Isn't both male and female to evolve separately?

Question

In man or in any other animals where reproductions happens only by sexual union of a male and a female of te same species, isn't it necessary that there should be at least two induviduals come to exist in the process of evolution or creation?

The science tells that every newly born individual with reproductive capacity (non-hybrids) would belong to the parent species only.

So if a new species has to evolve and continue its existence, as interpreted by evolution theory, isn't it a minimum requirement that at least two individuals should be evolved simultaneously or the second one of the opposite s*x evolved during the healthy life span of the other?

Will some of the experts explain the chance of such an evolution process?

Answer 1

No - both males and females evolve simultaneously.

I like this analogy I originally saw from Secretsauce (the answerer above here):

In ancient times, the people in Italy spoke Latin, but nowadays they speak Italian.

The change from one language to the next was gradual - there was no "first Italian speaker". Every generation, there were changes to the language introduced, but each generation would be capable of conversing with their parents, grandparents, and great-grandparents. But over time, the language changed radically - so nowadays it is a different language, and a modern Italian-speaker would not be able to converse with an ancient Latin-speaker.

It is the same with evolution:

Each generation small changes in gene frequency occur, and new traits are introduced by mutation. But each generation would be capable of breeding with its parents' generation, grandparents' generation, and great-grandparents' generation. But over time, larger-scale changes accumulate, so a modern individual would not be capable of interbreeding with an ancient member.

For chromosome number changes - you are correct that the number of chromosomes makes a difference, and that organisms with different numbers of chromosomes often are unable to interbreed. But it is actually the genes on those chromosomes which make the largest difference. So if an individual was born with a chromosomal fusion, and therefore had one less haploid chromosome number (two less diploid) than normal, they *might* be unable to interbreed with other individuals who hadn't had that change. This is what happens when a horse and donkey mate to produce a mule or hinny.

But if there were sufficient genes retained unchanged, then the might still be able to breed. Often, however, the only option is breeding with siblings and close family members, who share the same (new) chromosome number.

This is rarer in animals though in plants, the possibility of polyploidy events and self-fertilisation make it more common.

Answer 2

No.  It most definitely NOT is not a requirement that two individuals be evolved simultaneously of opposite s*x.

First, sexual reproduction does not require an organism to be *exclusively* sexually reproducing.  There are many examples of organisms that do *both* sexual and asexual reproduction.   Amoebas, for example, spend the majority of their lives reproducing asexually (just dividind), but then when resources get scarce, they collect in a colony that creates a stalk that reproduces sexually ... releasing spores into the air that fertilize other spores and start the process over.

Second, sexual reproduction does not require separate sexes.  There are many examples of organisms that use sexual reproduction, where every individual is both male and female.  For example, the majority of flowering plants work that way, and many aquatic species.

And third, the difference between male and female can be very very small at first, and slowly get more different over time:  

* First comes differentiation between two kinds of s*x cells ... some cells getting smaller and smaller so that they are more mobile and easier to mass-produce in huge quantities (sperm or pollen) and other cells getting larger and larger so that they are better equipped to start dividing once fertilized (egg cells).  

* Then come differentiation between the organs for producing and distributing each of these two kinds of cells, and making fertilization and early cell development easier.

* Finally comes differentiation between two kinds of individuals, one that contains only male organs and the other only female. And I won't get into the many different ways this occurs (it can be determined genetically, or in temperature during incubation, or by what it is fed during a larval stage).

All of the above took place very early in evolution, in aquatic species long before life emerged on land.   By the time the first amphibians began laying eggs in water, but living part of their lives chasing the insects on land, there was already differentiation in the sexes.   All subsequent animal species were partially or fully sexually reproducing with separate sexes ... and in mammals in one branch and birds in another branch, the difference between sexes slowly became more and more pronounced.

So at no point did there ever have to be the simultaneous birth of two members of a new species with opposite sexes.

Answer 3

No, because any individual will still be genetically close enough to it's parents to still be able to reproduce with them or anyone they're capable of reproducing with.  We like to try to think of it in terms like 'there are the parents who are one species, then an individual is born which is an entirely new species' because we like to think of things as black or white, this species or that.  But nature does not work that way, it's all a gray area and you can't pick out a single generation where speciation occurred.  There isn't enough change from one single generation to the next to make the two generations reproductively incompatible with each other.  The offspring will always be capable of reproducing with the rest of their population, however, after enough generations, the current population will be incompatible with the original population.  You can't say 'generation 500 was species A and generation 501 was species B.'  What you can say is 'generation 1 was species A and generation 1000 was species B, the change occurred during that time.'

The key to remember is that the individual doesn't evolve.  The *population* evolves.