Old and new views on human evolution
Professor Chris Stringer FRS, Natural History Museum, UK
Darwin and Wallace both wrote about human evolution when there was little relevant fossil evidence available, and none at all from the continent of Africa. Although both accepted that natural selection had produced many of the distinctive features of humans, Darwin developed a much more detailed model for the origins of bipedalism, tool-making, canine reduction and brain enlargement through feedback mechanisms stemming from selection to free the hands for manipulation, rather than locomotion.
Darwin also proposed that ‘racial’ features had largely been added through the action of sexual selection. Wallace demurred over the importance of sexual selection in Homo sapiens, arguing that consistent tastes were unlikely to have persisted long enough and widely enough for it to operate on any scale. He also doubted that the correlated ‘perfection’ of human characteristics could have been produced by natural selection alone, and that “unknown causes” must also have been at work. In the case of the human brain, in particular, he came to argue that spiritual, rather than natural, forces must have been responsible for the evolution of the highest human faculties.
In the light of subsequent fossil and archaeological discoveries, it is possible to critique the views of both Darwin and Wallace concerning human evolution. We now know that canine reduction, bipedalism, tool-making and brain enlargement did not evolve in concert, but were spread out over several million years of evolution in Africa, with different species showing distinct combinations of traits. Thus these features probably developed through multifarious causes, rather than being locked in a feedback system. And while natural selection does seem to lie behind the evolution of the human brain and of many regional (‘racial’) features, sexual (or cultural) selection also seems to have played its part. Some of the issues with which Darwin and Wallace struggled have been resolved by new evidence, while the richness of the current fossil, genetic and archaeological records has raised many new issues which they could never have contemplated.
Wallace and human evolution
Professor Ted Benton, University of Essex, UK
In 1859 Darwin had been evasive about human evolution, but Wallace addressed the question in 1864. His paper addresses two issues: the origin and significance of racial differences and the great mental gulf between humans and apes, despite the striking physical resemblances between them. The theory of evolution by natural selection can be used to explain both. At a certain point in the development of social dispositions and mental abilities in our ancestors, natural selection would have increasingly acted on these features, rather than bodily form. Cranial capacity would have increased greatly, leaving the rest of the body little changed. This approach was influential on Darwin’s argument in the Descent of Man, but in the meantime Wallace had become convinced that natural selection was insufficient to explain ‘higher’ human attributes. Darwin was horrified, but in fact their views had more in common than either recognised.
Wallace, Darwin and female choice
Professor Tim Birkhead FRS, University of Sheffield, UK
Wallace didn’t rate Darwin’s idea of sexual selection, at least, not as a much as Darwin. Wallace’s reservations – particularly with regard to female choice - anticipated the bumpy ride that sexual selection endured since Darwin. From the mid-1970s however, with a clearer view of how selection operates, sexual selection has enjoyed a spectacular Renaissance and is now considered to be as important as natural selection. I will explore the history of sexual selection, including Wallace’s criticisms, and discuss its rebirth, especially with respect to something neither Darwin nor Wallace even contemplated: the idea that sexual selection might continue beyond the choice of partner: post-copulatory sexual selection.
Wallace's understanding of species and speciation
Professor James Mallet, University College London and Harvard University, UK and USA
Soon after his return from the Malay Archipelago, Alfred Russel Wallace published one of his most significant papers. The paper followed many of the themes opened up by Henry Walter Bates 3 years earlier, and used butterflies as a model system to understand the evolution of mimicry and the origin of species. In a very important section, Wallace laid out what is perhaps the clearest definition by an early Darwinian of the differences between species, geographic subspecies, and local 'varieties.' He also discussed what is now termed 'reproductive isolation.' While he accepting it as a cause of species, he rejected it as a definition. Instead, species were recognized as forms that overlap spatially and lack intermediates, as had Darwin. This morphological distinctness argument appears to break down for discrete polymorphisms, but Wallace correctly diagnosed conspecificity of non-mimetic males and polymorphic female Batesian mimics in Papilio butterflies for the first time. Also in the 1860s Wallace wrote to Darwin about a suggestion that natural selection could lead to reproductive isolation, which the older man firmly rejected. When G.J. Romanes later published his theory of 'physiological selection' (a selective model for the origin of reproductive isolation), Wallace rebutted the idea in the pages of Nature. In his book Darwinism (1889), however, Wallace wrote up his own theory in a manner almost identical to what he'd outlined to Darwin in the 1860s, without apparently discussing why Darwin had rejected the idea. The problem with both Romanes' and some of Wallace's ideas is that they are inherently group selectionist; however, one part of Wallace's idea survives as today's model of 'reinforcement' in speciation.