Book: The Causes of Evolution

Overview
John B. S. Haldane sets out a systematic, quantitative account of the factors that drive evolutionary change. The narrative brings Mendelian genetics into dialogue with Darwinian natural selection, arguing that measurable changes in gene frequencies underlie the observable transformations of species. Mathematical reasoning is used to translate verbal hypotheses into testable predictions, with an eye toward explaining the pace and pattern of evolution.

Genetic foundations
Haldane emphasizes genes as the fundamental units of heredity that determine phenotypic variation subject to evolutionary forces. Mutation provides the raw material for novelty, but its rare, random occurrence cannot by itself account for directional change. Heritable variation, shaped by mutation and recombination, becomes the substrate on which selection and other processes act to alter population composition over generations.

Natural selection and measurable change
Selection occupies a central role: differential survival and reproduction of genotypes produces shifts in gene frequencies and thus in population traits. Haldane introduces and employs quantitative tools to relate selection intensity to the rate of evolutionary response, stressing that modest selective advantages summed over many generations can produce substantial change. He treats selection not as a vague force but as an operator with calculable effects on allele frequencies.

Other evolutionary forces
Migration, genetic drift, and hybridization are treated as additional causes that can accelerate, retard, or redirect evolutionary trajectories depending on population structure and size. Gene flow tends to homogenize populations unless counteracted by selection or isolation. Drift becomes important in small populations, introducing random fluctuations that can fix or eliminate alleles irrespective of their selective value. Haldane examines the interplay among these forces rather than elevating any single one to sole primacy.

Speciation and isolation
The emergence of new species is discussed as a population-genetic process in which reproductive isolation and divergent selection cause gene pools to separate. Geographic isolation and ecological differentiation are shown to foster accumulations of genetic differences that, over time, lead to reproductive barriers. Haldane considers hybridization as both a creative and a disruptive factor: it can introduce novel combinations but may also break down incipient isolating mechanisms.

Rate and pattern of evolution
A recurring concern is the tempo of evolutionary change and whether observed rates are compatible with genetic mechanisms. Haldane argues that accumulation of small genetic changes, acting over long periods, suffices to explain large-scale morphological and physiological transformations. He probes constraints on rate imposed by mutation supply, genetic correlations among traits, and the cost of selection, offering calculations to show when rapid change is plausible and when it is not.

Theoretical method and style
Haldane models theoretical problems with clarity and mathematical rigor while keeping biological relevance in view. Equations are used pragmatically to yield qualitative insights and quantitative limits rather than as abstract formalism. Empirical examples, historical cases, and thought experiments are interwoven with mathematical arguments to make the conclusions accessible to readers with biological training.

Impact and legacy
The synthesis articulated here helped consolidate population genetics as the quantitative backbone of evolutionary biology and paved the way for the later modern synthesis that unified genetics, systematics, and paleontology. Haldane's insistence on measurable causes and his demonstration of how multiple forces combine to shape populations remain influential. The work endures as a model for integrating theory, empirical observation, and mathematical reasoning in the study of evolution.