Evolution__3rd_Edition

408 PART 4 / Evolution and Diversity
A new hybrid species must
overcome reproductive problems
The Dobzhansky–Muller process
works in allopatry
and chromosomal complement that shows similarities to I. fulva and I. hexagona, but I.
brevicaulis also contributed genes to its origin. Genetic markers suggest that I. nelsonii
mainly resulted through repeated backcrossing into I. fulva, rather than from one
simple hybridization event in the manner of the Kew primrose (Primula kewensis). I.
nelsonii is not polyploid.
We have concentrated on the problem of how a new reproductively isolated hybrid
genotype can evolve. But some further problems are likely to arise in the evolutionary
transition from a rare new hybrid genotype to a full hybrid species. One is finding a
mate. When a fertile polyploid hybrid first arises, it is one hybrid (or perhaps one of a
small number) within two large populations of the parental species. It may simply be
infertile with both parental species because of the chromosomal difference; or the situation
may be worse if the parental species’ pollen fertilizes the hybrid’s eggs and they
then fail to develop or reproduce. The hybrid’s interfertility with other hybrids like
itself can only be expressed if other hybrids exist. Natural selection on the hybrid therefore
has a kind of positive frequency dependence (Section 5.13, p. 127): when it is rare
its fitness is lower because of the difficulty of finding a mate. It may have to reach some
threshold of abundance before natural selection favors it. (Strictly speaking, this is
number, rather than frequency, dependence; but there is frequency-dependent selection
in at least an informal sense.)
This problem is probably the reason why hybrid speciation has been much commoner
in some groups of plants than others. A new hybrid can more easily cross the
difficult transition stage, in which it is rare, if it has alternative reproductive options
besides sexual cross-fertilization. Stebbins (1950) has shown that hybrid speciation is
commoner in groups in which asexual reproduction or self-fertilization are possible.
Iris nelsonii, for example, can reproduce asexually by rhizome runners, in addition to
sexual cross-fertilization via pollen that is carried by bumblebees.
Hybrid speciation is a distinctive contribution to evolutionary biology that has come
from the study of plants. Hybrid speciation is probably commoner in plants than in
animals (though animal examples do exist, as Arnold’s (1997) book shows). It is certainly
much better understood in plants than in animals, and practically all our understanding
of the process has come from plants.
14.8 Speciation may occur in non-allopatric populations,
either parapatrically or sympatrically
In the theory we have looked at so far, reproductive isolation can evolve either as
an incidental consequence (or by-product) of divergence between two populations
or by reinforcement. What is the relation between these theories and the allopatric,
parapatric, and sympatric theories of speciation (see Figure 14.1)? Both prezygotic and
postzygotic isolation can evolve as by-products of divergence. Postzygotic isolation
evolves according to the Dobzhansky–Muller theory, and that theory is closely tied to
the allopatric theory of speciation. The Dobzhansky–Muller theory requires that
separately advantageous, but jointly disadvantageous, genes be fixed in two populations.
This is only likely to happen in separately evolving (and therefore allopatric)
..