Principios de Taxonomia

not reach the pupal stage. As the caterpillars cannot hibernate, they would perish. All
Swallowtails in Central Europe would become extinct if they all were to react to a
warm August in that way.
Therefore, one must conclude that these butterflies do not all react to a warm late
summer in the same way. Instead, their ability to produce a third generation cannot
be triggered by environmental conditions alone. Instead, the trivoltine Swallowtails
in Central Europe must be a genetically different population in comparison to the
bivoltine Swallowtails. Only the members of the trivoltine population hatch three
times a year. The members of the bivoltine population are genetically programmed
not to hatch in late August, even if it is warm (as in a Mediterranean environment).
This conclusion would explain why the species survives in Central Europe.
Another example is the Common Blue Butterfly (Polyommatus icarus) in Sweden
(Color Plate 1). This species consists of a univoltine population in the north and a
bivoltine population in the south of Sweden. Nygren, Bergstr€om, and Nylin (2008)
have shown that these two populations are genetically different in terms of a fairly
large number of traits. It is not possible to explain uni- or bivoltism as a reaction of
individuals to external climatic conditions.
The butterflies in the north have different metabolisms and significantly slower
development. They need more time to finish development of the egg, the caterpillar
and the pupal stage, as they must produce only a single generation. The butterflies in
the south develop faster because they must manage two generations per year.
Common Blues transferred from the north of Sweden to the south did not change
their behavior in reaction to the warmer climate. Instead, they maintained their slow
development and produced only one generation per year. The day-night rhythm and
other climatic conditions of the south, such as temperature, did not cause thenorthern
populations to transition into bivoltinism (Nygren, Bergstr€om, and Nylin, 2008).
These experiments clearly show that bivoltism and univoltism are genetically
driven. Several genetic differences characterize and differentiate the Common Blue
into two different populations. As with migratory and sedentary birds, this finding
again raises the question of whether univoltine and bivoltine populations of butterflies
would be genetically compatible if they were crossed. As clinal transition regions
exist in which univoltine and bivoltine individuals coexist, as with partially migratory
birds, it is tempting to assume that univoltine and bivoltine butterflies are different
morphs of the same species (Chapter 5). The frequency distribution of the morphs
differs between different geographic regions. In the northern populations, almost all
of the alleles correspond to univolism; in the southern populations, almost all of the
alleles correspond to bivolism, and in the overlapping geographic region, there are
populations where both morphs live side by side.
6.14
Speciation Genes, Pre- and Postzygotic Barriers
6.14 Speciation Genes, Pre- and Postzygotic Barriersj149
Reproductive incompatibility is the result of the mutual incompatibility of
the alleles of particular genes (Wu, Johnson, and Palopoli, 1998). Reproductive