Why do some species employ both mitosis and meiosis, whereas other species use only mitosis?
But just why do some species employ both mitosis and meiosis, whereas other species use only mitosis?
One option an organism has to grow is for cells to replicate themselves to create more cells (the alternative is for each cell itself to expand in size). A cell can replicate itself by creating an identical copy so the organism is left with two identical cells. Essentially, in mitosis, the nucleus of a cell divides and distributes the chromosomes to the two daughter nuclei resulting in each daughter cell having chromosomes that are identical to those of the parent cell. This is what is known as mitosis. Single celled organisms use this process in order to proliferate as a species. Multicellular organisms (like humans) employ meiosis to grow during development or to regenerate damaged tissue.
The other is for cells to make half copies where, through the process of fertilized, their genetic content will be made whole again. This is what is known as meiosis. Only organisms which perform sexual reproduction use meiosis.
So, why do some species employ both mitosis and meiosis? Because they themselves need to grow and repair (thus the need for mitosis) but they also reproduce and so need mitosis to produce animal gametes.