Análisis y Discusión para el objetivo uno

Trivers (1988) suggested that, in most species, the genetic variance in fitness of males is higher than the genetic variance in fitness of females. Typically, males undergo sexual selection, being chosen by females or competing for access to them. Sexual selection can in principle lead to stronger selection in males than in females for two complimentary reasons:

(1) Capture o f genetic variance by sexually selected traits

Sexually selected traits have been shown to be condition-dependent in expression; individuals in good condition produce better ornaments than individuals in bad condition. Rowe and Houle (1996) have suggested that the evolution of condition dependence has led to the capture of genetic variance by sexually selected traits. Their argument is quite straightforward; sexually selected traits have evolved to become condition-dependent. The condition of each male is dependent on his genetic quality. Therefore, his genetic quality is reflected in the sexually selected trait.

(2) A larger opportunity fo r selection

One male can inseminate many females. Therefore, males with good genes may achieve a very high level of fitness, impossible for females to achieve (Bateman 1948).

The overall variation in fitness of males that is observed is often higher than the overall variation in females. However, the difference is constrained by a number of factors. In

some species, such as Drosophila melanogaster, males invest a considerable amount of

resources into each mating (Partridge and Farquhar 1981), restricting the number of

females with which each male can mate. In other species, such as the red deer Cervus

elaphus, variation in male reproductive success in a single season is much higher than

variation in female reproductive success (Clutton-Brock, Guinness, and Albon 1982). However, the variation in lifetime reproductive success differs less dramatically because male reproductive success is much more strongly related to age than female reproductive success. Males are only able to compete in ruts with a high level of success in four years of their life, whereas successful hinds may produce offspring in ten or more seasons (Clutton-Brock, Guinness, and Albon 1982).

In practice, it is not clear whether either of these two factors necessarily results in the genetic variance in fitness being higher in males than in females:

(1) Females contribute directly to the success of their offspring. Variation in condition of females is also reflected in their reproductive success. Therefore female fitness may be as condition-dependent as male fitness.

(2) The only necessary connection between the opportunity for selection and the realised strength of selection is that the former gives a maximum bound on the latter (Crow 1989). Males may have a higher variance in fitness simply because the

stochastic variation in fitness is higher. The stochastic variation is likely to be highest in the sex in which the investment in each offspring is smallest. This is simply illustrated by a hypothetical example in which there is no selection in either sex, and each female has the same number of offspring, mating with a single male chosen at random. In the example there is no selection in either sex. There is no variance in fitness in females, but the variance equals the mean in males.

Because of these difficulties, there are currently no good theoretical or empirical estimates of the sex difference in the genetic variance in fitness. I list three possible avenues for future work to rectify this:

(1) Measurement o f the condition dependence o f fitness

It is possible to experimentally manipulate condition, for example by altering the amount of food available to individuals. The effect of an equivalent manipulation on the fitness of males and females could be compared. Assuming that condition and

genetic quality have similar effects on fitness, this should give an indication of the sex difference in the effect of genetic quality on fitness.

(2) Direct estimates o f the genetic variance in fitness

A more direct approach is to assess the fitness effects on males and females of

genotypes with known effects on total fitness. Estimates of the fitness of individual chromosomes can be obtained using population cage experiments.

For example, Fowler et al. (1997) competed twelve wild type chromosomes as heterozygotes against two balancer chromosomes in population cages. The

chromosomes each had a recessive lethal, but were otherwise chosen at random from a cage adapted population. The chromosomes varied in fitness substantially, and the fitness of each chromosome on the genetic background of one balancer was correlated with its fitness on the genetic background of the other. The variance in fitness of the chromosomes was estimated as 0.06. Of course, the proportion of the variance in fitness accounted for by the recessive lethal each chromosome contained is unknown. If it is assumed that the recessive lethals are responsible for little of the genetic

variation observed, then the total variance in fitness of Drosophila individuals is

estimated as 0.46 (Fowler et al. 1997). This experiment is significant because it indicates that normal chromosomes in heterozygous state show considerable genetic variance in fitness. This observation is consistent with a high mutational load, and with individual mutations having a large effect on fitness.

It is possible to assess the variance in fitness of each of the chromosomes for

individual fitness traits. For example, the same study found that, in an uncompetitive environment, the viability of each chromosome was uncorrelated with the fitness of that chromosome (although the statistical power may have been rather limited, since no between-replicate correlation was observed for viability). Male fitness traits, such as mating ability, are inherently more difficult to measure than female fitness traits, such as fecundity. Nevertheless, obtaining an estimate for the proportion of the genetic variance in fitness accounted for by easily measurable fitness traits would give an upper bound on the sex difference in the strength of selection.

(3) Evolutionary arguments

The degree of condition dependence in fitness is subject to evolution. Therefore it may be possible to use evolutionary arguments to relate the condition dependence in fitness to observable trade-offs in the life history strategies of males and females.