By George Beccaloni, October 2017
Wallace's contributions to science went far beyond 'merely' co-discovering the theory on which modern biology is based (evolution by natural selection)! Unlike Darwin, he always rejected Lamarckism - the inheritance of characteristics acquired during the life of a parent (for example the enlarged biceps developed by a blacksmith over the course of his career). In fact he was the first natural selectionist to reject this flawed theory and he was therefore in fact, (ironically) the first neo-Darwinian. Wallace devised the first modern definition of what a species is - a slightly modified version of which would later become known as the Biological Species Concept; in addition he believed that speciation typically occurs in allopatry or parapatry - when diverging populations are geographically separated or abutting. He also proposed what is known as the Wallace Effect (also called Reinforcement) to explain how natural selection against hybrids between incipient species could contribute to reproductive isolation and hence speciation. Darwin in contrast believed that speciation occurs largely as a result of competition in sympatry (within the same habitat) - a theory he called his Principle of Divergence. Given that it is now thought that most speciation is a consequence of geographical isolation, Wallace was therefore more correct about the origin of species than Darwin was! Interestingly, although many think of sexual selection as being Darwin’s theory, Wallace's ‘good genes’ argument (Wallacean Sexual Selection) to explain the evolution of sexual characteristics (i.e. that females select males seen to have genetic advantages that increase offspring quality) is regarded by many scientists today as more plausible than Darwin’s belief that females choose mates on aesthetic grounds, simply because they are more beautiful. Wallace could never accept that ‘simple’ animals such as butterflies have an appreciation of beauty.
Wallace made many other major contributions to our understanding of animal colouration. He devised the concept of warning colouration (aposematism) in animals (e.g. where caterpillars have evolved conspicuous colours to advertise their toxicity to potential predators); he was the first to propose how cryptic colouration (camouflage) in animals evolved; he proposed the concepts of luring colours, deflection markings and disruptive colouration; he devised the theory of recognition marks in animals (a study on facial patterns of monkeys recently gave support to this theory); and he also proposed a theory to explain sexual dimorphism in animals. He was the first to suggest mimicry in birds, snakes and Pachyrhynchini weevils; he discovered polymorphic sex-limited mimicry in butterflies; he proposed what is known as the morphological defence hypothesis to explain how the extremely hard exoskeletons of certain weevils have evolved to make them 'unprofitable prey' for potential predators; he also proposed that other beetles have evolved to mimic these weevils to benefit from the weevil's defence. He also was the first to suggest that the brilliantly-coloured fishes from warm seas are in fact "well concealed when surrounded by the brilliant sea-weeds, corals, sea-anemones, and other marine animals, which make the sea-bottom sometimes resemble a fantastic flower-garden”. In 1868 the proposed that natural rather than sexual selection could explain the striking differences in avian plumage dichromatism. Thus, he predicted that nesting habits, through their association with nest predation, could drive changes in sexual dichromatism by enabling females in cavity nesters to become as conspicuous as males. His hypothesis has recently been tested and it received some support. He also produced the first functional classification of animal colours.
In the field of evolutionary biogeography (a discipline he is regarded as being the 'father' of) Wallace not only discovered the famous Wallace Line, but his map of the World's Zoogeographical Regions has stood the test of time remarkably well. Wallace was a pioneer of the study of latitudinal gradients in species richness. In his 1878 book Tropical Nature, and Other Essays he attributed the greater diversity of the tropics to the greater age of tropical regions and their having escaped the catastrophic extinctions caused by glacial climates at higher latitudes. This is now known as the 'Species-Time Hypothesis'. He also originated what was later called the "Pleistocene pump hypothesis" of speciation in his 1880 book Island Life. He believed that the repetition of glacial and warm periods promoted the spread and subsequent isolation of populations, hence promoting speciation. His earliest biogeographical theory, known as the riverine barrier hypothesis, is still a topic of research today.
Another of Wallace's ideas was the Great American Interchange - where animals from South America moved into North America and vice versa, when the two previously isolated continents were joined together by the formation of the Isthmus of Panama about three million years ago. He also devised the first evolutionary theory to explain aging and death (now known as the Wallace-Weismann hypothesis); he was the first biologist to seriously attempt to evaluate the likelihood of life on other planets; and he was an important pioneer of statistical epidemiology. In addition he was the first Westerner to observe and document the spectacular mating displays (leks) of male Birds of Paradise; the first person to point out that the average size of beetles is no greater in the tropics than in temperate regions; and perhaps the first to argue that flightless ratites evolved several times, independently, from a flying ancestor.