Sexual and asexual organisms
To a first approximation, the situation is clear. Some species of organisms reproduce sexually, i.e. their descendants are formed by the merging of the sex cells of two organisms. Others reproduce asexually, i.e. their descendants are formed by splitting off of part of the parent organism (e.g. tubers for potatoes) or from individual specialized cells intended for this purpose (e.g. some species of stick insects and fish). On closer inspection, the location of the boundary between sexual and asexual reproduction is less clear and opinions of professionals on the difference between sexual and asexual reproduction need not completely agree. However, in this book, I will stick to the approach that considers asexual reproduction to be the formation of descendants with a genotype identical with that of one parent and sexual reproduction to be the formation of descendants with a genotype formed by the combination of the alleles of two parents. The fact that sexuality was almost certainly not originally connected with reproduction and fulfilled a completely different function is something I prefer to leave out here – this could even be the subject of a separate book.