Evolution__3rd_Edition

412 PART 4 / Evolution and Diversity
The apple maggot fly has only
recently moved on to apples
The races on apples show some
isolation from the ancestral races
on hawthorn
But the example is incomplete
them. Most models of sympatric speciation suppose that natural selection initially
establishes a polymorphism, and then selection favors prezygotic isolation between
the polymorphic forms. “Host shifts” in a fly called Rhagoletis pomonella provide a case
study that may illustrate part of the process.
14.10.2 Phytophagous insects may split sympatrically by host shifts
Rhagoletis pomonella is a tephritid fly and a pest of apples. It lays its eggs in apples
and the maggot then ruins the fruit, but this was not always so. In North America,
R. pomonella’s native larval resource is the hawthorn. Only in 1864 were these species
first found on apples. Since then it has expanded through the orchards of North
America, and has also started to exploit cherries, pears, and roses. These moves to new
food plants are called host shifts. In the host shift of R. pomonella, speciation may be
happening before our eyes.
The R. pomonella on the different hosts are currently different genetic races. Females
prefer to lay their eggs in the kind of fruit they grew up in: females isolated as they
emerge from apples will later choose to lay eggs in apples, given a choice in the laboratory.
Likewise, adult males tend to wait on the host species that they grew up in, and
mating takes place on the fruit before the females oviposit. Thus there is assortative
mating: male flies from apples mate with females from apples, males from hawthorn
with females from hawthorn.
The races are presumably about 140 generations old (given that they first moved on
to apples nearly one and a half centuries ago). Is this long enough for genetic differences
between the races to have built up? Gel electrophoresis shows that the two races have
evolved extensive differences in their enzymes. They also differ genetically in their
development time: maggots in apples develop in about 40 days, whereas hawthorn
maggots develop in 55–60 days. This difference also acts to increase the reproductive
isolation between the races, because the adults of the two races are not active at the
same time.
Apples and hawthorns differ and selection will therefore probably favor different
characters in each race; this may be the reason for their divergence. If it is, selection may
also favor prezygotic isolation and speciation. If flies from the different races are put
together in the lab, however, they mate together indiscriminately. Either reinforcement
has not operated when it might have been expected, or, alternatively, the differences in
behavior and development time in the field may be enough to reduce interbreeding
to the level natural selection favors. Selection would then not be acting to reinforce
the degree of prezygotic isolation. We do not know which interpretation is correct;
we need to know more about the forces maintaining the genetic differences between
the races. Once again, the evidence for reinforcement is the weak point in a theory of
speciation.
In the case of host shifts, we can be practically certain that the initial host shift, and
formation of a new race, has happened in sympatry. The shift took place in historic
time. However, it is not a full example of sympatric speciation because the races have
not fully speciated. Indeed, we do not know whether they will, or whether the current
situation, with incomplete speciation, is stable.
..