©Paul Handford, 1998
photograph of JBS Haldane by Dr. K. Patau from  "Haldane and Modern Biology" (ed. K.R. Dronamraju), Johns Hopkins University Press, 1968.

Haldane was distinct from Fisher and Wright in terms of his disciplinary background and interests:  he was fascinated by chemistry and biochemistry.  Throughout his career in developing mathematical population genetics in concert with Wright and Fisher, he was always at pains to consider what was, or might be, going on at the chemical level.  Thus he writes:

To my mind, it is probable that every gene produces a chemical effect, but we are very far from being able to prove this as yet.   (Haldane 1932)

..... The gene is spread out in a flat layer, and acts as a model, another gene forming on top of it from pre-existing material such as amino-acids.  This is a process similar to crystallization..........

Haldane's early career in genetics included some of the first studies in linkage in mammals, and an analysis of the mathematical consequences of selection in Mendelian populations  under a great variety of conditions and complexities.  In the process, he developed model systems very like those of Fisher.  He was initially persuaded, like Fisher, that deterministic selection, primarily of the effects of single genes, was the most powerful force in evolutionary change, and he saw populations as being effectively very large & panmictic.  However he differed from Fisher in supposing that some genes could have marked effects and give rise to large selective differentials (remember, Fisher saw tiny effects of allele substitution as the rule, conveniently giving him the possibility of very finely-tuned adaptations.)  Later, though, Haldane became convinced that interactions among genes could be important, and developed ideas which were very similar to those being produced, independently, by Wright.  Nevertheless, contrary to Wright, he believed that most populations, most of the time, are not highly subdivided and, therefore, not much subject to genetic drift, in the long run.

Haldane developed some ideas in evolutionary thinking which were, again, highly original and forward-looking - like his ideas on the chemistry of the gene & gene-replication quoted above.  As an example, we may note his adumbration of what was later to become (in 1964, with the papers of W.H. Hamilton) the theory of inclusive fitness and kin selection.  Quoting again from his 1932 book:

For in so far as it makes for the survival of one's descendants and near relation, altruistic behaviour is a kind of Darwinian fitness, and may be expected to spread as a result of natural selection.

At the time, (the 1920s and '30s), population genetics theory was concerned exclusively with processes internal to a population (it largely still is), and it was developing essentially separately from thinking about the larger-scale evolutionary phenomena, especially speciation - the evolution of new lineages.  Scientists studying speciation rarely had any acquaintance with, still less expertise in, population genetics:  their affinities were primarily with natural historians or systematists.  This disregard for matters concerned with diversification certainly characterised Fisher, and was largely true of Wright - indeed it was pretty much true of Darwin, too.  Haldane, on the other hand, did pay some attention to at least the earlier stages of lineage diversification.  Through his considerations of the conditions necessary for substantial evolutionary change, he came to believe that some form of isolation of a relatively small group of individuals may be necessary.  In discussing such isolated groups he says:

This case seems to me very important, because it is probably the basis of progressive evolution of many organs and functions in higher animals, and of the break-up of one species into several.    (Haldane 1932)

Note 1  Haldane, J.B.S.  1941  New Paths in Genetics.  Return to text.