I've been following the issue of same-sex marriage for some years now, and I've been sticking my nose into the debate on a number of occasions. As in every issue, there are good arguments to be made, and bad ones.
Among the bad arguments are those that involve an appeal to scripture. Another bad argument is "gays are icky". Both are arguments that will only convince those who are already convinced.
One of the things I've been asking about is what cultures actually practiced same-sex marriage before the turn of the millennium. (Or certainly before about 1990.) People have mentioned some cultures, in particular some Native American cultures where "Twin spirit" people will take on the lifestyle of the opposite sex, even marrying people of the same biological sex. I submit this is an example of a legal sex-change under the laws of the culture involved, and not an example of normalizing same-sex marriages.
Another discussion in support of same-sex marriage was a piece in the Washington Post, "Marriage, An Elastic Institution". It made the case that a generalized anthropologists' definition of marriage actually encompassed a great deal more than we think of as "marriage". As I mention in
this post, the definition Borneman used, "one person leaving a dependency relationship with one group and entering one with a different group", covers quite a variety of things including legal adoption and fostering out. In this case, same-sex marriage has always existed throughout Western culture.
In practice, I suspect this definition is overlooking defining features of marriage which are very important in practice.
So what does evolution have to do with the topic? Well, evolutionary processes leave traces which can be used to tell a lot about the underlying biology of a system.
One phenomenon that matters a great deal to evolution is the notion of genetic clocks and of a neutral mutation rate. Once we wrap our heads around the notion that all living things are related to all other living things, we can figure that any two living things have a common ancestor some distance in the past. Members of the same species have a very recent common ancestor. Members of related species, such as wolves and dogs, have a common ancestor in the recent past. The more distantly related any two species are, the further in the past we have to look for a common ancestor. Dogs and oak trees have a common ancestor hundreds of millions of years in the past.
As a result of this, when we look at equivalent genes and proteins in related species, we find a lot of similarities. We also find differences. In the case of closely related species, we can find areas of "junk DNA" and count up differences in the DNA sequences. Based on the observed mutation rate, we can work out how far in the past the two species' lineages separated.
Now one thing we notice in "working" genes and proteins is that while there will be differences, there will be some parts of a sequence that don't change. If we compare insulin from humans, dogs, squirrels, and kangaroos, we will find that the proteins have a number of slight differences. We will also find several areas that are identical or close to it. As we add more species to the list, we will find that certain parts of the insulin protein just don't change. They're "conserved".
When we examine the structure and function of insulin across different species, we find that the portions that are conserved invariably turn out to have some essential function. They form part of a receptor for enzyme activity. Or they are essential for the proper folding of the protein. The point is, if we change the amino acids in those conserved areas to something else, the protein works very badly or not at all. Creatures born without functioning insulin tend not to live very long, so examples of genes where the conservation is broken are quite rare.
Conservation works the other way around, as well. The fact that proteins in the lenses of the eyes of cave-dwelling fish are slightly conserved is an indication that these proteins serve some function unrelated to sight. We may not know what that function is, but we can be sure there is one from the fact that their mutation rate is less than the rate for junk DNA.
Now, in order for a system to be subject to the rules of evolution, we need two things. We need traits that are inherited, and these traits need to have some effect on the system's survival. When the system is an animal, these traits are things like the make-up of proteins, genes, and their expression in the animal's body. When the system is a human society, these traits are the rituals, ideas, systems, and processes throughout that society. Laws against murder are an example of a societal trait, and one that can easily have an effect on how well a society does. It seem obvious that societies that don't object to people randomly killing each other won't do very well.
Now, marriage is a system that exists in every known society in one form or another. I suppose it's possible that the form of marriage in a society has no effect on how durable that society is. I have my doubts.
It occurs to me that marriage is an example of a system with parts that are subject to variation, and more importantly, parts that are conserved. And the evidence is that the opposite-sex bonding part is very strongly conserved. It may not be quite universal, but the extreme scarcity of examples of same-sex marriage points to very strong conservation.
If this is so, then it would follow that a mutation – changing the definition of marriage to include bonds between participants of the same sex – could render the system nonfunctional. And if marriage is at all important to a society, rendering it nonfunctional could have serious repercussions.
So what do I expect to happen if marriage is redefined? I really don't know. It may well be that nothing happens, except that a tiny fraction of marriages will be between members of the same sex.
But...
I have some guesses, based on the assumption that neoDarwinian rules actually apply. Among them, instead of our society having
marriage, we may become a society that has
marriages. Instead of having one institution that most people of most faiths (or none) can compromise on and agree to mostly honor, we may have balkanized marriage, with each church, sect, and interest group having its own version of marriage it finds acceptable.
In extremis, a Baptist family may refuse to recognize a relative's marriage performed in a Methodist church. And forget about trying to convince your family that a Jewish, Mormon, or Buddhist marriage is a "real" marriage.
Another, related, possibility is that the social acceptance factor inherent in marriage will withdraw to the religious realm. Religions will continue to recognize each other's marriages, but the civil wedding will be seen as a legal mechanism by which people apply for special treatment under the tax code or in contract law.
And frankly, if gays find that being allowed to marry does not grant them the social status they crave, if they find that said acceptance retreats from them like water from Tantalus, perhaps they'd best not read the last line of this post where I say:
I told you so.
