Speciation and hybridisation

Rik Palmans (Rik.Palmans@ping.be)
Sat, 14 Oct 1995 19:13:47 +0100

After reading the articles on hybrisation of different Nepenthes species, I
would like to add a few general notes on hybridisation. I was highly
inspired by: GRANT, Verne "The Evolutionary Process -- A Critical Study of
Evolutionary Theory" (ed. Columbia University Press, New York, 1991, ISBN

The 'goal' of speciation is the generation of a combination of adaptively
useful genes. Natural selection favours organisms that are better adapted
to the environment they live in. Speciation is the most rapid and efficient
means of fixing a new adaptive gene combination. Each species possesses its
own unique set of adaptive gene combinations which guarantee the best
adaptation for its niche or habitat. Reproductive isolating mechanisms
protect this adaptive useful combination from intermixing with others, what
would result in less adapted organisms.

In plants these isolating mechanisms can be spatial: two species growing in
separated and different habitats. If by any action the two species get
intermixed the hybrids are ecological inferior to the parents: they are
less adapted to each of both habitats their parents live in. The 'pure'
species will always be better adapted to the ecological conditions in its
specific habitat. So, even if the hybrids are fertile, they will disappear
by natural selection and the 'pure' species will in the long or thort term
be favoured and 'drive out' the hybrid.

In nature hybrids can only survive if there is not only a mixing of genes
but also a mixing of habitats. A few years ago, I had the opportunity to
study this phenomenon on Melandrium dioicum and Melandrium album -- red and
white campion. (Not cp's, but maybe the principle is the same.) The first
one is a species of undisturbed habitats like wood edges; the second is
typical for higly 'dynamic' habitats like disturbed road verges and sides
of arable fields. The two species hybridise very easily and the hybrids are
fertile. When the two grow in each others neighbourhood and there are
habitats between with more or less disturbance, that are to places where
one may find their hybrids: they are indeed typical for those mixed
habitats. The more a habitat resembles the habitat of the pure species, the
more the hybrids resemble one of the parent species.

It would be interesting to look if the same principle can be observed in
Nepenthes hybridisation (in nature). They seem to occur mainly in regions
where human disturbance destroyed or altered their habitats. This removes
the spatial and ecological reproductive barriers between different species
and can benefit the occurence of hybrids.

Rik Palmans, Voeren (Belgium)