4.1 Phylogeny and Evolution

31

production of more numerous offspring—adaptation is clearly possible at any level

of organization, as evinced by the robust survival of very simple forms.

Although the classical theory ascribes competition between peers as a primor-

dial motor of change, decisive evolutionary steps seem to have occurred when the

relevant ecological niches were relatively empty, rather than in a period of intense

competition. ⁷

Arguments of this nature imply that the classical or orthodox view of evolution

does not offer a satisfactory explanation of the observed facts. At present, we do

not have one. It looks likely that principles of self-organization (Sect. 12.4), rooted

in the same physicochemical laws governing the inanimate world, are involved. It

would appear to be especially fruitful to focus on the constraints, on which a start

has been made by Gould (1977) with his picturesque image of spandrells in vaulted

rooms: in well-known buildings, such as the San Marco cathedral in Venice, the

decoration of the spandrells is a notable feature and contributes so significantly to

the overall aesthetic effect that one’s first impression is that they were designed

into the structure by the architect. They are, however, an inevitable consequence of

the vaulting and were used opportunistically for the decoration, much as feathers,

developed to provide thermal insulation, seem to have been used opportunistically for

flight—flight was an exaptation, not an adaptation. Other examples are now known at

the molecular level, where existing enzymes start to catalyse new, unrelated reactions.

The synthetic theory of evolution (sometimes called gradualism) asserts that spe-

ciation is a consequence of adaptation. Species are supposed to arise through the

cumulative effects of natural selection acting on a background noise of myriads of

micromutations. The genetic changes are not random (in contrast to classical natural

selection), nor are they directed toward any goal. Change is opportunistic; that is, the

most viable variants (in a given context) are selected. Selection takes place in vast

populations. The sole mechanism is intraspecies microevolution.

The synthetic theory is not in accord with the facts of palaeontology. Ruzhnetsev

has emphasized that change is concentrated in speciation events. The time needed

for a new species to become isolated seems to be negligible in palaeontological (let

alone geological) time: a few hundred years. Transitional forms are not observed (on

the other hand, certain species have been stable for more than 100 million years).

Speciation precedes adaptation. This theory is now usually called punctuated equi-

librium (Fig. 4.1). It is in sharp contrast to gradualism, which predicts that the rate

of evolution (i.e., the rate of speciation) is inversely proportional to generation time.

There is little evidence for such a correlation, however. On the contrary, for example,

the average species duration upper D overbar ^¯D for mammals is about 2 million years. ⁸ Their initial

Cenozoic divergence took place over about 12 million years, but this would only

allow time for about 6 speciations, whereas about 20 new orders, including bats and

whales, appeared. Punctuated equilibrium interprets this as the rapid occupation (by

speciation) of niches vacated by dinosaurs in the great mass extinction at the end of

the Cretaceous era.

7 See Kirchner (2002) regarding limits on the rate of the niche-filling process.

8 See Stanley (1957) for a full discussion.