American Scientist

To the Editors:

I enjoyed Bradford Harris’s column “Evolution’s Other Narrative” (Macroscope , November–December), not the least because it mirrored part of a lecture I have been giving to my Introduction to Biology students for the past 20 years, as well as some concepts I explored with my upper-level students in a course on symbiosis and cell evolution that I taught with a colleague. I think Harris’s point that American and European evolutionary biologists have not fully explored the role played by cooperation (mutualism) in natural selection, while focusing perhaps too intently on competition, is well taken.

However, I am not sure that the contention that genetic variation emerges primarily through symbiosis can be sustained in the face of the great body of evidence indicating the importance of mutation and recombination. Evidence for the fusion of nuclear genomes remains scarce and seems to be limited mainly to the transfer of some genes from a symbiont to the nucleus of its host, as has obviously happened in the case of mitochondria and also in mutualistic nitrogen-fixing bacteria and some plant pathogens related to them.

I am more skeptical of a “symbiosis-driven history of speciation”—I am not sure what that phrase means. If it is taken to mean that new species arise through symbiosis, I find no examples given in the text of Harris’s article, nor is it clear what concept of species and speciation Harris is using. The example of Euplotidium, which seems to be the lynchpin of his argument, is not, at least in terms of the leading concepts of speciation and species, an example of speciation. If one subscribes, as do most biologists, to the Biological Species Concept, a species consists of a population that is reproductively (or genetically) isolated from other such populations. Harris does not demonstrate that such isolation can arise from symbiogenesis, much less that it is responsible for most or even a significant fraction of speciation events.

Regarding natural selection and cooperative evolution as opposites or in conflict with one another sets up a straw man. The theory of natural selection, as originally proposed by Charles Darwin and as it stands today, much modified and augmented, has plenty of room to accommodate cooperation. After all, any inherited character of an organism that confers an advantage will increase the fitness of that organism, be it an ability to compete more effectively or to cooperate.

To effectively react against what has undoubtedly been an unbalanced approach to evolution, we need to take care not to attribute too much to contrary emergent views. Yes, cooperation plays a huge and underappreciated part in evolution, but there can be no question that competition, even if it is between tightly integrated consortia of organisms, is also important.

William A. Shear
Hampden-Sydney College
Hampden Sydney, VA

To the Editors:

I enjoyed Bradford Harris’s column on evolution and symbiosis. But I was disappointed to see the Victorians’ overly individualistic, conflict-based view of evolution blamed on Adam Smith. Smith was more of a moral philosopher than a libertarian economist, and both Charles Darwin and Alfred Russel Wallace mention reading Thomas Malthus’s influential Essay on Population just before arriving at the idea of natural selection. I would suggest that Malthus (later echoed by Charles Dickens’s Scrooge, who suggests that the poor should hurry up and die in order to reduce the “surplus population”), together with Herbert Spencer, are the more important sources for the idea that evolution is all about conflict and competition.

In the end, Malthus was just wrong about humans: birth control (what he called “vice”) allows substantial improvements in living conditions without the population explosion that he thought would inevitably follow. But Darwin and Wallace were right about other living things and were not quite so focused on competition as Harris’s essay suggests. Darwin wrote, “We behold the face of nature bright with gladness, we often see superabundance of food; we do not see, or we forget, that the birds which are idly singing round us mostly live on insects or seeds, and are thus constantly destroying life; or we forget how largely these songsters, or their eggs, or their nestlings are destroyed by birds and beasts of prey; we do not always bear in mind, that though food may be now superabundant, it is not so at all seasons of each recurring year.” Struggle and competition for access to limited resources between members of the same species are implicit here. But cooperative responses to these challenges also naturally come to mind, including flocking to reduce predation and pair bonding to improve nestling survival.

Bryson Brown
University of Lethbridge
Lethbridge, Alberta, Canada