Why are there more or less equal numbers of males and females in most species?

In species without male parental care - it would be more efficient to produce mostly females (what type of argument is this?)

What would the best Individual strategy be in a population with four females to every male?

Males in such a population will have four times the reproductive success of each female.

If we reverse the bias (4 males to every female), then each female will (on average) have four times the reproductive success of each male.

Hence, a 50:50 sex ratio is a stable equilibrium point - natural selection will cause the population to shift back to it if there is a biased sex ratio.

The argument really applies to the sex ratio of Parental Investment - the amount of parental investment provided to offspring of each sex - so if males cost more to produce than females, there should be a bias towards higher Numbers of females, but not in total investment in females.

HOWEVER - there are circumstances under which natural selection favors a biased sex ratio.

W.D. Hamilton - Local Mate Competition

In some species, males may mate only with their sisters - common in parasitic wasps - females lay eggs in the larvae of other insects. Females lay a female biased sex ratio. Parental perspective - competition between brothers to mate with a sister is wasteful - parents genes passed on either way.

Hamilton's hypothesis predicts that secondary females will lay a more male-biased sex ratio (because they will be less likely to compete with siblings for mates) - this has been tested in several species of wasp.

R.L Trivers and H. Hare - sex ratios in ants

Hymenoptera (ants, bees and wasps) - have a "haplo diploid" sex determination system: males are haploid, females are diploid.

Relatedness under haplodiploidy:

sister to sister r = 3/4, sister to brother r = 1/4

workers (always females, and typically sisters) should favor a female biased sex ratio. The queen is equally related to her sons and daughters, and hence should favor a 1:1 sex ratio. If the workers control the sex ratio, there should be a 3:1 bias in investment toward females. Trivers and Hare confirmed this by looking at sex ratios of investment (by weight) in a variety of ant species.

Recent research indicates that workers in the ant Formica exsecta will kill male larvae in order to bias the sex ratio in favor of females.

In contrast, in slave making ants, the workers are slaves, unrelated to any of the brood they care for - slaves should not favor one sex over the other. Trivers and Hare predicted and found unbiased sex ratios in slave making ants.

Parental manipulation of the sex ratio

Trivers/ Willard Hypothesis: if size or vigor influences the reproductive success of one sex much more than the other, then natural selection may favor parents that produce the more expensive sex when they (the parents) are in good condition, and hence can afford to invest heavily in each offspring.

Example 1: Red deer (T.H. Clutton-Brock)

Example 2: Seychelles warblers (Jan Komdeur)

Seychelles warblers are cooperative breeders - female offspring of previous broods sometimes stay and help parents to raise later broods.

These ÒHelpersÓ only really help when territory resource quality (food availability) is high - helpers are actually costly to parents when resource quality is low because they compete with parents for scarce resources.

Komdeur et al. determined sex in hatchlings with RAPD DNA markers.

The sex ratio (proportion of females) correlates with territory quality - parents on high quality territories produce female biased sex ratios (when they have few or no helpers), parents on low quality territories produce male biased sex ratios.

Transplantation experiments: breeding pairs transplanted from low quality to high quality territories (experimental treatment) switched from producing mostly males to producing mostly females, whereas breeding pairs transplanted from one high quality territory to another (control treatment for the effect of transplantation) produced a female biased sex ratio before and after transplantation.

Removal experiments - removing helpers from pairs in high quality territories causes sex ratio shift toward production of females.

Seychelles warblers adjust the primary sex ratio facultatively in response to the quality of the territory they inhabit and to the number of helpers present on the breeding territories.

Intra-genomic conflict: The inheritance pattern of genetic elements determines how they will be selected to influence the sex ratio. Cytoplasmically inherited factors,such as mitochondria and a variety of microorganisms, which are passed through females but not through males, are generally selected to produce 100% females, whereas nuclear genes (autosomal genes) are selected to produce offspring of both sexes. This leads to evolutionary "conflicts of interest".

Cytoplasmically inherited factors that skew sex allocation towards females are known from a variety of plants and animals. Examples include mitochondrial male sterility factors in plants, microorganisms that cause selective male lethality, feminization, parthenogenesis, or primary sex ratio shifts in animals.

An evolutionary "arms race" may arise between sex ratio distorters and autosomal genes (which suffer a cost from sex ratio distortion). Example: Woodlouse, Armadillidium vulgare: females - heterogametic sex chromosomes (ZW), males - homogametic (ZZ). Cytoplasmically inherited bacterium (F) - causes feminization of genetic males by suppression of the male determining gland. These ÒfemalesÓ are fertile - transmit the bacterium to offspring. Some populations have extremely female-biased sex ratios, and the extreme scarcity of males imparts a huge selective advantage to any genotype that is able to produce males. Autosomal suppressors of the F bacterium have evolved, which push the sex ratio back toward 50:50.