Why do some species employ both mitosis and meiosis, whereas other species use only mitosis? see concept 13.2 (page 258)?

Sex cells divide through meiosis and non sex cells divide through mitosis in animals while in plants sex cell divide through both mitosis and meiosis

Explanation:

In humans and all other animal species, the non sex cells such as skin cell, muscle cell, blood cells etc. divided by mitosis. It is so because these cells deplete at a fast rate and hence their retrieval also needs to be fast. Also these are diploid cells hence cell divide through mitosis. Sex cells such as male and female gametes are haploid cells and hence their divide through meiosis.  

In plants gamete formation requires both mitosis and meiosis cell division.