Sex Ratio Secondary Article

Natural Selection: Sex Ratio Secondary article Jon Seger, University of Utah, Salt Lake City, Utah, USA Article Contents . Introduction Natural selection acts strongly on the proportional numbers of male and female offspring . Darwin on the Numbers Game among the progeny of adults. The equilibrium is often one at which males and females are . Conditional Indifference, Not Optimality produced in roughly equal numbers, but under certain ecological and genetic . Cost versus Numbers circumstances there may be dramatic population-wide biases, and even in populations . Benefit Versus Numbers with balanced sex ratios, parents are often selected to produce strongly male- or female- . Mode of Inheritance and Locus of Control biased progenies in response to variations in their individual condition. Condition-Dependent Strategies . Sex Allocation in Hermaphrodites Introduction Darwin on the Numbers Game Why are approximately equal numbers of male and female In Chapter 8 of The Descent of Man, after a review of sex offspring produced each generation in most species with ratios in mammals, fish, birds and insects, Darwin (1871) separate sexes? A seemingly obvious mechanistic explana- opens a section called ‘On the Power of Natural Selection tion suggests itself in species where sex is determined to regulate the proportional Numbers of the Sexes, and chromosomally, for example by sperm carrying either an X General Fertility.’ Its second paragraph begins as follows or a Y chromosome.In such species, meiotic segregation of (p.316):Let us now take the case of a species producing _ the chromosomes carrying female- and male-determining an excess of one sex – we will say of males – these being genes will give rise to equal numbers of female- and male- superfluous and useless, or nearly useless.Could the sexes determining gametes, and thereby to equal numbers of be equalised through natural selection? We may feel sure, female and male zygotes.But this cannot be a general from all characters being variable, that certain pairs would explanation, because many species with nonchromosomal produce a somewhat less excess of males over females than sex determination also have balanced sex ratios.Con- other pairs.The former, supposing the actual number of versely, biased sex ratios are sometimes produced in species the offspring to remain constant, would necessarily with chromosomal sex determination, demonstrating that produce more females, and would therefore be more the Mendelian mechanism can be overridden.A seemingly productive.On the doctrine of chances a greater number of obvious functional explanation suggests itself in mono- the offspring of the more productive pairs would survive; gamous species where a father and a mother must and these would inherit a tendency to procreate fewer cooperate in brood care to rear more than minimal males and more females.Thus a tendency towards the numbers of offspring (for example, in most passerine equalisation of the sexes would be brought about. _ The birds).But this functional advantage cannot provide a same train of reasoning is applicable _ if we assume that general explanation either, because monogamy is taxono- females instead of males are produced in excess, for such mically restricted and rare while balanced (or approxi- females from not uniting with males would be superfluous mately balanced) sex ratios are widespread and common. and useless. A very general principle of sex-ratio evolution was This passage expresses the most important elements of described by R.A.Fisher in The Genetical Theory of the modern theory as later stated more precisely and Natural Selection (1930).Fisher’s account of the principle generally by Fisher.In the following paragraph (pp.317– is traditionally considered to mark the beginning of 318) Darwin even seems to anticipate the concept of modern understanding of sex allocation, but Edwards parental expenditure or investment (as ‘force’), explicitly (1998) has recently shown that almost all elements of noting the trade-off between offspring number and off- Fisher’s argument can be found in some long-neglected spring quality that now plays a central role in many models works by several late nineteenth and early twentieth of sex allocation.Today we summarize the argument (in its century authors including Darwin.Edwards suggests that simplest form) by noting that members of the minority sex Fisher understood his own account of the principle to be enjoy greater reproductive success than members of the derived from these earlier sources, and that he assumed his majority sex, regardless of the mating system, because interested contemporaries would have been as familiar every offspring must have a mother and a father.Parents with these sources as he was.This may explain why Fisher that produce a relative excess of the underrepresented sex (1930) presents the principle so casually. do not have more offspring than typical parents, but their offspring have more offspring.Parents that overproduce the underrepresented sex will thus enjoy greater than ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net 1 Natural Selection: Sex Ratio average genetic representation in the generation of their authors who have summarized research in this large and grand-offspring. active field.The paragraphs that follow mention just a few major extensions of the basic principle, which has been applied to an amazingly broad (and ever-growing) range of Conditional Indifference, Not situations. Optimality The fitnesses associated with production of male and Cost versus Numbers female offspring are strongly frequency dependent.When equal numbers of males and females are produced in the If offspring of one sex are individually more costly than population at large, the parents of sons and the parents of offspring of the other sex, such that a brood composed daughters enjoy equal representation in the second (and entirely of the more costly sex can include fewer of them, subsequent) generations; parents are indifferent to the then the equilibrium will be one at which the total cost sexes of their offspring, in the sense that no one could do (effort, expenditure, investment) allocated to each sex is better (or worse) by producing any other sex ratio.But if equalized over the population as a whole.To be more the population sex ratio is not balanced, then some precise, if the individual cost ratio is R : 1 (females : males), parental sex ratios (those favouring the underrepresented then the equilibrium numerical sex ratio will be 1 : R, and sex) will produce more grand-offspring than others and the investment ratio will be (R Â 1) : (1 Â R) 5 R : R 5 1:1. absolutely more than are produced by any sex ratio in a Thus, more generally, the principle is that net allocation to population with equal numbers of males and females.Thus (investment in) each sex will be equal at equilibrium, even if a balanced sex ratio is an evolutionary equilibrium, not an this means that the numbers of the two sexes will be optimum; it is often considered to be the canonical unequal.For example, males and females are of very evolutionarily stable strategy (ESS). different sizes in some species of wasps and bees (usually This verbal account of sex-ratio evolution makes several females are larger than males), and parents tend to produce simplifying assumptions.For example, it assumes that sons more of the smaller sex (usually males). and daughters are equally costly to produce; that parents This cost-versus-numbers principle can be derived from do not differ in their abilities to produce viable sons and a simple formalization of the basic argument.Because the daughters; and that individuals mate randomly through- summed reproductive success of all females must equal the out the population (or at least that the population is not summed reproductive success of all males, a parent structured differently for males and females).When any of producing f females and m males will have an expected these (or several other) assumptions is relaxed, the fitness (number of grandchildren) proportional to W 5 equilibrium sex ratio can change.In some cases, the (f/F) 1 (m/M), where F and M are the population average equilibrium becomes heterogeneous in the sense that some or total numbers of female and male offspring.If the parents are favoured to produce progeny sex ratios possible progeny compositions for individual parents (f, m) different from those of other parents in the same are constrained by differential costs (R : 1) as defined population; such patterns of differential sex allocation above, then W will be the same for all feasible progeny sex are often expected to reflect variations in individual ratios if and only if the population-wide offspring condition, ecological circumstance, or social status, among productions F and M are in the ratio 1 : R.Otherwise, other factors that may affect the reproductive prospects of individual parents will be able to maximize W by making parents or their offspring. broods consisting entirely of the sex with less net The selection pressures associated with sex ratios are investment over the population as a whole; offspring of very strong, so real species and individuals are expected to this underinvested sex will produce more grandchildren, respond to them.Theorists, field workers, and laboratory per unit investment, than offspring of the overinvested sex. experimentalists have explored hundreds of these possibi- This formal development of the sex-ratio principle has long lities during the last few decades, and this work has yielded

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