I propose that 'Haldanes Dilemma" be renamed 'Haldanes Selection Rule'. I think what Haldanes analysis really shows that evolution basically can't happen to populations that live entirely in homogeneous environments. The hypotheses implicit in 'Punctuated Equilibrium' directly contradict the hypotheses of Haldanes Dilemma. Therefore, Punctuated Equilibrium is actually what happens when Haldanes Dilemma is invalid.

Haldanes Dilemma considers the case where there is a homogeneous environment that deteriorates. A de nova mutation creates an allele which has a slight advantage over its alternative allele. Haldane basically showed that the new allele can't replace the old allele before the population goes extinct. Unless the fitness advantage is VERY big, the deteriorating environment kills off the new species before it has fully emerged.

'Haldanes Dilemma' has been presented as a paradox. A paradox would imply that there is no plausible hypothesis that contradicts the 'dilemma'. However, it seems to me that 'the dilemma' relies on the hypothesis that the habitat is homogenous. The fitness of the individual does not vary over the entire habitat of the population. In actual fact, habitats are highly heterogenous.

'Punctuated Equilibrium' is said to occur because of 'peripheral isolates'. A peripheral isolate is a subpopulation of a larger population that lives in a region where the mutated gene has an advantage over the unmutated gene. Thus, punctuated equilibrium only occurs when there is a big inhomogeneity in the environment.

Punctuated Equilibrium seems to imply a significant inhomogeneity in the habitat of an environment where an interbreeding population lives. There has to be a region where the new allele has a significant advantage over the old allele. The organisms living in this region are referred to as the 'peripheral isolates'. However, there also has to be a larger region where both old allele and new allele are fit. Hence, there is a significant inhomogeneity implicit in punctuated equilibrium.

Homogeneity is what physicists would call a 'translation symmetry'. The fitness of an organism in an in homogenous environment can't explicitly vary with translation of the organism within the homogenous environment. Symmetry properties are associated with selection rules. A conservation law is a type of selection rule. It seems to me that 'Haldanes Dilemma' is almost like a conservation law that emerges from homogeneity. A species in a homogeneous environment either is conserved or goes extinct due to the 'cost of fitness'. The homogeneity, which is a symmetry property, eliminates the effect of selection on the genome of the population. Hence, the dilemma is really a selection rule.

This would explain why cladogenesis is so much more common than anagenesis. Anagenesis is when one species evolves into another species without splitting up. Haldanes Dilemma describes conditions under which anagenesis may occur. However, the species is likely to go extinct before the new species emerges. Cladogenesis is when one species splits up into many species. It seems to me that the hypotheses of punctuated equilibrium are much more consistent with cladogenesis than anagenesis.

A heterogenous environment has a lot of local environments where a peripheral isolate population can live. A peripheral isolate can evolve into a new species. So a heterogenous environment could cause many species to emerge in peripheral habitats. The new species would be 'hidden' in the peripheral habitat until something like a mass extinction occurs. So the common mode of evolution is cladogenesis, where each peripheral isolate becomes a new species.

I think this representation of 'Haldanes Dilemma' is very revealing. I have never heard a good explanation of why cladogenesis is so common compared to anagenesis. I think 'Haldanes Dilemma' a good pedagogical explanation for why cladogenesis is more common than anagenesis.

How do people feel about my juxtaposition? Edsner, how do you like this?