Genes similar to yours are good. A woman’s sister has many of the same genes as hers; sis’s kids -her nephews and nieces- have some as well. Kinship rules say that she values them more than a random stranger.
A woman picks the overall best man she can to mate with. She has judged his genes valuable to pass on to her children.
So: wouldn’t combining 1 and 2 mean that of all females that a woman’s husband could sleep with, the least objectionable would be her female relatives? Since then the resulting nieces and nephews would be one-quater her genes and one-half the genes of the man she judged fit to combine with her own genes. Yet if anything, women become even more outraged when the other woman is a sister or still fertile mother. So where’s the flaw in the reasoning?
Yeah, the flaw is that complex human behaviors like that aren’t so simple. There are other factors to consider, such as the betrayal of trust (both the husband and the sister), the embarrassment the woman faces in her social group, to name just two things that immediately come to mind.
You are missing supposition 3- genetic variety is also good.
You may work hard to protect your genetic relations in order to pass your genes along, but there is also a drive to introduce genetic variety into those offspring in the first place.
With respect to sexual reproduction, variety is generally better than similarity (in a complete over generalization!).
Also, the premise of the whole “kin selection” thing is non-competition. Your sister/mother sleeping with your husband is a pretty egregious violation of the non-competition ideal.
From a non-random mating perspective, a mate that can provide sufficiently to fling his genes far and wide may garner more respect than a mate that is looking into how that whole diminishing-returns thing can be stretched within one group of siblings.
Or, y’know, it could be a socially adapted behavior related to keeping track of parentage. Or the idea of one male decreasing a family’s prospects by consuming multiple daughters by exploiting familial access.
Have you seen some of the people reproducing these days? Have you seen the results? It doesn’t look like too many people are exercising much genetic discrimination.
The flaw is that you are driven to want to create more of your own genes, not those of your kinship. Just yours. That’s why they are called selfish genes. They don’t care about your sister. They care only to make more copies of themselves and if the person who is helping you copy them (your husband) is not only not helping but actually making copies of other genes, well, that’s just plain wrong according to your genes.
Of course genes don’t actually “care” about anything. But people with genetic traits that make them jealous have a better chance of making copies of those genes than people who don’t have them.
There is also the matter of resource allocation. If a couple are exclusive, then all the materials that they can gather goes to our offspring. If the male has other mates, then his efforts would be divided between raising more offspring.
My old bumper sticker (sadly lost when I traded the car in) read:
Stupid People Shouldn’t Breed
Unfortunately, the folks this sentiment was directed to never seemed to comprehend. They just stared at it, apparently tried to understand the symbols, then sauntered off with their brood.
This is a flaw, in my opinion. It is not an evolutionary principle, as there is no judgment to be made on the relative value of genes. Either a gene (or, more accurately, a phenotype) is advantageous in a given environment, or it is not (i.e., it can be neutral or disadvantageous). There is no consideration for “similarity” between your genes and others; natural selection operates on each individual independently.
This is wrong. First of all, the metaphor of “selfishness” is a post hoc description of the result of genes being successful; that is to say, individual genes that are most prolific are those that are most adaptable and most widely transmitted, even if this occurs at the expense of an individual organism. One can demonstrate this quantitatively with eusocial insects like bees, where sterile workers will sacrifice themselves in order to perpetuate the hive genes that are held in common. Similar behaviors that are lethal to the organism but successful in the context of reproduction can be seen in other organisms, including fig wasps, cephalopods, and many other sexually reproducing organisms (arguing for gene-centric behavior in asexually-reproducing species is a significantly more difficult proposition, albeit not conceptually untenable.) , and can be explained strictly in terms of the benefit to the gene insofar as ensuring its overall proliferation rather than a more ambiguous kin-based altruism. Of course, in many situations, what benefits the gene also benefits the carrier, hence why the “selfishness” of genes is not detrimental to the whole organism; by providing benefit to the carrier in terms of improving its reproductive fitness, the gene is also reproduced.
So selfish genes “want” to reproduce themselves, even if in other organisms via interchanges that look (and in effect, are) cooperative, even if there is no rationale for volition on the part of the gene or organism. This also delves into another neo-Darwinist concept of an extended phenotype, or an influence of the gene beyond the organism itself. Bee hives are an example of an extended phenotype; the structure of the hive–which is produced automatically and cooperatively by the bees without training or experience–is an example of a phenotype that is demonstrably the result of information encoded in the genome but external to the individual organism.
Note that the metaphor of “selfishness” is misleading if entirely appropriate jargon in a limited strict game theory sense. Successful genes may act to their own benefit, and that benefit may generally coincide with a benefit to the organism or kin group, but there is, as you note, no decision-making or cognition on the part of genes. Unfortunately, some people have taken the comparison as a literal statement of causality and used this as an arguing point against gene-centric theory and indeed, natural selection and the modern evolutionary synthesis from which it emerged. In fact, the alleged selfishness is simple an after-the-fact baldly inarguable expression of effectiveness; the gene that is most successful is the one that has provided the capability for carriers to most widely disseminate it. Some tenants of specifics based on this hypothesis are debatable (particularly overextension or oversimplification of extended phenotypes), but the fundamental statement is as solid as the Central Dogma of molecular biology.
Jealously (as a genetic trait) is only beneficial insofar as it promotes the proliferation of one set of genes instead of those of a competitor. When a male bear or tomcat kills the offspring of a sow or female cat in order to put her in estrus and eliminate competition with his offspring, that “jealousy” can be explained in terms of its reproductive benefit. On the other hand, when you get jealous because your girlfriend makes an offhand comment about Brad Pitt’s butt, resulting in an argument and no hoochie-coo, it provides no benefit to you whatsoever. Such jealousy may have an innate component that triggers it, but much of it is also socialization.
To answer the o.p., from a strictly genetic point of view, a woman would want to pick a man who is not in her immediate kin group (to avoid consanguinity and reinforcement of detrimental recessive genes) but who has phenotypes that she recognizes as both reproductively beneficial and accepted by her kin group, giving the best balance between fresh genes and those that are identifiably beneficial (insofar as one can tell by phenotypical expression). However, socialization has a large impact on what one finds desirable in a mate, often in competition with reproductive suitability. This is especially true (in primates) with males, for whom the greatest benefit is in mating with the most females. Females of nearly all species, on the other hand, are far more picky about who they care to mate with; understandable since they invest substantially more resources in rearing offspring that males of most species.
Unfortunately, the fundamental statement is also little more than a tautology: the most successful genes are those that are most successful (wherein “most successful” can only realistically be defined as “most widely disseminated”, since there is precious little else at which a gene can “succeed”). This is one of several reasons why I don’t feel that the so-called “selfish gene” theory really contributes much to evolutionary biology in general. It works, at best, as a metaphor for explaining altruism, but fails as a general descriptor for evolutionary processes.