13 June 2008

Is there a "zone of maladaptedness" between the sea and the land?

In an essay titled Does microevolution explain macroevolution?, Ernst Mayr, wrote:

What is so often forgotten in the discussion of evolutionary discontinuities is that they represent not only a structural discontinuity but also an ecological one. A new niche and, particularly, a new adaptive zone is often separated from the ancestral one by a pronounced gap. There is no well-adapted condition for the area between the two adaptive zones. Hence, when a new adaptive zone is first invaded, a zone of maladaptedness has to be crossed.
Ever since I read this passage about 2 months ago, I've been trying to understand what exactly Mayr meant by "zone of maladaptedness". I think he implied one of the following mutually exclusive meanings:

(1) There is a permanent transition zone between the ancestral and descendant zones where no species is ever well adapted.

(2) Initially, no ancestral species is well adapted to the transition zone and only after they have evolved preadapations to life in the transition zone, can they enter it.

The problem is Mayr's 2nd from the last sentence seems to support the 1st meaning, while his last sentence seems to support the 2nd meaning.

Furthermore, in the next paragraph, Mayr wrote:
No organism can invade a new adaptive zone unless it has a minimum of structural, physiological, and behavioral attributes that preadapt it to succeed in this shift.
This statement also supports the 2nd meaning.

My interest is in the transition zone between the sea and the land. Specifically, I am trying to understand how ancestral marine gastropods evolved to become terrestrial. If Mayr meant that coastal habitats are zones of maladaptedness where no gastropod species is well adapted to live, then I totally disagree with him. The semi-terrestrial gastropods of coastal habitats are as well adapted to their habitats as all other gastropod species are to their specific habitats.

But if, on the other hand, Mayr meant that ancestral marine gastropods needed to evolve preadapations before their descendants started to move into the coastal habitats, then, yes, he was correct.

Some gastropods of the transition zone between the sea and the land that were featured on this blog: Assiminea succinea, Cerithidea scalariformis, Melampus bullaoides, Littoraria angulifera.

*Towards a New Philosophy of Biology. Harvard U. Press, 1988.


Anonymous said...

I haven't read the paper you quote, but I suspect Mayr wasn't referring to physical space at all. He may have meant a gap in the fitness landscape (you might want to look this up if you're not familiar with the term). His point is that in many cases ecological niches are discrete, not continuous, so individuals with intermediate traits are selected against. This is the basic problem of speciation: since (by definition) intermediates between species tend to be lost, the conditions for gradual evolution of new species are restrictive.


Even if Mayr was talking about the fitness landscape, his arguments still don't make much sense to me. I don't see a "pronounced gap", adaptive or physical, between the aquatic & terrestrial snail species; instead there is a continuum of adaptations.