Sexual Reproduction in nature is reinforced by Natural selection in many ways. What we need to know about sexual reproduction is this: Sexual reproduction leads to diversity due to its mechanism. The first reason for this diversity is crossing-over, random gene changes, which happens during Prophase I. This and phenomena like this are responsible for offsprings being different than their parents. This can not be seen in Asexual Reproduction, since the genetic material (DNA) is cloned and transferred to both offsprings.
An example why asexual reproduction has negative aspects is this:
Let us think that lions would use asexual reproduction (that a lion would dublicate itself to two small lions, as funny as it might be). The big lion which dublicated, would be the same as the two small lions both genetically and physically (if we ignore mutations). So we would have a population of the exact same individuals, hundreds of the same lions, identical in every way! Now, let’s think of a fatal virus entering one of the lions. This virus is highly contagious and kills the animal in a short period of time. Since all the lions are identical or very similar, the virus would be able to kill every single lion contaminated to the virus, since the lions had no genetical diversity and no tolarence to the virus. So the great line of lions would perish, because of one simple, single virus.
However; today is the chances of such a situtaion pretty slim. For instance, we all had suffered influenza or even the swine flu. Because of this illness hundreds of people lost their lives but nothing close to the lion example had happened, the human race had not suffered extinction. This is because of our different genetic materials: some of us are more immune to certain influenza types, whereas others to malaria, and so on… The main reason is diversity caused by sexual reproduction.
Sexual reproduction has a bunch of more benefits. Some of them are:
- Since different organisms have different chances of survival, sexual reproduction increases the chances of of survival of their offsprings.
- Meiosis (sexual reproduction), prevents the deleterious mutations, which are generally harmful to the individual, to pass to the offsprings.
- Thanks to meiosis, corrupted parts of the DNA can be repaired and passed on to the new generations accordingly. This actions can not be seen during Mitosis.
- Owing to those reasons, natural selection supported sexual reproduction billions of years ago. The primary reason is of course the increase in diversity. Other reasons can be seen as secondary. An article about the advantages of sexual reproduction from J. F. Crow from the University Wisconsin is below.
But when did the first sexual reproduction occured? Which species evolved meiosis from mitosis?
We certainly know that meiosis evolved from mitosis. In general, but not in all cases, the simple evolves to the complex and meiosis is much more complex then mitosis.
Comparative evidences show that meiosis evolved in the early stages of the eukaryotes (cells containing complex structures enclosed within membranes).[n]M. A. Ramesh. (2005). A phylogenomic inventory of meiotic genes; evidence for sex in Giardia and an early eukaryotic origin of meiosis. Current Biology.[/n][n]A. M. Schurko. (2008). Using a meiosis detection toolkit to investigate ancient asexual “scandals” and the evolution of sex. Bioessays.[/n] Besides, the high similarity of the progress among all species support the idea that meiosis evolved only once.
How meiosis evolved from mitosis can be seen on the gorgeous article here, step by step and in great detail.
In short: Meiosis evolved billions of years ago in the bacteria, the first eukaryotic organisms, and moved on until today.
When and on which species have the sexual organs evolved first?
This question can be answered by scientist as well. The first sex organs might had evolved on two different organisms:
1) Funisia dorothea
These tubular invertebrates, that lived 565 millions of years ago, are believed to be the first organisms which developed sex organs.
It is believed that F. dorothea, the ancestor of sponges and coral reefs (sponges are in the kingdom of animalia) were the first organisms which developed organs to keep sperms and egg cells. In the animals before them were no such specialized constructions.
But some scientists of the University of California state that there is no doubt that these animals use sexual reproduction but they have doubts of the specialized sex organs of these animals.
2) Materpiscis sp.
This fish is believed to have developed the first sex organs. It is believed that this animal evolved the sexual reproduction 410- 400 million years ago.
The scientists of the Los Angeles Natural History Museum, who think that the evolution of jaws were more related to reproduction rather then nutrition, state that a great many of sharks use their jaws and teeth to fix the female during copulation. Scientist believe that the specialization of the os coxa lead to the development of sex organs.
Furthermore, another reason for the development of the primary sexual characteristics is that the sea is chaotic and a lot of fish have to swim constantly in order not to sink. This decreases the chances of sperms and eggs to meet and fuse. Because of this reason, fish developed structures like hooks in order to hold on to the egg sacs of females. Millions of years of evolution led to the development of the female sex organ. So, the sex organs evolved to increase the chance of reproducting by limiting the area.