Social evolution

One of the most intriguing features of animals is their behaviour towards conspecifics (social behaviour). This behaviour can vary from cooperation and altruism to competition and lethal combat. Cooperation and altruism, in particular, have been the focus of attention because selfish entities are, by-and-large, expected through natural selection (Chapter 2). However, several mechanisms have been identified by which cooperation and altruism can evolve (see also Chapter 11). The most famous, kin selection, occurs when interactions occur between relatives, and is largely due to work by Hamilton (1963,1964a,b).

Hamilton derived from population genetics a general rule that would predict when a gene for altruism would be favoured: when br > c. The three terms of this inequality are respectively, the fitness benefits accrued to the recipient of altruism (b), the fitness cost accruing to the altruist (c), and Wright's coefficient of relatedness (r) which measures the probability that a gene in the altruist is also in the recipient. We have already encountered the idea of relatedness in passing in Chapter 5 and live with these values from day-to-day: for sexual diplo-diploids it is 0.5 between parents and offspring, 0.5 between siblings sharing both parents, 0.25 between grandparent and grandchild (Figure 7.4). The effect of r in Hamilton's rule is that higher values make it more likely that the inequality will be satisfied, hence more likely

Generation 2

Generation 1

Generation 3

Fig. 7.4 Working out coefficients of relatedness. The arrows trace the possible lines of descent of a gene, while numbers indicate the probability of that line of descent under Mendelian inheritance. The relatedness of a grandchild to its grandparent is 0.25 (left), while that of sibs is 0.5 (right).

that altruism will be favoured. This of course is a very intuitive result: we are nearly always nice to ourselves (r = 1), we look after our offspring very well (r = 0.5), while the concept of the wicked stepmother (r = 0) is found in many cultures. Hamilton's rule has found wide support in a number of animal systems, but it is worth remembering that it is only exactly correct under certain conditions, which are not always fulfilled (see Grafen 1984) whereby one may have to resort to basic population genetics.

While Hamilton's rule is widely acknowledged to define limits to altruism, the flip side is that it also defines limits to selfish behaviour. Imagine a situation where the decision is to keep a resource or to donate it to a relative. If the resource has the same value for donor and recipient (b = c), then an individual should prefer the resource to belong to itself than another individual, even a sibling or offspring. The lower the relatedness between individuals, the stronger the selfish tendency. Thus, the interests of relatives frequently may not coincide, leading to conflict between family members. Clarence Darrow's famous saying, that 'the first half of our lives is ruined by our parents and the last half by our children' holds some truth for many organisms.

Plants have, in this sense, plenty of opportunity for social interactions between relatives. The application of Hamilton's rule, and especially the concept of relatedness, helps our understanding of many reproductive phenomena. The reproductive biology of angiosperms outlines many opportunities for conflict (see Mock and Parker 1997). Pollen consists of two haploid cells (microspores) produced by a meiotic division encased in a

Fig. 7.5 The formation of an ovule in angiosperms (thick lines are chromosomes, dotted circles are

Fig. 7.5 The formation of an ovule in angiosperms (thick lines are chromosomes, dotted circles are nuclei, solid circles are cells). A diploid mother cell (a) undergoes meiosis to form four haploid megaspores (b). Three disintegrate, while the surviving functional megaspore undergoes three mitotic divisions producing a single cell with eight haploid nuclei (c). An uneven cytoplasmic division then produces six haploid cells and one diploid cell (d). Once fertilized by the two pollen sperm, the diploid cell becomes the triploid endosperm, and one of the haploid cells the egg nucleus (e).

tough capsule. On arrival at the female stigma one cell grows as a pollen tube, while the other divides by mitosis into two sperm. Eventually the pollen tube reaches the ovary which consists of one or more ovules, which when fertilized will develop into seeds. Like pollen, ovules are rather complicated entities (Figure 7.5).The ovule starts out as a single diploid mother cell which divides by meiosis into four haploid megaspores. Three of the four mega-spores disintegrate, leaving a single functional megaspore. This undergoes mitosis three times without cytoplasmic division to give a single cell with eight haploid nuclei. Then a cytoplasmic division occurs leaving six small haploid cells and one central cell with two haploid nuclei. When fertilized by one of the sperm, this cell will become the triploid endosperm that provides nutrition for the developing seed. Meanwhile, one of the haploid cells (the egg nucleus) is fertilized by the other sperm to become an embryo.

The triploid endosperm is a curious phenomenon. The endosperm provisions the seed by drawing on maternal resources. It contains two identical copies of the maternal genome and one paternal. There are at least two reasons why this might have happened. First, it could have arisen to try to counter sexual conflict between mother and father. We will encounter sexual conflict again in a later chapter in a different context, but we have already encountered one of its consequences: genomic imprinting (Chapter 2).

If embryos all have different fathers, then the paternal genome is selected to gather resources for the embryo it has fertilized at the expense of other embryos, to which it is totally unrelated. A mother, however, is related to all embryos equally since she is mother to them all. In short, fathers and mother conflict about apportionment of resources between embryos. Genomic imprinting results from this conflict: it is where the expression of a gene is conditional on the sex that transmits it (Chapter 2). Fathers, for example, should be selected to activate genes that result in resource transmission to the embryo; mothers should be selected to counter this. This is found in maize where the number of paternal and maternal genomes in the endosperm can be manipulated to some extent (Haig and Westoby 1991). Genes on chromosome 10 are only active when paternally derived and result in normal sized kernels if the endosperm genome ratio is normal, small kernels if there is maternal overdose, and when the endosperm is tetraploid with equal maternal and paternal representation, the embryos abort. The latter is possibly a result of active maternal countermeasures. From this example, the doubling of the maternal genome could therefore be a 'trumping' mechanism for the maternal sporophyte to reassert control over the paternal genome.

The alternative view stems from conflict between kin. From an inclusive fitness perspective, the mother is indifferent to which embryo receives resources (but see below), since she is equally related to all of them (r = 0.5). The embryo, however, is related to other embryos by between 0.5 (if they all have the same father) and 0.25 (if they all have different fathers) so prefers a skew of resources towards itself. Thus, embryos should be selected to try to gain resources from the mother at the expense of other embryos. There is thus a potential for kin conflict, and hence parent-offspring conflict. At the centre of all this is the endosperm. The relatedness of the endosperm to these players is key. If the ancestral endosperm were a diploid maternal genome, then that would have put the mother in control of resource provisioning, since the endosperm and the maternal interests would be exactly congruent. If the ancestral endosperm was a diploid identical twin of the embryo, that would put the embryo in control. What we have instead is an intermediate situation: a gene in the endosperm is guaranteed to be in the embryo (r = 1), but a gene in the endosperm has a two-third chance of being in the sporo-phyte (r = 0.67), and is more related to other embryos than its own embryo is. Hence, things obviously do not go all the mother's way, but she is much better off than if she only had a half share in a diploid endosperm.

There is evidence as well for kin conflict in plants. In Dalbergia sissoo, a tropical tree from the pea family, multi-seed pods become single seeded by progressive seed abortion, caused by water-soluble chemicals that diffuse from the focal sibling. This increases the weight of the focal seedling, and the removal of its sibs enables the pod to disperse farther by the wind (Ganeshaiah

Fig. 7.6 A flower (a) of M. guttatus, and (b) plants growing by a copper-polluted stream at Copperopolis, California. Photos Courtesy of Mark Macnair.

and Uma Shaanker 1988). Sibling conflict of this kind is also probably in maternal interests in many plants. It is characteristic of most flowering plants that there is extravagant overproduction of early reproductive structures in most years. There is strong evidence that mothers are able to select which should stay, and which should go. These include selfed embryos, which are in many plants more likely to be dropped than outcrossed embryos. However, other characteristics may be selected. Amazingly, Mimulus guttatus growing in copper-stressed soil can selectively abort offspring which are copper sensitive (Searcy and Macnair 1993) (Figure 7.6). In general there is good evidence that selective abortion improves progeny fitness, that is, that it is adaptive. For example, plants of Lotus corniculatus allowed to abort seeds naturally have more fecund offspring than ones which have equivalent numbers hand thinned at random (Stephenson and Winsor 1986).

Even when the fruit has left the parent plant, interactions between the parent and offspring may continue. As Ellner has pointed out (1986), parents may be selected towards making a certain proportion of seeds dormant (Chapter 6). If there is competition between germinating sibs, parents may be selected to produce dormant seeds to reduce sib competition. She values all her offspring equally. Each seed, however, is expected to give priority to itself. This can select for reduced dormancy, giving rise to conflict between parent and offspring over whether seeds should be dormant or not. Rather interestingly, dormancy is commonly caused by a tough seed coat, which is broken down by mechanical or chemical means. The seed coat is a maternally derived tissue, giving her control over seed dormancy.

So the strange reproductive life cycles of plants leave much room for family interactions, through (1) maternal over-production, (2) multiple siring of offspring on a plant, and (3) provisioning of seeds from maternal resources. The concept of kin selection enhances our ability to understand these peculiarities of life.

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