By George Beccaloni, October 2017
Wallace's contributions to science went far beyond 'merely' co-discovering the theory on which modern biology is based. 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 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 could have an appreciation of beauty. Even the concept of warning colouration in animals (e.g. where caterpillars have evolved conspicuous colours to advertise their toxicity to potential predators) and the idea of 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) were ideas originally conceived by Wallace.
In addition, Wallace was the first to propose how cryptic colouration (camouflage) in animals evolved; he produced the first functional classification of animal colours; he devised the theory of recognition marks in animals (a scientific paper about facial patterns in monkeys recently supported this theory); and a theory to explain sexual dimorphism in animals. He also devised the first plausible evolutionary theory to explain aging and death; he was first to propose mimicry in birds and in snakes; and he discovered polymorphic sex-limited mimicry in butterflies.
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.
Then there was his discovery of the Wallace Line, and the fact that he collected about 5000 animal species new to science in the Malay Archipelago. He is also acknowledged as being the founder of astrobiology and evolutionary biogeography.
The above is an incomplete summary, more will be added in future.