Brian Charlesworth

The population dynamics of transposable elements

This paper describes analytical and simulation models of the population dynamics of transposable elements in randomly mating populations. The models assume a finite number of chromosomal sites that are occupable by members of a given family of elements. Element frequencies can change as a result of replicative transposition, loss of elements from occupied sites, selection on copy number per individual, and genetic drift. It is shown that, in an infinite population, an equilibrium can be set up such that not all sites in all individuals are occupied, allowing variation between individuals in both copy number and identity of occupied sites, as has been observed for several element families in Drosophila melanogaster . Such an equilibrium requires either regulation of transposition rate in response to copy number per genome, a sufficiently strongly downwardly curved dependence of individual fitness on copy number, or both. The probability distributions of element frequencies, generated by the effects of finite population size, are derived on the assumption of independence between different loci, and compared with simulation results. Despite some discrepancies due to violation of the independence assumption, the general pattern seen in the simulations agrees quite well with theory. Data from Drosophila population studies are compared with the theoretical models, and methods of estimating the relevant parameters are discussed.

The cost of sex in relation to mating system.

This paper analyses some models of selection for asexual reproduction, orincreased rates of self-fertilisation, in a variety of mating systems. It is shown that mutations to asexual modes of reproduction have a strong advantage in dioecious species and outbreeding hermaphrodites. This advantage is lost if the sex-ratio is heavily biased in favour of females (with dioecy), or if there is a high level of inbreeding (with hermaphroditism). It is also shown that there may be an advantage to asexuality, but not to increased self-fertilisation, in isogamous species. The biological implications of the results are discussed.

Sex differences in fitness and selection for centric fusions between sex-chromosomes and autosomes

A model of centric fusions between the X or Y chromosome and an autosome carrying a selected locus is studied. It is assumed that fusions are not associated with any fitness effects, and that all chromosomes disjoin regularly from their homologues. It is shown that a necessary condition for the fusion to be favoured is that there is a selectively maintained sex difference in allele frequencies at the selected locus. If this condition is satisfied, the initial rate of increase of a rare Y-autosome fusion is about three times that of an X-autosome fusion, with the same parameter values. Computer calculations of the final equilibrium states reached by populations containing such fusions were done.

The Evolutionary Genetics of Sexual Systems in Flowering Plants

Population genetic studies of the evolution of breeding systems in flowering plants are reviewed. The selective advantage of a gene’s increasing the selfing rate is stressed. In the evolution of outbreeding mechanisms, some strong disadvantage to selfing must therefore be acting; it is suggested that this disadvantage is inbreeding depression. Populations with no absolute barrier to selfing, and with intermediate levels of self-fertilization, appear to be the most likely starting state for the evolution of outbreeding mechanisms. There is some evidence for inbreeding depression in such populations. The evolution of distyly and dioecy are considered in some detail. An explanation for the existence of supergenes controlling these systems is proposed. The breakdown of distyly and tristyly are also considered. The evolution of recombination rates in selfing and outcrossing species is examined briefly.

Population genetics of partial male sterility and the evolution of monoecy and dioecy

SummarySelection on reproductive phenotype in plants is studied, using a theoretical model. Fitnesses of mutant phenotypes with altered male and female fertility are derived, assuming an initially hermaphrodite or monoecious population with no self-incompatibility mechanism, with partial self-fertilisation, and with some inbreeding depression. These fitness expressions are used to derive conditions for the spread of such mutations, in terms of the minimum increase in opposite-sex fertility that will give a phenotype with reduced male or female fertility a selective advantage over the original type, taking into account the effect on the selfing rate of altering the sex phenotype. Conditions for polymorphism are also obtained. The conditions are used to study the evolution of dioecy from monoecy, and the evolution of gynomonoecy and monoecy from the hermaphrodite state.

Some models of the evolution of altruistic behaviour between siblings

Abstract Some simple genetic models are developed for investigating the evolution of altruism between siblings as a result of either kin selection or parental manipulation. It is shown that the critical value of the cost/benefit ratio of fitness effects, which must be exceeded if altruism is to evolve, is in general dependent both on the cost of altruistic behaviour and on the probability that an individual behaves as an altruist. For small values of the latter, the critical values of the cost/benefit ratio with kin selection approach those given by Hamilton (1972). They are always lower with parental manipulation than with kin selection. It is also shown that both kin selection and parental manipulation may lead to the establishment of an evolutionarily stable strategy, with an intermediate frequency of altruists within a sibship. For a given cost/benefit ratio which exceeds the critical value, there is usually a tendency for selection to increase to its limit of unity the amount of fitness sacrificed by the altruists, with a consequent increase in the fitness of their sibs. This may help to explain the origin of sterile castes. The relevance of these findings to the evolution of the social insects is discussed.

The evolution and breakdown of S -allele systems

SummaryWe have examined a model proposed by East (1929) for the evolution of gametophytic self-incompatibility allele systems, starting from populations with a self-fertility allele. The original self-fertility allele is not eliminated from the population when three active S alleles have been incorporated, but self-fertility alleles can coexist with S alleles. In order to examine this co-existence more fully, we studied the invasion of self-incompatible populations containing various numbers of S alleles (all assumed to be equally frequent) by three possible types of self-compatibility factors that could arise by mutation at the S locus. We find that there is always some critical number of active S alleles that ensures the elimination of mutant alleles with no activity in pollen; this number depends on die inbreeding depression. Alleles abolishing the stigma activity can spread, irrespective of how many active alleles are present, provided the inbreeding depression is small enough. Some of the biological implications of these results are discussed.

Genetic variation in recombination in Drosophila . I. Responses to selection and preliminary genetic analysis

Artificial selection was practised for increased and decreased rates of recombination between two pairs of 3rd chromosome genes Gl, Sb and Sb,Ser, in female Drosophila melanogaster. These genes had been placed on a genetic background derived largely from a natural population (IV), prior to the start of selection. One line out of sixteen showed a significant response to selection; recombination between Gl and Sb was increased from 13 per cent to 22 per cent. There was no evidence for directional dominance in the F1 between the Gl–Sb high recombination stock and the unselected control, but there was a small (≈1·5 per cent) but significant reciprocal difference. Backcross experiments showed that this was due to a grandmaternal effect of the nuclear genotype, probably controlled by genes on chromosome 3. These experiments also demonstrated that chromosomes 2 and 3 contributed significantly to the difference between the stocks; there was suggestive but not conclusive evidence for an X chromosome effect. Salivary chromosome studies ruled out any role of cytologically detectable chromosome rearrangements in these effects.

The population genetics of anisogamy.

Abstract This paper analyses the population genetics of anisogamy controlled by a single locus, in both the haploid and diploid cases. The conclusions of Parker et al. (1972), based on computer calculations, are confirmed analytically. The effects of the existence of two mating types on the evolution of anisogamy are examined. Close linkage between a mating type locus and the gamete size locus may produce non-random associations of alleles, leading to disassortative fusion with respect to gamete size. With loose linkage, there is random association of alleles, but selection favours closer linkage.

Kin selection in age-structured populations

Abstract There have been several discussions in the literature as to how to weight interactions between individuals of different ages in models of kin selection. It has commonly been assumed that the reproductive value of a given age is the most appropriate weight, for the purpose of calculating its contribution to inclusive fitness. This paper analyses a model of kin selection in an age-structured population. It is shown that reproductive value is relevant to behavioural interactions involving effects on survival, although the reproductive value of a given age does not provide an exact weighting of its fitness contribution in either discrete- or continuous-time populations. Reproductive value is not relevant to interactions involving effects on fecundity. The results are discussed in relation to observations on behavioural asymmetries involving age differences.