Before Darwin, when species were thought to be immutable, naturalists believed that membership in a species was determined by whether the organism possessed the qualities that defined the essence of the species. For the pre-Darwinian naturalist, variations were of little interest, except as curiosities. It was, after all, the “standard” specimen that exemplified the external essence of the species, which the naturalist was trying to learn about. The essence was something real and determinate, fixed by nature itself, and the systems of classification devised by biologists were viewed as accurate or inaccurate depending on how well they corresponded to the fixed order of nature.
Evolutionary biology implies a very different view. Darwin argued that there are no fixed essences; there is only a multitude of organisms that resemble one another in some ways but differ in others. (Moreover, variations are no longer to be regarded as mere curiosities; on the contrary, they are the very stuff of nature – they are what make natural selection possible.) How those individuals are grouped – into species, varieties, and so on – is more or less arbitrary. In The Origin of the Species Darwin declared:
I look at the term species, as one arbitrarily given for the sake of convenience to a set of individuals closely resembling each other, and that it does not essentially differ from the term variety, which is given to less distinct and more fluctuating forms. The term variety again, in comparison with mere individual differences, is also applied arbitrarily, and for mere convenience sake.
Thus Darwinian biology substitutes individual organisms, with their profusion of similarities and differences, for the old idea of determinate species
(James Rachels, Created from Animals: The Moral Implications of Darwinism, p. 195)
The pre-Darwinian view of species, as described here, reminds me of Plato’s Forms: the idea that for every individual example of a thing, there exists a Form, an essence, to which it corresponds. Thus a particular dolphin is an instantiation of the form of dolphin; it is an example of dolphinness.
So the pre-Darwinians believed that each species had its own essential nature and identifying characteristics. It is why they so prized the perfect specimen; it was a tangible example of that species’ essential nature. Perhaps this idea was also undergirded by the doctrine of special creation, which states that each species was independently created by God at the beginning of time. In that case, then, each species literally had its perfect specimen, i.e., that first one created by God in the Garden of Eden.
Darwinian evolution calls all of this into question. The species were not created independently, but arose from a common ancestor. And far from being merely interesting errors, the process of speciation is actually driven by variations. While classification by species is obviously useful, it is not to be taken as an eternally valid catalog of creation.
If, then, species classification is a matter of organizational convenience and each species does not exist within impermeable barriers delineating its essential nature, then the idea of something being natural (i.e., according to its nature) becomes problematic. Naturalness seems to be rooted in pre-Darwinian ways of thinking, in which each creature is an instantiation of the essential Idea of that creature.
If species are clusters of characteristics then variations are perfectly “natural”. To put it another way, it is factually wrong to call these variations “unnatural”. Which leads me to quote Fr James Alison:
In the last fifty years or so we have undergone a genuine human discovery of the sort that we, the human race, don’t make all that often. A genuine anthropological discovery: one that is not a matter of fashion, or wishful thinking; not the result of a decline in morals or a collapse of family values. We now know something objectively true about humans that we didn’t know before: that there is a regularly occurring, non-pathological minority variant in the human condition, independent of culture, habitat, religion, education, or customs, which we currently call “being gay”. This minority variant is not, of course, lived in a way that is independent of culture, habitat, religion, education and customs. It is lived, as is every other human reality, in an entirely culture-laden way, which is one of the reasons why it has in the past been so easy to mistake it as merely a function of culture, psychology, religion or morality: something to get worked up about rather than something that is just there.
We still have a great deal to learn about this regularly occurring minority variant in the human condition. However we know enough about it now to recognise that it is something like a category mistake to talk about homosexuality, or heterosexuality for that matter, as though we were talking about a sort of desire. It seems to be closer to the mark to talk about these things as being particular configurations, a minority and a majority configuration, of the conditions of possibility of desire being human.
If we want to talk about what is or is not “natural”, then it seems clear that homosexuality is a perfectly natural variant within the human species. Those who would call it unnatural are, in many cases, operating within a pre-Darwinian understanding of nature.
And consider people who identify as genderqueer. The human species cannot be divided neatly into male and female; there are variations. This is precisely what we should expect given the evolutionary view of the world.
Now, of course, we must mind Hume’s Guillotine: we can’t make ethical statements about what ought to be based on scientific statements about what is. My point is simply that modern science makes judgments about “naturalness” problematic, especially when those judgments are made within an outdated conception of species.