Fertilization is the impregnation, the state when<span> a sperm cell penetrates the egg cell and the genetic material of both cells combines.
A few days after fertilization i</span><span>mplantation happens. Implantation is the attachment of the fertilized egg to the uterus, happens on average about 9 days after ovulation/fertilization (between 6 and 12 days) and is required for the fetus to continue to grow.</span>
The important point is that the surface area<span> to the </span>volume ratio<span> gets smaller as the </span>cell<span> gets larger. Thus, if the </span>cell<span> grows beyond a certain </span>limit<span>, not enough material will be able to cross the membrane fast enough to accommodate the increased cellular </span>volume<span>.</span>
Fitness is the answer.
Evolution is the process of a species changing.
Mutation is also a species changing.
Variation is the differences between animals of the same species.
It is hypoxia, or low levels of oxygen
