Asexual Reproduction
Early Protozoans adopted this method as the primary means of multiplication.
Enabled them to build up large populations in shortest possible time.
Binary fission, multiple fission, budding, plasmotomy etc.
Best suited to these animals which possessed delicate bodies, no apparent means of defence and therefore, needed a speedy way of multiplication.
In asexual method, all individuals produce offspring without the help of other individuals.
Saves energy and time.
Disadvantages: Produces clones and does not produce diversity.
Sexual Reproduction
Sexual reproduction involves two types of individuals producing two types of gametes whose primary aim is to exchange the genetic material and to bring about genetic diversity in the population.
Power of recombination is so great that chances of two individuals being identical are almost nil.
Such a trait is advantageous and is favoured by natural selection.
In sexually reproducing populations, only 50% individuals produce offspring, while the rest half just contribute their genes, thus reducing the reproductive capacity, whereas in asexuals all individuals produce offsprings with no help from others.
Evolution of Sex
First there was asexual reproduction – fast and simple but produced clones and could not get rid of harmful effects of mutation.
Meiosis evolved to reduce the no. of chromosomes to half.
Nucleus and the cell division apparatus were necessary to invent the process of meiosis that produced
gametes carrying half the number of genes.
Isogamy produced equal sized gametes that had equal amount of energy investment.
They were large cells, had low searching capacity and did not have particular attraction towards each other to fuse and form zygote.
Anisogamy: Different mating types originated.
Anisogamy to some extent tried to solve the problem of cell fusion.
Large number of mating types was reduced to two.
Larger gametes (females) specialized in the storage of nutrients and development of embryo. Could now be produced in small numbers. Being in small numbers became limiting resource, triggering competition, and in turn selection.
Small gametes were in large number and competed to fertilise the large gametes.
Evolution of anisogamy from the primitive isogamy happened due to two selection forces:
1) Increased efficiency in finding mating partner in anisogamy.
2) Supply the zygote with sufficient amount of reserve food for development of embryo.