Unicellular organisms, posseses relatively simple organisations. So, the asexual mode of reproduction is common in them. It is so because through 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 gamets have to fuse, while in asexual reproduction, cell division takes place.
Heterotrophic organisms such as 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.