different species, partly during the process of speciation, and partly after the repro-
ductive barriers evolved. Because genetic differences continue to accumulate long
after two species achieve complete reproductive isolation, some of the genes, and
even some of the traits, that now confer reproductive isolation may not have been
instrumental in forming the species in the first place. Such information can be
obtained by studying populations that have achieved reproductive isolation only
very recently.Prezygotic barriers
In many plants and animals, prezygotic barriers are the most important isolat-
ing mechanisms. There are many kinds of barriers, depending on the biology of
the organism (FIGURE 9.9). Species may be temporally isolated by mating at dif-
ferent times of year, or even in different years. Ecological isolation results when
ecological differences, for example habitat preference, contribute to genetic bar-
riers [64, 93]. For example, two Japanese species of herbivorous ladybird beetles
(Henosepilachna) feed on different genera of host plants (Cirsium and Caulophyl-
lum). Each species mates exclusively on its own host plant, and this ecological seg-
regation appears to be the only barrier to gene exchange [41]. Sexual isolation is
an important barrier to gene flow among sympatric species of animals that fre-
quently encounter each other but simply do not mate. Commonly, females will not
respond to inappropriate male vocalizations or other display signals. Many birds,
fishes, and jumping spiders are sexually isolated by visual signals. In many groups
of animals, sexual isolation is based on differences in sex pheromones.FIGURE 9.9 Prezygotic isolation takes many forms, illustrated
by some species that have been extensively studied. (A–C)
Three modes of prezygotic isolation. (A) Seasonal isolation: the
band-rumped storm-petrel (Oceanodroma castro) includes two
genetically different populations that mate at different times of year
[28]. (B) Temporal isolation: related species of periodical cicadas
(Magicicada) have either 17- or 13-year life cycles, and rarely
emerge in the same year [10]. (C) Ecological isolation: closely re-
lated species of ladybird beetles (Henosepilachna) feed and mateon different species of plants [41]. (D–F) Examples of sexual isola-
tion based on different sensory modalities. (D) Female Physalae-
mus frogs respond almost exclusively to the calls of conspecific
males [88]. A calling male P. pustulosus is shown here. (E) In moths
and many other animals, sexual isolation is based on different
chemical signals. Two forms of the European corn borer (Ostrinia
nubilalis) are strongly isolated by responses of males to different
female sex pheromones [85]. (F) Males of Heliconius pachinus rec-
ognize conspecific females by their wing color pattern [48].Futuyma Kirkpatrick Evolution, 4e
Sinauer Associates
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