About Gender. Notice: You can find the original article here
The basis of sex is that two individuals exchange genetic material and produce offspring that have some from each, but different, through recombination, from either of them. How sex came to be is still the subject of much scientific speculation.
Single cell organisms exist in their billions, reproducing by division. Clearly they also evolve. Any mutation may enable them to bypass their host's defences, such as the flu virus, or the new antibiotic resistant strains of tuberculosis. In recent years, however, it has been found that, in fact, bacteria do sometimes join and exchange genetic material.
Given that the more complex species multiply by sexual means, one may assume that the appearance of sex facilitated their evolution. One theory is that mutated alleles are usually recessive. If individuals that are homozygous for the mutation are successful and go on to breed, there will be an increase in those heterozygous for the allele. If these are, in turn, especially successful, perhaps with changing circumstances, its frequency will increase in the population. If on the other hand, it is harmful, it will affect only a quarter of the population and will gradually disappear.
Most plants can multiply either sexually or asexually - so-called vegetative reproduction. The offspring in the latter case grow very close to the parent plant which may produce overcrowding. Sexual reproduction, by producing masses of pollen is wasteful and chancy, since only a small amount of the material is likely to grow. However, fertilised seeds containing the genetic material may be, for instance, transported in the gut of an animal which eats the fruit. It does, therefore, allow plants to spread over a wider area.
The simplest multicellular organisms are clusters of identical cells which congregate together, like the slime moulds. In more complex organisms different cells become specialised for various purposes and thus become interdependent on each other. The successful way for such organisms to propagate themselves, is by producing a single undifferentiated cell which contains the information for the cells produced at each division to specialise as necessary. In plants, these are known as meristems.
Some animal species reproduce in this way, a process known as parthenogenesis. Parthenogenesis is well known in such species as aphids, bees, and Daphnia but is rare in vertebrates. The offspring are clones, genetically identical to the mother. In many social insects, such as the honeybee and the ant, the unfertilized eggs give rise to the male drones and the fertilized eggs to the female workers and queens.
There has been much in the news lately about fishes being affected by estrogens in our rivers. Given these were largely press reports, one wonders whether they actually "changed sex," but simply didn't develop properly. However, different species of fish use a wide range of sexual processes. Parthenogenesis is relatively common, while in many shoaling species, the sex of an individual depends on the sex ratio of the shoal, where females may change to a male role. In other species, individuals change roles at certain stages of their lives.
Crocodiles and turtles do not rely on sex chromosomes, but on the temperature at which the eggs are incubated. It appears to be related to the production of hormones within the egg. Attempts to incubate alligator eggs for reintroduction to the wild invariably produced males, although it has been recently found that females could be induced by the use of estrogens. Some writers have speculated that this contributed to the extinction of the dinosaurs. Changing global temperatures, they suggest, skewed the sex ratio, adding to the problems that all species were facing at that time.
Insects such as fruit flies seem to use genetic sex on a cell by cell basis. Mosaics occur relatively frequently, as in the fruit fly which, with their short lifespan and high reproductive rate, have been models for much genetic research.
In mammals, though many cells are sexually dimorphic, their development is controlled largely by the circulation of hormones on a whole body basis.
While therefore, sex is varied and flexible among many species, it is a flexibility which mammals, with the complex physiology of placental nurture, have sacrificed
The sex chromosomes would appear to be a relatively recent evolutionary "bolt-on goody" which, in mammals, might explain why, though serious autosomal anomalies are almost always lethal to the organism, many anomalies of the sex chromosomes are not.
Whereas the sperm produced by the male contains little else but the germ cell, the egg produced by the female, in almost all cases, also contains the nutrients required for initial development. In mammals there is also the energy investment involved in gestation and after-birth care. A central principle in ethology, then, is that the energy investment by the female means that relatively few eggs are produced, while a male can produce vast numbers of sperm. Further that, while a male is potentially able to mate with many females, the female has constraints on when and with which individuals she will mate.
Citation
Bland, J., (2004) About Gender: Reproduction.
http://www.gender.org.uk/about/10ethol/a1_repro.htm