The Origin of Sex

Eukaryotic cells reproduce sexually, involving the fusion of two gametes. Each gamete contains one set of chromosomes and is called haploid where as the zygote formed when two gametes combine is known as a zygote and is diploid (contains 2 sets of chromosomes). Today's gene pool consists of genes that have been passed down many generations from common ancestors. The species is an evolving unit sharing a common gene pool.

The sexual process and reproduction are opposite processes. In sex one cell divides to create two cells where as in reproduction two cells fuse and form one. In some species, reproduction can occur without sex and this production of eggs that can develop without being fertilized is known as parthenogenesis. Although this process is common for plants and some reptiles and insects, it has not been observed in mammals. Reproduction without sex is not possible.

Sex is advantageous to populations . Selective advantages can occur at three levels. Firstly, entire populations can benefit at the expense of others. Secondly, individuals might be advantageous in survival over others, some genotypes having a better chance of success in a particular environment. Individual selection is effective in short term. Lastly, within an individual, sex may favour some genes over others.

Selection is advantageous to sexual populations because they can evolve at a rapid rate and asexual populations accumulate deleterious mutations. On the individual level, production of a variable progeny is favourable to an individual, DNA damage is easier to repair due to its double helical structure. Also, within a population, individuals might be favoured if they are better able to adopt to certain environmental conditions or in a constant environment, their offspring may have a reduced amount of harmful mutations. On a genetic level, selection favours genes that enable individuals to produce gametes that fuse because a gene present on a fusing cell has the ability to transfer to the other.

Population cycles can change between haploid and diploid states. A haploid can transform into diploid by replicating each chromosome while undergoing cell division in a process called endomitosis. On the other hand, the diploid can be converted to haploid by the process of meiosis in which each daughter cell has a single copy of the chromosomes of the parent cell. First homologous chromosomes combine in the parents, then separate and two new cells are produced. Crossing over is pointless since the two homologues are identical.

Diploids are advantageous because DNA repair is possible since two sets of the genetic material is present. Haploids are also advantageous because they are usually smaller in size and have a higher surface area to volume ratio and can reproduce faster than diploids.

First, DNA from different ancestors combine in a single descendent. Stage two involves cell fusion which replaces endomitosis as a means of restoring diploidy in a genuine sexual cycle. This change has benefits because fusion covers the effects of recessive deleterious mutations and a selfish transposon, for the purpose of its own propagation caused cell fusion. After diploidy is re-established, the two homologues chromosomes will produce new genotypes from crossing over despite their similarities. One step meiosis is thus replaced by two step meiosis with recombination.

Recombination is advantageous. In a changing