Evolution__3rd_Edition

..
. . . but also open to alternative
interpretations
Hybrids, after polyploidy, may form
a new species
CHAPTER 14 / Speciation 405
allopatric populations by bringing individuals sampled from them into the lab. The
figure would be some average for the range of values for particular pairs of populations.
Now imagine also that the geographic distributions of some of the populations
change, and some of the formerly allopatric populations become sympatric. If two
populations that become sympatric had already evolved a high amount of reproductive
isolation, then the two will propably continue to coexist. But if they happen to have
evolved a low level of isolation, the processes that we looked at in Section 14.6.2 will
start to operate. Either the rarer of the two populations will be lost, or gene flow
between the two populations will cause them to fuse into one. Either way, the population
pair is lost from the dataset. The only population (or species) pairs we are left with
in sympatry are the pairs with high isolation. Then the amount of isolation between
pairs of species living in sympatry will be high because the pairs with low isolation have
been lost, not because reinforcement has increased isolation.
The argument does not show that reinforcement has not operated in cases such
as Figure 14.10, it only shows that the evidence is inconclusive. Coyne & Orr (1989)
ingeniously subdivided the evidence in various ways, making a stronger case for reinforcement;
but their evidence is still explicable without it (Gavrilets & Boake 1998).
Therefore, the biogeographic evidence, like the evidence from artificial selection, is
currently inconclusive. Evolutionary biologists remain undecided about reinforcement.
Few would say that it never operates, but the theoretical and empirical case for
its importance is unconvincing. Of the two processes that can drive the evolution
of reproductive isolation a (i) divergence with isolation as a by-product, and (ii)
reinforcement a the first is well documented and is almost certainly important in
speciation, but the second is not well documented and its influence in speciation is
indeterminate.
14.7 Some plant species have originated by hybridization
We encountered the origin of plant species by hybridization in Section 3.6 (p. 53),
where we saw how a new species of primrose, and a natural species of Galeopsis,
were artificially produced by hybridization. Interspecies hybrids are largely sterile, usually
because the chromosome pairs, which consist of one chromosome from one
species and another chromosome from the second species, do not segregate regularly
at meiosis. In order for a new species to evolve, this sterility has to be overcome. One
famous mechanism is polyploidy. If the chromosome numbers are doubled, each
chromosome pair at meiosis contains two chromosomes from one species, and regular
segregation is restored. Polyploids arise naturally, by mutation, and may lead to the
evolution of a new species. The polyploid hybrids are interfertile among themselves,
but reproductively isolated (by the mismatch in chromosome numbers) from the
parental species; they are therefore well defined new species.
The simplest cases to identify are those, like Primula kewensis, in which the new
species is a simple 50 : 50 hybrid, produced from two parental species, with 50% of its
genes coming from one parental species and 50% from the other. Within the past century,
the natural evolution of four new species of this sort has been recorded, two in