ABSTRACT
Our planet teems with life. In the sky, on the land, in the sea, and in between, there are some millions of species of plants, animals, fungi, bacteria, and viruses, and almost all thrive as they develop and reproduce using energy directly or indirectly derived from the sun. is is possible because each species is appropriately adapted to the environment in which it lives: its ecosystem provides it with food, and it, in turn, provides nourishment and a context for other organisms, sometimes while it lives and always when it dies. Unicellular organisms, be they prokaryotic or eukaryotic, can reproduce by simple division, whereas multicellular organisms mainly reproduce by mating – no multicellular individual survives or reproduces in the long term unless it is part of a population.
