Amoeba and yeast reproduce asexually by fission and budding respectively, because they are
1. microscopic organisms
2. heterotrophic organisms
3. unicellular organisms
4. uninucleate organisms
(c)
Unicellular organisms have a relatively simple organisation. So, the asexual mode of reproduction is common is them. It is so because by asexual reproduction unicellular organisms can multiply very fast. In Amoeba it occurs by binary fission and in yeast by budding to be described first.
In the sexual reproduction, both male and female gametes have to fuse, while in asexual reproduction, cell division takes place.
Heterotrophic organisms (humans, animals, and decomposers) can reproduce either asexually or sexually, e.g., in bacteria sexual reproduction occurs via conjugation and asexual reproduction occurs via binary fission.
Uninucleate organisms, like Ulva (algae) reproduce asexually by zoospores and sexually by the fusion of gametes.
