evolutionarily ancient form of reproduction on the planet - is asexual reproduction.It is a division of the single-celled organism (or cells of a multicellular organism) to form subsidiaries individuals completely identical parent.This form of reproduction is most frequently observed in prokaryotes, fungi, plants, protozoa, and is also found in some animals.
Among the types of asexual reproduction can be called by dividing (a doubling of ring chromosomes in prokaryotes, and mitosis in the simplest single-celled algae), sporulation in fungi and plants (higher and lower), vegetative propagation of higher plants.For asexual reproduction also include fragmentation of worms, some algae, fungi, budding freshwater hydra and corals.
Asexual reproduction in favo
rable conditions can dramatically increase the number of individuals of a given species.However, all the descendants of the parent have an identical genotype and increasing genetic diversity hardly occurs while changes acquired during sexual process, could be useful for adaptation to the new environment changed conditions.That is why most of the living organisms continuously or periodically reproduce sexually.
In sexual reproduction new individuals are the result of the merger of two haploid sex cells - gametes, and forms a diploid zygote, which develops from the embryo.The gametes are formed in the genitals of males and females.The genetic information of the parent species combined, increasing the diversity and vitality of the offspring.
The body hermaphrodites - bisexual animals - can form simultaneously two types of gametes - male and female.Historically, these animals were older.These include coelenterates, flat and segmented worms, a number of shellfish.But later appeared dioecious species in evolution began to dominate and achieved better development, although self-fertilization hermaphrodites in some cases also has its advantages (for example, when the low probability of finding a sexual partner).
primitive form of sexual reproduction found in bacteria and protozoa.So, ciliates shoes sexual process called conjugation, during which the two converge ciliates and partially communicate with each other hereditary material.At the same time they may be new, useful adaptive properties.But the number of individuals as a result of conjugating ciliates not increased, so it is called the sexual process, not a reproduction.
Another type of sexual process - copulation.It is observed in a number of single-celled organisms, their cells are transformed into the same gametes, and fuse to form a zygote.Only one kind of germ cells formed in the most ancient organisms (isogamy), these gametes can not be distinguished or to say whether they are male or female.When heterogamy male and female gametes (sperm and eggs) are very different from each other, they have different sizes, structure and functions.