultivated apples (Malus pumila) in North America, R. pomonella infested the fruit of native hawthorns (Crataegus spp.). However, sometime in the mid-1800s the fly formed a sympatric race on apple. The recently derived apple-infesting race shows consistent allele frequency differences from the hawthorn host race for six allozyme loci mapping to three different chromosomes. Alleles at all six of these allozymes correlate with the timing of adult eclosion, an event dependent on the duration of the overwintering pupal diapause. This timing difference differentially adapts the univoltine fly races to an 3- to 4-week difference in the peak fruiting times of apple and hawthorn trees, partially reproductively isolating the host races.
Feder et al (2011) Evidence for Inversion Polymorphism Related to Sympatric Host Race Formation in the Apple Maggot Fly, Rhagoletis pomonella, http://www.genetics.org/cgi/content/full/163/3/939 , 14.01.2011
Temporal (or time related) barriers
This can come about because some members of the species breed at different times of the year.