The zygote is representative of an organism's earliest stage of development. Once fertilization takes place between the female ovum and the male sperm cell, a single diploid cell is formed. The DNA of both parents contributes to this diploid cell. The specifics of reproduction in different eukaryotic organisms vary and in plants reproduction is often asexual. Asexual reproduction produces identical copies of the parents except when mutations take place.
In eukaryotes, there are two main processes in sexual reproduction. First there is meiosis and then fertilization. During fertilization the two gametes are fused together, which regenerates the number of chromosomes of the newly formed cell back to normal (46 chromosomes), since during meiosis the number of chromosomes is halved.
Sexual reproduction allows for the development of complex organisms and typically the new zygote will possess traits and adopt survival strategies which are suitable for the environment. Through sexual reproduction mutations can be passed on and through genetic recombination certain mutations and traits can also be eliminated. New allelic combinations get generated through genetic recombination. One evolutionary advantage of sex is thought to be related to the increased genetic variation which is produced through sexual reproduction. This increases the fitness of offspring.
Sexual reproduction is a critical process in eukaryotic organisms. There is a eukaryotic common ancestor which all eukaryotes are derived from. It is an important process for passing on genes through generations and it is a mechanism tightly linked to natural selection.