The Difference Between Prezygotic & Postzygotic

The terms prezygotic and postzygotic are used to describe the chronology of conception. They can be used to pinpoint the time that an event took place during the conception and fertilization process. When a conception moves from prezygotic to postzygotic, the attitude of scientists towards the cells involved changes, and the newly formed zygote is recognized as a living organism.

Prezygotic

• Prezygotic basically refers to any stage in the reproductive process before a zygote is formed. A zygote is a genetically unique fusion of the male and female reproductive cells, also known as a fertilized ovum. Despite both sexual cells carrying a half-cargo of chromosomes, neither are considered to be genetically unique organisms until they combine. An ovum has been described as some scientists as an inert globule of matter, until it is fertilized by the sperm.

Postzygotic

• The term postzygotic refers to any point of life beyond the fertilization of the ovum. This is not to be confused with the beginning of a pregnancy; medical practitioners to not acknowledge the beginning of gestation, until the zygote attaches itself to the uterine wall, usually occurring 10 days after conception. Although pregnancy is not considered to have occurred at this stage, the zygote is now considered to be a living organism, as it has a metabolism, exhibits growth, reacts to stimuli and reproduces. These are considered the four criteria for life.

Reproductive Isolation

• One of the theories that uses the concepts of the pre and postzygotes is that of reproductive isolation. This is the theory that explains why species are generally unable to fuse via interbreeding. The theory states that certain mechanisms are in place to prevent this from happening; these mechanisms are divided into prezygotic and postzygotic isolating mechanisms. Prezygotic mechanisms include temporal isolation -- where species do not interact because they are active at different times, ecological isolation -- where differing habitat preferences means different species don't interact, and behavioral isolation -- where species interact but decide not to mate with each other. Other prezygotic mechanisms are mechanical isolation -- where the two species do have intercourse but sperm is not transferred -- and gamete incompatibility, where sperm is transferred but the egg is not fertilized.

Postzygotic Isolation Mechanisms

• Postzygotic mechanisms occur after fertilization and prevent the interspecies hybrid, either from being born or taking hold as a species. These mechanisms include zygote mortality -- where the fertilized egg dies in the womb -- and hybrid inviability, where the embryo is created but the hybrid species is handicapped or in someway disadvantaged. Other postzygotic isolation mechanisms are hybrid sterility -- where the hybrid species survives gestation and is able to survive in the wild but is sterile, and hybrid breakdown, when first generation hybrids are able to survive and reproduce, but subsequent generations become sterile.