Archive for the ‘breeding’ Category
more on the complexities of breeding behaviour – how do we know?
The Hapsburg, and sickly, Charles II of Spain (1665-1700). All in the family.
Here’s a simple thought experiment, if perhaps an unlikely scenario. Imagine two children born to the same parents, a male and a female. For some reason they’re both separated from those parents at a very early age, before they know each other as siblings. For the next twenty years or so they’re brought up in separate households, unaware of each others’ existence.
Guess what happens next – they meet, they ‘fall in love’, because after all they’re each as good-looking as the other, and they share interests, politics and ambitions, and they each want to start a family. What could go wrong?
Or more to the point of these explorations, how would these two individuals have any sense that they might be entering into dodgy territory? Of course they might start comparing backgrounds and entertaining suspicions, but what if they both share a tendency to bullshit about their past? Obviously this is a ridiculously rare scenario, but hopefully it makes a point.
So, when I try to learn about inbreeding I’m quickly taken into the inherent dangers, and the complexities of autosomal inheritance, whether dominant or recessive – all of which is fascinating in itself, but of little interest to bonobos, bats or budgerigars – or bodgies and widgies – when mating season arrives. Inbreeding avoidance is all very well if you know who to avoid. But a lot of the info I’m gathering tells me that we do know who to avoid, and I don’t just mean we humans. According to Wikipedia’s article on inbreeding avoidance, ‘there have been numerous documented examples of instances in which individuals are shown to find closely related conspecifics unattractive’. But this bald statement strikes me as totally unconvincing. I’m sure we could find plenty of ‘documented examples’ of the exact opposite too.
However there does seem to be something, beyond choice or preference or awareness, that promotes inbreeding avoidance, whether it’s pheromones, MHC (major histocompatibility complex) genes, or other evolved mechanisms. We generally manage it – and by ‘we’ I mean just about all breeding entities – but not in a foolproof way, it seems. What interests me is the kind of set process for inbreeding avoidance that we find in chimps, bonobos, Tiwi Islanders and other human societies, which rises to the point of incest taboos in humans. With humans there is knowledge. We know that breeding with siblings and other close kin is problematic, yet we probably didn’t always know that, and our taboos are more about it being ‘icky’ and ‘creepy’ than about giving birth to unhealthy children. What gave rise to those feelings, which seem to be shared by other apes, and perhaps further down the evolutionary bush?
Let’s look at MHC genes, though I don’t think they’re going to provide an answer that satisfies me. Here are the opening lines from a 2008 PubMed article entitled ‘Major histocompatibility complex alleles, sexual responsivity, and unfaithfulness in romantic couples’:
Preferences for mates that possess genes dissimilar to one’s own at the major histocompatibility complex (MHC), a polymorphic group of loci associated with the immune system, have been found in mice, birds, fish, and humans. These preferences may help individuals choose genetically compatible mates and may adaptively function to prevent inbreeding or to increase heterozygosity and thereby immunocompetence of offspring.
So, yes, I find nothing wrong with this finding, but maybe it’s a bit of a correlation-causation problem for me. I’d like a bit more info on the causal. And obviously all these creatures have no idea whether a potential mate has similar/dissimilar MHC genes, so why mention preference, which is about choice, or intention? But then, doesn’t love, or desire, strike us below the level of awareness? Ain’t love blind? Not to our MHC genes, apparently. And then again, is our preference for sweet rather than sour, or vice versa, really a personal choice or something we find out about ourselves? Do I contradict myself? Very well…
Anyway I’m glad to note the phrases ‘may help individuals choose…’ and ‘may adaptively function to prevent inbreeding…’ here, an acknowledgement, methinks, of the fact that they’ve observed a correlation which hasn’t yet been found as determinative – though probably everyone thinks it is. Meanwhile, we – human and myriad other species – seem pretty good at avoiding inbreeding, mostly, so why worry…
References
https://pubmed.ncbi.nlm.nih.gov/17100780/#:~:text=Abstract,HLA%2DA%20Antigens%20/%20genetics
Touching on the complexities of breeding behaviour – introductory…
It does seem that the more we examine male-female relations historically and culturally, in terms of matriarchy and patriarchy, matrilineality and patrilineality, and patrilocality and matrilocality, we find complexity, variety and riddles. ‘Why’ questions. And not just with humans. Why do young female bonobos move to another group or troupe? To avoid interbreeding, I’m told. But how do they know to do this, and why the females?
So there’s another term I should learn – philopatry. Whether male or female, it means staying put in your natal zone for breeding. It’s always explained in terms of heightening genetic diversity, but how did apes know about this, including human apes in earlier times? When I try to research this, I always come up with what we, as post-Darwinians, know, but what do bonobos know?
Inbreeding avoidance, that’s the term. My culture tells me not to mate with my sister or brother. We’re told to be disgusted by it, and to call it ‘incest’. There are religious taboos of course, and as we grow up we learn about birth defects – but that knowledge comes up way afterwards. Interestingly, Joseph Henrich’s The WEIRDest people in the world tells us that the Church, when it was all powerful in medieval Europe, eventually went as far as banning marriage between fifth cousins!
But I’m more interested here in the knowledge than the coercion (I’ve just started reading Darwin’s Descent of Man – wondering if he had any thoughts on this). So here’s something from Wikipedia:
The inbreeding avoidance hypothesis posits that certain mechanisms develop within a species, or within a given population of a species, as a result of assortative mating and natural and sexual selection, in order to prevent breeding among related individuals. Although inbreeding may impose certain evolutionary costs, inbreeding avoidance, which limits the number of potential mates for a given individual, can inflict opportunity costs. Therefore, a balance exists between inbreeding and inbreeding avoidance. This balance determines whether inbreeding mechanisms develop and the specific nature of such mechanisms.
So what is assortative mating? Well, in human terms, it’s mating based on physical or phenotypic similarity, though a distinction has been made between phenotype and social congruence – yes, we like to mate with individuals as beautiful as ourselves, but also with those with whom we share our socio-political values (three criteria are given – socio-economic background, ‘race’ and/or ethnicity, and religious beliefs, or not).
Such mating, though, doesn’t rule out incest – especially when extended to fifth cousins. In fact it might encourage it – think, in an extreme case, of a pair of beautiful same-sex loving monozygotic twins. But then, that’s not breeding, that’s just sex.
So the danger of assortative mating is that it might reduce the diversity that may be of value to a species in tough times. Such mating surely follows natural inclinations, and yet, with bonobos, chimps, and some human cultures (I know the Tiwi Islanders follow this practice), natal dispersal – yet another term – occurs as a method of inbreeding avoidance, and, with humans, it becomes a fixed cultural thing. I suppose it has the value of raising phenotype above genotype. Again, though, I wonder about how we know about genotype connections. I have cousins, like most humans, but I only know they’re my cousins because I’ve been told so. Language is rather useful in this regard. But are there other ways of knowing, utilised by our bonobo and other cousins? Do hormones/pheromones play a role?
This question – how we know, or even how we sense, that we should not mate with x, seems to be mostly avoided online. The Wikipedia quote above comes close, in telling us that assortative mating may encourage inbreeding, but there must surely be some mechanism for knowing who to avoid sexually, and perhaps even why, if even zebrafish can manage it. I mean, it must be something basic, visceral, chemical. This is extremely frustrating. Citing ‘familiarity’ as a motive for inbreeding avoidance makes no sense to me whatsoever. To me it must be something far more basic, something which doesn’t require ‘thought’, and the activities we and other species engage in must constitute a quasi-cultural layer on top of that basic instinct.
So what about those zebrafish? An article from Animal Behaviour in June 2006, entitled “Kin recognition and inbreeding avoidance in zebrafish, Danio rerio, is based on phenotype matching”, sounds promising, but think about phenotype matching. Matching with what? It seems to be about having a similar phenotype, or ‘look’, which is likely to match with similar genotypes. Here’s the article’s introduction:
Differentiating kin from non-kin enables organisms of many species to allocate resources or altruistic behaviour towards related conspecifics and to avoid mating with close relatives. Kin recognition mechanisms can vary among species and may reflect the social environment. Learned familiarity with nest or shoalmates may serve as a good indication that individuals are related, as long as the social system is sufficiently stable to avoid intermingling of unrelated individuals with siblings. Phenotype matching allows for recognition of even unfamiliar kin because individuals establish an olfactory, visual or acoustic template for their kin during early development and compare this template to cues from unfamiliar individuals later in life. We tested which kin recognition mechanism is used by zebrafish and we found that the preference for kin changes with sexual maturity. The olfactory preference of laboratory-bred juveniles and reproductively active adults were tested in an odour choice flume. Juveniles of mixed-sex groups spent more time on the side of unfamiliar kin than unfamiliar non-kin, indicating that kin recognition and preference are based on a phenotype matching mechanism.
A ‘conspecific’ is simply a member of the same species, so I’m not sure what the difference is here between a ‘related conspecific’ and a close relative. What this opening sentence appears to be saying is that we feel friendly and altruistic towards our relatives and avoid mating with them, which seems pretty contradictory. The sentence ‘Kin recognition mechanisms can vary among species and may reflect the social environment’ is also unhelpful. Am I asking or expecting too much? What I’m trying to get at is the physical basis of cultural taboos that we know to be useful/essential to avoid inbreeding. And it’s probable that zebrafish, however interesting, are not going to provide the solution I want, being as I’m human, all too human.
So let’s look at a concept known as the Westermarck effect, about which I’m a bit dubious (at first glance). It was first touted in a late 19th century book by the anthropologist Edvard Westermarck, and I note that Wikipedia doesn’t give it a lot of attention, which is not a good sign. Another term for it is ‘reverse sexual imprinting’, and it hypothesises that ‘people tend not to be attracted to peers with whom they lived like siblings before the age of six’. One has to wonder why of course, but regardless of causes, there is some evidence for this in the Israeli kibbutz system. From Wikipedia:
In the case of the Israeli kibbutzim (collective farms), children were reared somewhat communally in peer groups, based on age, not biological relations. A study of the marriage patterns of these children later in life revealed that out of the nearly 3,000 marriages that occurred across the kibbutz system, only 14 were between children from the same peer group. Of those 14, none had been reared together during the first six years of life. This result suggests that the Westermarck effect operates from birth to at least the age of six.
This is certainly interesting but it doesn’t explain why, as well as suggesting that it’s not an incest taboo. Is there some physio-chemical something that operates, on humans, other apes, birds, fish, etc, to reduce inbreeding? And if so, what exactly is it? It kind of astonishes me that bonobos, chimps and humans send or push individuals, whether male or female, out of their social group to a ‘stranger’ group when they reach breeding age, without knowing why.
So, enough for now. To be continued…
References