ASEXUAL REPRODUCTION

Asexual reproduction can occur by following mechanisms:

(i) Fission: Many simple organisms like protozoa and bacteria just divide or split into identical halves, forming new organisms. It can be binary fission, i.e., division of original or parental cell into two daughter cells or multiple fission, into many daughter cells, multiple fission, into many daughter cells, e.g., amoeba, malarial parasite, etc.

(ii) Fragmentation: This involves breaking  off of parts of fragments of the parental organism. Each part subsequently grows to form a complete new organism, e.g., flatworm.

(iii) Budding: In this type of reproduction, a small outgrowth or "bad" sprouts from the parent organism, which later grows into a complete individuals, e.g., sponges, Hydra, etc.

SEXUAL REPRODUCTION 

The basic precondition for this type of reproduction is fusion of male gametes known as "sperms" and female gametes- the "ova" or eggs, to form the zygote or embryo. In simple terms, zygote is  a "fertilized ovum" or fertilised egg. The union or fusion of male and female gametes to form a zygote, during sexual reproduction, called fertilisation.

  Sexual Reproduction leads to increased genetic variation in population because offsprings receive genes from both the parents, which get mixed during fertilisation  (meiosis). Further by providing genetic variation, sexual reproduction leads  to origin of new species, i.e.,  evolution.