Mark R. Macnair

Models of parent-offspring conflict. IV. Suppression: Evolutionary retaliation by the parent

We have earlier analysed ESSs for the amount of parental investment (PI) that offspring are expected to solicit from their parents, given that parents acquiesce to offspring demands. The present paper considers evolutionary retaliation by the parent for species where only one parent provides PI. Two genetic loci are envisaged: one (the ‘conflictor’ locus) determines the extent of offspring solicitation; the other (the ‘suppressor’ locus) determines how parents retaliate. Solicitation is assumed to carry a cost which may affect a particular offspring uniquely if time and energy are the major costs, or may affect all offspring in a brood equally if the main cost is predation risk. Two kinds of parental retaliation are possible. Parents may supply PI in proportion to offspring demands, or may ignore solicitation altogether and give a fixed PI. Analytical models of conflict in which the parent supplies PI in proportion to solicitation yield pure ESSs with PI at a compromise level between parent and offspring interests. These are termed ‘pro rata’ ESSs. Where solicitation costs are high, an ‘offspring wins’ ESS (offspring get all they ‘want’) is possible especially for forms of conflict that affect future sibs, and a ‘parent wins’ ESS (parent supplies its optimum) is possible especially for conflict that affects contemporary sibs. When parental retaliation takes the form of ignoring offspring solicitation, this can lead to a ‘parent wins’ ESS if costs of ignoring solicitation are negligible, but where parental insensitivity carries costs, the result is an unresolvable evolutionary chase with cycling frequencies of alleles coding for parent and offspring strategies. ‘Pro rata’ ESSs cannot be invaded by ‘ignore solicitation’ mutants but ‘pro rata’ mutants can often invade at certain stages in ‘ignore solicitation’ limit cycles. We therefore conclude that the probable evolutionary end product for most species will be the ‘pro rata’ ESS in which the parent supplies more PI than would be optimal in the absence of conflict, but less PI than would be an ESS for the offspring in the absence of parental retaliation. Such ESSs will be characterized by solicitation costs; offspring will ‘ask’ for more PI than they get. In nature, under similar conditions, highest conflict will occur when both parents sustain equally the effects of conflict, or when conflict affects contemporary rather than future sibs.

Zinc tolerance and hyperaccumulation are genetically independent characters.

The hyperaccumulation of metals by a rare class of plants is a fascinating and little understood phenomenon. No genetic analysis has been possible since no intraspecific variation is known for this character. Here, we report on crosses between the zinc–hyperaccumulating and –tolerant species Arabidopsis halleri and the non–hyperaccumulating, non–tolerant species Arabidopsis petraea. The F2 segregates for both characters and it appears that the two characters are genetically independent. The data for tolerance are consistent with a single major gene for this character (although the number of genes for hyperaccumulation cannot be determined) but is probably not very large.

Models of parent-offspring conflict. III. Intra-brood conflict

Abstract A genetic model of parent-offspring conflict is developed for the situation where conflictors cause the parent to redirect resources from contemporaneous siblings to themselves, by increased solicitation. It is shown that there is no restriction on the spread of genes causing such conflict if there is no direct cost associated with it. We examine the effect of imposing on the conflictors a cost of the extra solicitation when the cost is felt (a) by the individual conflictor only, and (b) by all members of the brood equally. It is shown that the Evolutionarily Stable Strategies for the two situations allow a greater degree of solicitation when its cost is borne by the whole brood. Multipaternity of broods also increases the degree of conflict.

Models of parent-offspring conflict. I. Monogamy

Theoretical models for Trivers (1974) concept of parent-offspring conflict are examined for species in which the effects of the conflict are felt by full sibs. A rare conflictor gene will spread if (f(m) greater than 1/2(m + 1), where f(m) is the fitness gained by a conflictor relative to a non-conflictor offspring (f(m) greater than 1), and m is the amount of parental investment taken by a conflictor relative to m = 1 for a non-conflictor. The range of m alleles which can spread against the parent optimum decreases as the cost to the parent increases until a point is reached where there is no conflict of evolutionary interests. There would be no polymorphism for conflictor: non-conflictor alleles unless special conditions prevail. The conflictor allele which spreads most rapidly as a rare mutant against the parental optimum is not an evolutionarily stable strategy (ESS). The ESS for parent-offspring conflict in monogamous species has mo = f(mo)/2[df(mo)/dmo]. The analytical solutions are confirmed throughout by simulations.

Heavy Metal Tolerance in Plants: A Model Evolutionary System

Evolved tolerance to toxic concentrations of heavy metals in plants inhabiting spoil heaps of mines is a well known phenomenon that has been the subject of much research in the last two decades. These plants are useful models for studying processes involved in the early stages of the speciation of edaphic endemics. Recent work has revealed the importance of several phenomena in the differentiation of tolerant populations, including natural selection, founder effects and hitch-hiking, and has demonstrated the early evolution of morphological differentiation and reproductive isolating mechanisms. Further studies of the biochemistry and molecular biology of heavy metal tolerance will help to show why some plant groups, such as Agrostis, are far more prone to evolve tolerance than others.

Why the evolution of resistance to anthropogenic toxins normally involves major gene changes: the limits to natural selection

Standard population genetic theory suggests that adaptation should normally be achieved by the spread of many genes each of small effect (polygenes), and that adaptation by major genes should be unusual. Such models depend on consideration of the rates of acquisition of adaptation. In practice, adaptation to pollutants and anthropogenic toxins has most frequently been achieved by the spread of major genes. A simple model is developed to explain this discrepancy, in which the determining factor is not the rate of spread, but the maximum response achievable under the two contrasting models of polygenic or major gene inheritance. In the short term, for a given mean and genetic variance, characters in which the additive genetic variance is produced by the segregation of many genes of small effect at intermediate gene frequencies are unable to produce as large a response to directional selection as characters in which the variance is caused by genes of large effect at low frequency. If the ‘target’ for selection is a long way from the mean prior to selection (as it may well be for adaptation to novel anthropogenic stresses) then adaptation can only be achieved by species possessing major genes. The model is discussed with reference to the example of heavy metal tolerance in plants.

Models of parent-offspring conflict. II. Promiscuity.

The population genetics of Trivers (1974) concept of parent-offspring is examined for species in which the effects of the conflict are felt by future half-sibs, as in promiscuous mating systems in which the male shows no parental care. Whether or not a rare conflictor gene will spread in a non-conflictor population depends on f(m) greater than (m + 1)/(0.5m + 1.5) for a dominant gene, and on f(m) greater than 1/4(7 + 3) for a recessive gene; f(m) is the fitness gained by a conflictor relative to a non-conflictor offspring [f(m) greater than 1], and m is the amount of parental investment taken by a conflictor relative to m = 1 for a non-conflictor. The ESS value for conflict (mo) in promiscuous species with zero male parental care has mo = f(mo)/4[df(mo)/dmo]. However, where the male maintains the same harem for several breeding seasons, or where there is promiscuity but both sexes contribute equally to parental care, conditions for conflict are equivalent to monogamy.

The evolution of plants in metal-contaminated environments

There are many areas of the world where the soil is naturally, or through anthropogenic activity, contaminated with metals. Metals are toxic to plants in excess, and the consequence has been that many species found on normal soils are excluded from these areas. The plants that can grow on these soils can normally be shown to have evolved tolerance to the metals in excess, though there are some species which may be constitutively able to tolerate high metal levels, and there is some evidence for environmentally induced tolerance in some species. Tolerance is generally under major gene control, though the degree of tolerance shown by a plant will be affected by minor genes as well, at least some of which act hypostatically to the major tolerance locus. The major tolerance loci generally are specific, so that where plants show tolerance to more than one metal, it is because they have evolved independent tolerances to more than one metal. Co-tolerance, where one gene gives pleiotropic tolerance to more than one metal, is probably rare. The mechanism of tolerance is in most cases unknown, and the problems of studying this phenomenon are discussed. There is circumstantial evidence that tolerance involves a cost, in that tolerant plants are at a selective disadvantage in an uncontaminated environment. It has not, however, been possible to establish the reason or basis of this cost. The ability of a species to evolve tolerance seems to depend on the presence of tolerance genes at low frequency in normal populations prior to the selective agent being imposed. In areas which have been naturally contaminated for very long periods, endemic species restricted to the toxic environment are found (edaphic endemics). The evolutionary processes leading to these, and the difference between an endemic and an ecotype, are discussed.

The distribution of postmating reproductive isolating genes in populations of the yellow monkey flower, Mimulus guttatus

Postmating reproductive isolating barriers are generally believed to arise as the chance by‐product of genetic differentiation. The classical view is that these barriers normally involve differentiation at many loci, and therefore require long periods of allopatric isolation. The formal genetics of, and the distribution of genes responsible for, such barriers are known in very few cases. This paper examines the distribution of the genes responsible for two different postmating barriers in 18 populations of the yellow monkey flower, Mimulus guttatus. The genetic relatedness of the populations was measured by a morphometrical analysis. Widespread polymorphism was found for three of the four components of the two genetic systems responsible for the two barriers, with at least 13 populations possessing genes for one or both of the barriers. In one system (the C7/U8 system; Christie and Macnair, 1984), the distribution of the two component genes was correlated with the morphometrical similarity and geographical location of the populations. This pattern could be produced by a historical association or by an adaptive response. In the other system (the Cerig/C10 system; Macnair and Christie, 1983), the genes were more widely dispersed, and there was no obvious morphometrical or geographical association. Populations possessing the complementary factors causing partial reproductive isolation are not always widely separated geographically. These results indicate that the spread of postmating reproductive isolating genes through drift, selection, or hitchhiking could readily cause reproductive isolation to evolve in this species.>

An ESS for the sex ratio in animals, with particular reference to the social hymenoptera☆

Abstract It is argued that a complete description of the evolutionary stable strategy (ESS) for sex ratio must allow three factors, investment ratio, investment in individual males, and investment in individual females, to be varied. A formulation of the ESS along these lines is presented. This approach can be extended to other breeding systems, and the ESS for haplodiploid breeding systems is derived. If any one of the three variables is constrained, however, the ESS will be altered. One example of such a constraint is given by the social Hymenoptera in which the primary sex ratio is produced by the queen, but the bulk of the investment by the workers. The solution to the ESS in this situation is considered, and related to data presented by Trivers & Hare (1976).