How does Evolution account for the 2 sexes?

Question

In order for evolution to begin (through reproduction) isn't it necessary for the complex reproduction system to already be in place? How are females and males so different, yet work together so well to reproduce? I understand how evolution can account for a change in a species, but I don't get how it can account for the origin of the sexes.

Answer 1

>"In order for evolution to begin (through reproduction) isn't it necessary for the complex reproduction system to already be in place?"

No. Like anything else in evolution, complex systems can evolve from simpler ones. Just as, say, a circulatory or a nervous system can get more complex in each new species, so too can the reproductive system. You need to break it down into smaller substeps:

(a) the development from asexual reproduction, to sexual reproduction;
(b) the development from sexual reproduction with no distinguished sexes, to two different sexes; and
(c) the slow development of those two sexes so that they become more and more different from each other.

Just the development of (a) could take a long description, because this itself needs to be broken down into substeps ... such as the development of exchange of genetic material (as occurs even in asexual bacteria); the development from haploid to diploid organisms; the development of fission or meiosis (the splitting of a diploid cell into separate haploid cells); fusion or fertilization (the joining of two haploid cells to produce a single diploid cell).

All of that took more than a billion years (longer than all multicellular life has been around); and seems to have been well in effect before multicellular life arrived (as most multicellular life seems to use some sort of sexual reproduction).

But the part you are asking about is (b) and (c). This also takes a LOT of substeps. First, it's important to note that an organism doesn't *have* to use either asexual or sexual reproduction ... an organism can do both. For example, amoebas can reproduce asexually in a plentiful environment, but if food gets scarce, the amoebas congregate into a slime, build a fruiting stem with a spore cap on the top, which bursts and releases haploid cells into the air (spores), which can travel in the air and find and fertilize spores from other amoebas (sexual reproduction). There are many other examples, even in modern organisms, that can reproduce both asexually, and sexually (for a multicellular example, check out aphids).

The second step is understanding that once sexual reproduction is in place, at first there may be no difference between the types of haploid cells (gametes) involved. That's what we call "spores" ... undifferentiated cells that fertilize each other. But there can be advantages if there is slow differentiation between two types of cells ... small mobile cells designed for mass production and wide dispersal (by water, wind, or other methods) ... what we call 'pollen' in plants and 'sperm' in animals; and larger cells with built-in food supplies, designed to be fertilized *by* the other kinds of haploid cells and then survive long enough to begin mitosis and producing new diploid cells ... the larger gametes we call 'seeds' in plants' or 'egg cells' in animals (or 'ova').

Next, it's important to understand that there can be sexual reproduction, with the two kinds of sex cells, and still all individuals can be both 'male' and 'female' ... i.e.. all individuals have parts for making pollen or sperm, and parts for making ova and receiving pollen or sperm to fertilize the ova. Most flowering plants still use this system, and many non-vertebrate animals (molluscs, echinoderms (starfish)) do too (called hermaphodites).

The next step, is the slow development from sexual reproduction with undifferentiated sexes, to slow differentiation of dedicated males and females. Even there this gets broken down into many substeps ... as there are many organisms in which individuals go through phases where they change from male to female; or become male or female depending on what other individuals are available to mate with (some slugs); or become male or female depending on what they are fed as larvae (some ants); or males are haploid and females are diploid (bees and wasps); or become male or female depending on temperature during gestation (most amphibians and reptiles); or become male or female depending on whether they have an X or Y chromosome (the system used by mammals).

Every single one of these developments deserves a lot more description, and examples, than I can provide here ... and as you are probably most interested in mammal reproduction, I provide a link to the evolution of the Y chromosome (the second link below).

And the final topic (c) is understanding the slow development where the two sexes slowly become more and more different from each other (what's called 'sexual dimorphism'). In many animals, such as most amphibians and reptiles, and even many mammals, there are NOT a lot of differences between males and females, except specifically in the reproductive organs. What happens in species that have a Y chromosome is the development of secondary characteristics that make males and females more *visibly* different. This is as a result of mating behaviors, competition for mates, etc. It is quite obvious in primates (and we humans are specifically atuned to sexual differences between fellow humans), but it is not nearly as pronounced as it is in some other mammals (look at walruses, or ungulates like deer, moose, cows, or goats), and nothing to how pronounced it is in birds. Again this is a long evolution that I can't do justice to here. (See last link for a start.)

So I see my "short" description already got pretty long.

So summary: This happened long before there were mammals, and is the result of *MANY* substeps occupying over 2 billion years. To get a proper explanation we need to keep breaking it down into substeps and asking (a) what are the advantages of this substep?; and (b) do we see any examples of organisms living *today* that still use a system that represents this substep.

This represents over 2 billion years worth of evolution, so it takes a bit of study if you really want to understand it.

Answer 2

No, it's not necessary for the reproductive system to be in place. Evolution works equally on organisms that reproduce asexually, hermaphrodites, etc.
Female and male *humans* are only slightly different. We all start out as female, and maleness comes about by a series of molecular events in the embryo. All of our "parts" are analogous, believe it or not.
Sex determination, it turns out, is just a more labile process than you'd imagine. It gets done very differently in different sexually reproducing species. It's not as primal a thing as you'd imagine.

Answer 3

Evolution has nothing to do with frogs mating with fish and man with monkeys. Reports say that such experiments tried out in Russia failed. Evolution occurs on rules based on Philosophy and deeper truths like Natural Selection and Origin of Species. A true New Species of evolutionary Origin, if you have Darwin's book, is born like Jesus, from a Virgin Mother. This proves Natural Selection, since two recessive genes express. Without knowledge, your questions are very baseless and inappropriate.

Answer 4

Remember that many organisms - including the simplest, like bacteria - reproduce *asexually* (binary fission).
So sex is *not* required for evolution - just reproduction.
Additionally, many single-celled organisms can undergo a form of sexual reproduction by fusing together, and then subsequently dividing. This allows for the mixing of genes, and also provides a mechanism for evolutionary variation.

The idea would go that the appearance of multicellular, diploid organisms allows for the first "sex cells" as haploid cells produced by meiosis. These sex cells wouldn't even neccessarily be specialised, but evolutionary traits like the ability to be motile, or containing an energy store would be advantageous, and would lead to the development of separate egg and sperm gametes.
This still doesn't require entirely separate sexes though - as the organisms would likely be hermaphrodites, or might be male one season, and female the next.

The appearance of separate genders would be a late-stage development.

One theory is that it is the endosymbiotic relationship with mitochondria that gave rise to entirely seperate genders:
Since mitochondria are *only* passed down in egg cells, it is to the mitochondria's advantage to be in a "female" organism. As a result, mitochondria evolve to "attack" the male gonads of hermaphroditic organisms by actions like oxidative radical production (this has actually been observed in hermaphroditic species). Evolutionarily, this maximises their chances of being passed down in an egg.
This means that instead of having a population of totally 50/50 male/female hermaphrodites, you get organisms that are "more female than male", with possibly a few (sickly) "more male" individuals.
Eventually, the population consists of many entirely female organisms, with a few male individuals. As soon as this happens, there is lo longer any evolutionary pressure on the mitochondria to sabotage the male gonads, because they have no chance of being passed on from a male organism anyway. Therefore, the evolutionary pressure on the "host" organism takes over, where it is more advantageous to have a roughly equivalent number of males to females, and eventually, with more evolution, the proportions equalise.

Answer 5

Different sexes started vary early in evolution. It probably started with simple organisms consuming other organisms or two morphing together, then taking in their DNA as they reproduced. This would help for genetic diversity. Eventually the ability to combine genetics in a reproductive way evolved.

Answer 6

Evolutionary Theories on Gender
and Sexual Reproduction
http://www.trueorigin.org/sex01.asp

Comment on Grace's answer. You can believe anything you want. Science is science. What you are actually doing is called denial. If you reject evolution, you have to reject a major part of biology. Not only that, but other theories and laws are aligned with evolution. You'd have to reject those as well. You also make the common error many people make. You believe the misconceptions about evolution, maybe because your faith is stronger than logic.

Answer 7

If everyone had the answers, there would be no God. The is always the pros and cons and arguments. Science does not know it all and could still not explain it all, not yet perhaps. Some things just need faith. Science cant explain God but still, almost everyone believes in God.

Evolution isnt really confined to one explanation only. Circumstances might occur during evolution of species and voila, male and female. Maybe, there was only one type of human. Then maybe, the cells got hyperactive and decided to divide! So, one half splits with an extending thing in front while the other has a hole and breast. it makes sense doesnt it?

Answer 8

Without opposite sexes there would be deformities due to inbreeding. Everything needs a + and a -.

Answer 9

evolution doesn't account for many things such as the eye. This is why it is still a theory and not a law. There has been no evidence to prove that evolution occurred only hypothesis and guesstimating. I have found many flaws within this theory that's why I don't believe in it.

Addition: Jim I don't have to reject the subject of biology just because I don't believe evolution. Maybe I just have more education of the subject than you do. On a biochemical level evolution begins to have many flaws. Do some research on it.

Answer 10

THE STORY OF ADAM AND EVE TELLS IT .SO READ THAT FROM THE BIBLE.