Most things that promote the survival and/or reproductive prospects of an organism also promote the replicative prospects of that organism’s genes, and vice versa. For example, being able to escape predators increases your chances of living longer, and hence of having more off spring; being more attractive to the opposite sex increases your chances of having more off spring and so on.1
But both of these mean that your genes have a greater chance of being passed on. Only a randomly selected2 50 per cent of your genes are passed on to each
off spring, but since it is random, every increase in the number of off spring you have is an increase in the chance of any one of your genes being passed on. In terms of what is explained and predicted, then, it may seem that there is very little diff erence between Darwin’s own view of selection – where a trait is selected in virtue of its eff ect on individual organisms – and the gene- selectionist view. However, there are some crucial points of diff erence. Kim Sterelny and Philip Kitcher (1988) argue that the gene-centred approach to natural selection has the advantage over the organism-centred approach of greater comprehensiveness. Th at is, they claim that there are cases that can be handled by the gene-centred approach, but not by the organism-centred approach. For the requirement of greater comprehensiveness to be met, there must also be no cases that the gene- centred view cannot handle that the classical Darwinian view can. Sterelny and Kitcher claim that this is indeed the case. On the gene-selectionist view, genes are the primary benefi ciaries of adaptation. Th at is, even if an adaptation happens to benefi t an organism or a group, the reason it is there is that it ben- efi ts the genes. So we should not expect to fi nd traits that benefi t an organism or group at the genes’ expense. If no trait that could be found that favoured either over the other – that is, if every trait that we found promoted both gene replication and the organism’s survival and reproduction at the same time, if it promoted either – then there would be nothing to choose between the two views. However, there are situations where the two do confl ict, and – according to Sterelny and Kitcher – in such situations we fi nd that the genes always win. Th is should not be thought of as the outcome of some struggle – even in the attenuated Darwinian sense of “struggle” – between genes and organisms, which genes always win because they are so powerful. Rather it should be thought of as the outcome of the situation described above; it is in virtue of contributing to the “success” of genes that traits get naturally selected, so the traits that get naturally selected are those that contribute to the success of genes.
Yet another metaphor found in the gene-selectionist literature is of the geno- type of an individual organism being a temporary alliance or cooperative venture, each gene for the most part having eff ects that enhance the replicative prospects of the whole genotype, rather than specifi cally its own replicative prospects. But this is not always the case: sometimes individual genes have eff ects that increase
their own chances of being represented in the next generation, over and above any contribution they make to the survival and reproductive prospects of the genotype as a whole. Because they may be thought of as “defecting” from the alliance, such cases are referred to as “outlaw genes”. Examples of outlaw genes include segregation distorters and sex-ratio distorters.
Segregation distorters aff ect the process of meiosis: that is, the splitting of a diploid (i.e. in human beings, 46-chromosome) cell into haploid (23- chromosome) cells. Normally, this process involves random “shuffl ing” of genes, so that the 50 per cent of the parent-cell’s
dna
that each haploid cell ends up with is a random selection. However, segregation distorter genes interfere with the shuffl ing process, with the eff ect of giving themselves a better than average chance of being represented in the haploid cells produced by meiosis. It is as if a certain card in a pack was somehow able to increase its own chances of coming near the top of the pack when the pack is shuffl ed. Th is eff ect is independent of any eff ect the gene may have on the survival and reproductive prospects of an organism containing it – it may have no eff ects, or it may reduce those prospects. Nonetheless, such genes will stand a better than average chance of being represented in the next generation. Th us, the persistence of such genes is a product of natural selection, but they are selected in virtue of their eff ect ontheir own replicative prospects, not their eff ects on the survival and reproductive
prospects of the organism.
To see the signifi cance of sex-ratio distortion genes, one must bear in mind that, if you are female, it is to your advantage for there to be as many males as possible in the population, whereas if you are male it is to your advantage for there to be as many females as possible, because this will maximize your reproductive prospects. As always in biology, there are exceptions. In this case, there are many exceptions, ants and bees being perhaps the most obvious. But in a great many species the ratio is 50:50, and the symmetry of the ratio is due to the interests of both sexes. However, Sterelny and Kitcher ask us to consider genes that have the eff ect of distorting this ratio. For example, a gene on the Y-chromosome (the chromosome that, in human beings, is present in all males but not females) might have the eff ect of making sperm with that gene more successful in the race to get to the egg. Th is – entirely hypothetical – gene has been called the “speedy-Y” gene. Since it is on the Y-chromosome only, the resulting off spring will be male. But if the speedy-Y gene became reasonably widespread that would mean that there would be more males than females in the population, to the disadvantage of the male’s reproductive prospects. Nonetheless, the speedy-sperm eff ect of the gene could still be great enough to become widespread despite putting males in the population, including its possessors, at a disadvantage. Th us the speedy-sperm eff ect would be naturally selected to the benefi t of the speedy-Y gene, at the expense of the organisms that possess it. Less hypothetically, evidence has been found of a gene on the
X-chromosome of Drosophila mauritania that reduces the production of Y- bearing sperm in males carrying the gene. Th ere is also a Y-linked gene that counteracts this eff ect; if there was not, Drosophila mauritania would be extinct. (Th e sex-ratio distorting eff ect was only discovered when the gene was trans- ferred to a diff erent species, Drosophila simulans; Tao et al. 2001.)
Yet another hypothetical gene goes under the name of the “Green beard” gene. Th is concept was developed by Hamilton (1964a,b). As we have already seen, a single gene may have two or more eff ects, so there could be one that has the following three eff ects: (i) causing all its possessors to have a distinctive externally visible mark – say, a green beard; (ii) causing its possessors to rec- ognize that mark in other individuals; (iii) causing all its possessors to behave in a way that benefi ts other individuals with green beards, and/or a way that is harmful to individuals without green beards, even to the point of self-sacrifi ce. “Benefi t” here means something that can be reasonably expected to increase that individual’s chances of surviving and reproducing. Th e individual making the sacrifi ce is not benefi ted, but the replicative prospects of the green beard gene are enhanced. An actual example of the green beard eff ect has been found in red fi re ants (Keller & Ross 1998). Th ese ants’ nests have multiple egg-laying queens at any one time. Th ere are two relevant genetic variants: BB and Bb (bb females die prematurely of “natural causes”). Workers can distinguish the two variants by odour (a pheromone), and those with Bb attack and kill queens with BB. However, some of the workers involved in the attacks pick up the “BB odour” from them, and are subsequently killed by their Bb fellows. Th us, Bb individuals benefi t their own kind, even at the expense of their own lives.
Perhaps the best-known example of gene-selectionist reasoning in action is
Hamilton’s rule (Hamilton 1964a,b). You share a proportion of your genes with
close relatives, over and above those that are common to the species in general. Th us, it is “in the interests” of your genes that those close relatives survive and reproduce. Hamilton devised an index of relatedness, which specifi es how much benefi t your genes get from a benefi t to one of your relations instead of a benefi t to yourself.
• For brothers, sisters, sons and daughters it is 50 per cent;
• for half-brothers, half-sisters, nieces and nephews it is 25 per cent; • for cousins it is 12.5 per cent;
• and so on.
Th is means that natural selection would favour behaviour that benefi ted your relations, even if it was harmful to yourself, but that it tails off when it comes to more distant relatives. A sibling is “worth more” to your genes than a cousin, and so on. Th is mathematical index of relatedness leads us to Hamilton’s rule: a behaviour that is costly to the individual is worth the cost if C < R × B, that is, if the cost to the individual is less than the benefi t to some other individual multiplied by how closely related that other individual is. Th is means that even
if the cost to the individual is 100 per cent – that is, if the individual gets killed – it can still be worth it. For example, a suffi ciently large benefi t to three siblings – saving their lives, say – can add up to 150 per cent. Hamilton’s rule, inciden- tally, is what makes sense of the “altruistic” behaviour of the non-reproducing workers in many species of ants and bees.
Th e advantage of gene selectionism with regard to Hamilton’s rule is not so much that the organism-selectionist view is unable to accommodate it, but that gene selectionism is able to give a much simpler account. To accommodate Hamilton’s rule on the organism-selectionist view, the formulation “survival and reproduction of the organism” has to be modifi ed to something like “sur- vival and reproduction of the organism, plus survival and reproduction of its relatives, in a scale determined by the genetic relatedness index”. (In fact, this was how Hamilton originally understood his rule.) With gene selectionism, Hamilton’s rule is accommodated by simply focusing on the replication pros- pects of genes, because this includes tokens of those genes that occur in other individuals, as in the “green beard” eff ect.