How does evolution explain two separate sexes?

Granted that with asexual reproduction, there is no need to find a mate. The chances of any given organism reproducing will be significantly higher as it does not require finding a mate. However, the offspring will be genetic clones of the original organism, making adaption and evolution entirely dependent on mutations influenced by outside conditions. This strategy will not work good if any major geological or atmospheric changes occur.

Sexual reproduction creates greater genetic variety and allow organisms to change over generations. The result is the huge variety of species on the earth today. It encourages change when the environment changes. Another huge advantage with 2 genetic strands is that organisms have backup info in case harmful mutations occur.

The actual origins of sexual reproduction can only be guessed at, but it probable that it goes back to very early eukaryotes. Perhaps it started more as a selfish viral way for one organism to insert its dna into anothers offspring.

>"It seems the only way possible for this to have occurred is if TWO single cell organisms were created in roughly the same area at the same time and evolved into the same species."

Not at all.

First, sexual reproduction itself obviously does NOT require two separate sexes. (Look at flowering plants for an obvious example ... or some single-celled protozoans, like some amoebas, that reproduce via sexual reproduction, but do not have "males" and "females".)

So you need to separate the origins of sexual reproduction, from the origins of separate sexes.

In fact, when you really look at it, your question is actually four questions ... four separate developments in the history of living things:

1. the development of sexual reproduction to begin with (cells from two different individuals combining to form a new individual);

2. the development of separate male and female gametes (sperm or pollen vs. egg cells or ova);

3. The development of separate male and female *organs* (one for distributing large numbers of male gametes vs. one for receiving male gametes and allowing fertilization);

4. The specialization of *individuals* within the species, that have only one type of organ vs. the other.

Here's a *rough* outline:

First came the separation of the diploid state (where all chromosomes are paired) and a haploid state (where the chromosome pairs are separated, so each cell has half the full number of chromosomes). For example, slime mold amoebas are normally diploid, but can form small haploid cells ... called spores ... that can travel and combine with other spores to produce new diploid individuals (fertilization). That's basic sexual reproduction.

The next step is in the slow specialization of two kinds of haploid cells. Some get smaller and are specialized for mass-production, and for lightness (so they can be carried farther by air or water) ... while others get larger as they contain all the nutrients needed to start a new individual after fertilization. The smaller cells we call "male gametes" (sperm or pollen), and the larger ones we call "female gametes" (egg cells or ova).

The next step are organs that specialize in either mass-producing and distributing male gametes, or specialize in producing female gametes and providing an environment for growth after fertilization (e.g eggs in the case of animals or seeds in the case of plants).

And finally comes the specialization of individuals to have only one kind of sexual organ or the other, rather than all individuals having both (the way most flowering plants are). Male and female organs will continue to get more and more differentiated from each other, but will always work together. That's because those individuals of any generation that do *not* work well with the sexual organs of the opposite sex, don't reproduce, and therefore those genes don't last long. But other than that ... any slight alteration in the sex organs of either sex that makes it a little better at doing what it does best (e.g. the way that placental mammals slowly kept the egg internally for longer and longer until the egg is never laid outside the body, but becomes a placenta ... and the young "hatch" directly from the mother).

Again, that is a *ROUGH* outline of about 3 billion years of evolution. Each of those steps, has many, many different substeps .... and as you learn more about biology, you will encounter MANY different variations of these different methods for reproducing ... but I hope that gives the basic idea.

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Dealing with extant organisms …

The unicellular eukaryote Chlamydomonas undergoes sexual reproduction. However, the gametes are not specialized into eggs and sperm.

In the volvocaceans, sexual reproduction includes gametes that are specialized into eggs and sperm, and there are male and females.

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“Unicellular protists and the origins of sexual reproduction

Sexual reproduction is another invention of the protists that has had a profound effect on more complex organisms. ...

The union of these two distinct processes, sex and reproduction, into sexual reproduction is seen in unicellular eukaryotes. Figure 2.8 shows the life cycle of Chlamydomonas. This organism is usually haploid, having just one copy of each chromosome (like a mammalian gamete). The individuals of each species, however, are divided into two mating types: plus and minus. When a plus and a minus meet, they join their cytoplasms, and their nuclei fuse to form a diploid zygote. This zygote is the only diploid cell in the life cycle, and it eventually undergoes meiosis to form four new Chlamydomonas cells. This is true sexual reproduction, for chromosomes are reassorted during the meiotic divisions and more individuals are formed. Note that in this protist type of sexual reproduction, the gametes are morphologically identical; the distinction between sperm and egg has not yet been made.”

(http://www.ncbi.nlm.nih.gov/books/bv.fcgi?highligh...

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Although all the volvocaceans, like their unicellular relative Chlamydomonas, reproduce predominantly by asexual means, they are also capable of sexual reproduction, which involves the production and fusion of haploid gametes. In many species of Chlamydomonas, including the one illustrated in Figure 2.10, sexual reproduction is isogamous ("the same gametes"), since the haploid gametes that meet are similar in size, structure, and motility. However, in other species of Chlamydomonas as well as many species of colonial volvocaceans swimming gametes of very different sizes are produced by the different mating types. This pattern is called heterogamy ("different gametes"). But the larger volvocaceans have evolved a specialized form of heterogamy, called oogamy, which involves the production of large, relatively immotile eggs by one mating type and small, motile sperm by the other (see Sidelights and Speculations). Here we see one type of gamete specialized for the retention of nutritional and developmental resources and the other type of gamete specialized for the transport of nuclei. Thus, the volvocaceans include the simplest organisms that have distinguishable male and female members of the species and that have distinct developmental pathways for the production of eggs or sperm.

(http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=dbio...

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I think it was that the cells evolved into two sexes because that is the only other way that Evolution could continue :)

"And even in this case, there really wouldn't be a benefit to having a sexual partner."

Did you take 8th grade biology?

Did they not teach you about genetic recombination or anything?

"Is there simply no current answer for the origins of the sexes? Or is there a reason?"

Frankly, do you know what 'google' is?

http://www.google.com/search?client=safari&rls=en&...