I like working with clear and precise definitions not for some love of rigid conventions, which I don’t have, nor because I think science is immutable and made of fixed certainties, which is not true. For me, clear and precise definitions provide a basis for thinking and planning a research, for scientific communication and for the development of scientific activity. We all like to talk and exchange ideas with smart people, using logical and consistent reasoning, especially regarding our academic life. However, it is not uncommon for many of these people, by indolence or even overconfidence, to neglect the importance of some definitions and some fundamental principles of the science they work with.
That being said, I tend to be very cautious when dealing with selection. In non-specialized literature, it’s not uncommon to find all sorts of inaccurate definitions of selection — which is not, however, a justification for the journalists from these large-circulation magazines and websites writing all kinds of pseudoscientific nonsense. But even in the specialized literature there are particular uses of the term selection, which end up compromising the uniformity of the scientific language and its concepts.
The definition of selection that I favour is quite mathematical, and therefore very disappointing to some. Here it is, as it appears at Futuyma’s “Evolution”:
Selection is any consistent difference in fitness among phenotypically different classes of biological entities.
Thus, even risking to sound as a reductionist (which I am not), I define and use selection as a difference: given two values, the difference between 4 and 3 is 1, the difference between 3 and 5 is -2, the difference between 2.6 and 2.6 is zero, and so on. For me it makes no sense statements as “the modern medicine gets in the way of selection”, sometimes heard when people take as an example the diagnosis and treatment of genetic diseases like phenylketonuria (PKU) or hemophilia A. There is no way to hinder a difference, because that is just a number: you can only change it. Note that when one says that medicine is hindering selection, one is making a dangerous value judgment. The only thing we have here is that the difference of reproductive rates between normal and PKU individuals was a given number x1 before the rise of modern medicine, and now moved to a (smaller) number x2, that’s all.
What’s more, what do they mean when they say that modern medicine hinder selection or even evolution? The most common answer is that the treatment “increases the frequency of lethal allele”. But if the medicine is treating this condition and avoiding double recessive individuals from dying, what is the problem? Furthermore, if the frequency of a given allele was reducing before the emergence of modern medicine and now the frequency is rising, this is exactly evolution (actually, I’ve written about this before).
Because I favour this mathematical definition of selection I try, whenever possible, not to use the adjectives “natural” and “artificial”, that we so often find describing selection. If selection is only (no reductionism intended) a difference in fitness among different biological entities, what would be the distinction between natural selection and artificial selection? Mathematically speaking, no distinction at all. The only difference we could assign is regarding the “selection” agent. But precisely here lies my disagreement: if we are dealing with selection as a mathematical difference, a number, what we mean by an agent? What is, if the question makes any sense, this selection’s agent?
The common explanation — that natural selection has as selection’s agent the nature (whatever it is) and the artificial selection has as selection’s agent the man — lacks meaning to me, for two reasons: first, it is not appropriate to seek an agent for selection. The mathematical definition of selection refers mainly to the result of the process, not to how it unfolds or even less to its causes. The second objection is a bit more complicated philosophically speaking, and a very boring one, regarding which countless works were written: is not the human being part of the nature?
Despite these objections, the terms natural selection and artificial selection are commonly used in the scientific literature. So, how to proceed? I think it is appropriate to begin by analysing how natural selection differs from artificial selection regarding their results. In the former, the biological entities struggle for life on what we might call the real external world, their environment of evolutionary adaptedness (yes, I like Bowlby). In these circumstances, all the characteristics of the biological entities adds up to determine their dynamics in that environment, and all of these characteristics end up contributing in higher or lesser degree to the differences in reproductive rates. But in the latter (artificial selection), an organism (in this case an human being) chooses the mating individuals because of just one characteristic, or sometimes a few, and that do not necessarily contribute to increase the fitness that that variety would have in its natural environment.
Thus, it seems clear that artificial selection promotes a much faster quantitative change compared to natural selection, relative to a given specific characteristic. Furthermore, as almost all biologists know, artificial selection, by targeting just one trait, makes a whole series of genes and their related phenotypes fluctuate randomly. For example, if I choose to cross corn plants with bigger kernels, avoiding the cross of individuals that produce smaller kernels, I do not care (initially) if those corn plants with bigger kernels are tall or small, if they have narrow or large leaves, if the kernels are yellow or red, if they are resistant to a given fungus or not… None of this matters to the farmer, or to the person cross-breeding the plants. Thus, we end up creating a substantially different selection compared to the selection that would occur in that same environment, if the corn plants were left to their own care.
Therefore, if the fundamental difference between natural selection and artificial is their results vis-à-vis the natural environment of that population or organism, I’d rather use the term selection alone, without any adjective (neither “natural” nor “artificial”), to refer to the selection process in general, and using the term selection experiment to refer, when deemed necessary, to artificial selection in particular. Mark Ridley in his “Evolution” often uses the term artificial selection experiment. I believe that this is a more appropriate way to refer to artificial selection, always keeping in mind that an experiment is not necessarily conducted in complex laboratories or associated to modern science: the human being has been conducting artificial selection experiments since the agricultural revolution, ten thousand years ago.