Theory of Asynchronous Evolution


IntroductionChromosomesTheory of Sex | Asymmetry | Questions | FAQ | Articles | Glossary | Site Map |


The Evolutionary Theory of Sex: Three Main Types of Reproduction

Asexual — a form of reproduction, such as budding or simple fission that does not involve the fusion of gametes.  With this type one kind of organisms, produce the same progeny.

Asexual type is common amongst bacteria, algae, amoebas and infusorians.

Hermaphroditism — the coexistence of both male and female sex organs in the same organism.

Worms, many mollusks, some fishes, vast majority of plants.

Dioecious (bisexual) — two kinds of organisms, male and female.

Female — an individual that bears young or produces large usually immobile gametes (as eggs).
— a pistillate plant.
Carries Venus sign ♀

Male — an individual that produces small usually mobile gametes (as spermatozoids) which fertilize the eggs.
— a staminate plant.
Carries Mars sign ♂

Primitive organisms are asexual.

Hermaphrodite types can be considered as an intermediate or transitional stage to dioecy

All advanced species (mammals, birds, insects, dioecious plants, and others) are bisexual.

Three Main Types of Reproduction

First important event - appearance of crossing (fertilization) processes - lead to creation of sexual forms.  Crossing made possible to combine the genetical information of parents. With crossing all types of reproduction, existing in animals and plants was divided by two main types—asexual (no crossing) and sexual (crossing is present).

Second important event was appearance of sexual differentiation—division into two sexes with prohibition of crossing between the organisms of the same sex. The sexual process and sexual differentiation are different and, in essence, opposite phenomena. The first creates (increases) diversity of genotypes, and this is its evolutionary role; the second, in fact, decreases it in half, and nobody can explain its evolutionary role.

Depending on the presence or absence of crossing and differentiation, the existing ways of reproduction can be divided into three main forms: asexual, hermaphrodite, and dioecious (Figure).

Types of Reproduction

Advantages and Disadvantages of  Different Reproduction Types:

Asexual types:

Advantages: There is no need to find a partner. Simplicity.

Disadvantage: Slow changes only by accidental mutations.

Advantages of fertilization:

Genetical recombination

Avoid inbreeding

Amount of combinations for  N  organisms:

Asexual                                                  N

Hermaphrodite    N * (N - 1) / 2  ~=  NČ / 2

Bisexual              (N / 2) * (N / 2)  =  NČ / 4

What are the Advantages in Having Two Separate Sexes?

There is a clear advantage of sexual forms compare to asexual, but nothing so far can justify transition to dioecy.

Bisexual forms have 2 times less combination potential compare to hermaphrodites, so advantages of differentiation are still unclear.

Back to The Evolutionary Theory of Sex



Copyright © 2005-2009 S. Geodakyan. All rights reserved.


  www Site

html hit counter
html hit counter code