Stacy Morrow e-mailed me the question below:-

My name is Stacy and I am a senior at Houston High School in the U.S. I love biology and genetics and have read a lot of material by Richard Dawkins, Matt Ridley, etc.

I really enjoy your posts at Ask a Biologist, and it seems you have some expertise in this area. Instead of posting on the site, I decided to e-mail you because it is quite long and detailed.
I was wondering if you could help me in an argument, I want to check my facts.

So, I got into a rather interesting argument with my friend about which sex came "first" in the evolutionary scheme. She seemed to be very "sexist" in the way she presented her side.

Basically, she argued that females obviously did because the Y chromosome was a "broken" X and because there are all-female species that do not need males. She said the first cells, since they were asexually self-replicating, were by definition "female". She contends that females are the natural primary default sex, and that males serve a very limited purpose, mainly evolved just to provide "genetic variation" within the species. She furthered her argument by pointing to embryology, that females are the developmental default and you have to have a Y chromosome to make a male. She told me to ask "any biologist" and that they would tell me the same.

It seems to me that she is twisting certain biological truths to purport a ridiculously sexist agenda.

Now, I know something about the evolution of sex myself, and I argued that there really is no basis to saying that one sex came 'first'. If my knowledge is correct, females are NOT defined by giving birth or even by chromosomal structure, but strictly by the types of gametes they produce (females produce large, immobile ones, and males produce smaller, mobile ones). Now, after reading a bit of Dawkins, I came across the theory of how differing gametes might have evolved. You originally have cells replicating themselves, no males, females or sex. Then, cells start fusing and exchanging genes - sex had arrived on the scene! For some reason, this conferred an evolutionary advantage. So, you now have "isogamous" cells that are the same size reproducing sexually (but, by definition, still no "males" or "females"). So right off the bat, you don't really need males OR females for reproduction, either sexually or asexually. I think some fungi still reproduce this way. Anyhow, among these medium-sized mobile cells, some gametes just happened to be larger than others. This may have been favored, because larger cells have more nutrition. So, cells got larger. Then, the smaller cells decided to cash in and "exploit" the resources of the larger cells. This became advantageous (to be smaller), because it cost less energy to make these cells if they fused with bigger ones. The large cells become larger to compensate for the nutrition-scarce smaller cells. So, in a runaway fashion, you get smaller, faster cells and larger, less mobile ones. Eventually, the really large ones become immobile (because they are constantly chased by small exploiters) and the smaller ones get even smaller and faster. These become, in time, eggs and sperm respectively. So the gametes seem to evolve in response to the other. In reality, you do not have one without the other.
Then, when you get to organisms, originally individuals produce both types of cells (hermaphrodites) and at some point there is a split where one individual produces only one type of sex cell. In response to my friend, in some worms, I think there are only males and hermaphrodites, so it is possible that either sex "split" first.

As far as her chromosome argument is concerned, I think it is just an evolutionary coincidence that females are homogametic, and therefore "default" in development and males heterogametic. In birds, I read that default development is male and it is females that have the smaller Y-like chromosome (termed W). Males are not losing any genetic information (they also have an X chromosome), and we had "males" before the Y chromosome (some types of mammals have lost the Y chromosome and still have males!) There is no reason to think that females are more "basic" or fundamental in anyway.

And finally, with regard to parthenogenesis, the reason these species are all-female is simply because females produce nutritional gametes, therefore any selection pressure that favored asexuality would, out of necessity, have to go through the female line. It does not prove that females came first, because in actuality these asexual species were once sexually reproducing - in other words, they originally had males! It raises, however, an evolutionary question of "why have males at all," simply because it seems a better way to reproduce than by chucking away half your genome to have sex. It is not about how males evolved, or why they appeared, as my friend states, by why they do not disappear! Why would a female put up with the cost of sex, when males don't contribute much materially? There have been several reasons given, the parasite theory of Ridley's book, or the theory that sex covers up bad mutations. Whatever it is, the cost of sex (and incidentally, males) is HUGE, and the fact that sex is ubiquitous means that males are maintained for a very, very important reason, certainly no "limited purpose".

Nevertheless, I argued that no organism is interested in anything beyond spreading its own genes - males did not originate for female's purposes or the purposes of the species - the fact that they perform a vital function in evolution is purely incidental (as is females'). Also, it doesn't really matter in our own species, since natural asexuality is genetically impossible (think genomic imprinting). From a gene's perspective, the male strategy and the female strategy are, on average, equally good. Nature, therefore, values the two sexes equally and will invest equally in them (even if it isn't for the "good of the species"), so arguments for overall superiority of one sex are futile.
That was basically my argument, and it's been a while since I've read any good books on biology. So are my facts accurate? Did I make a good case? That's basically why I'm e-mailing. Sorry that this was so long, but I'm sure you'll find it interesting!

I think that Stacy  made an excellent case, and she clearly has a good understanding of the biology behind sex and evolution. A female is defined as being the gender which produces the egg cell as opposed to the sperm. This is the only consistent differentiation between males and females, as one can find examples of males carrying the developing embryos (seahorses), no investment in parental care from either sex (many species of fish, where fertilisation is external), and as correctly pointed out the swapping of the sex determining chromosomes in birds, or in many reptile species the fact that their is no sex determing chromosome, and development is sent down the pathway of male or female based on the temperatures the egg is exposed to. In tropical clown fish sex is determined by social status, with individuals being capable of changing sex through their lifespan.
One can argue to some extent that the Y chromosome is a 'broken' X, as it contains the SRY gene, inheritance of which determines your fate as a male. The result of having this extra gene is that Y and X are no longer compatible for recombination (where pairs of chromosomes swap genetic material, allowing gene shuffling and repair). The Y chromosome is truncated when compared to the X because it has no opposite number with which it can recombine and no genetic template which cellular repair mechanisms can use to repair damage. Hopefully you can see how the Y chromosome is therefore more readily susceptible to accumulating harmful mutations and losing genes, so genetic information is being lost, though males still possess an X chromosome which picks up the slack (though it makes males more susceptible to genetic diseases caused by mutations in this region, instances of R-G colour blindness are much higher in men than women). Eventually the Y chromosome will be lost, though this will not be the end of males as has been claimed in the past. Other species have already passed this point in their evolution and either other sex determing mechanisms evolved or the SRY gene has fused with another autosome to start the process all over again.
The presence of the Y chromosome tells us nothing about whether females or males came first, sex is believed to have evolved before multicellularity evolved  roughly a billion years ago, and the XY sex determination system obviously is a much newer development (around 160 million y.a.) as it is present only in marsupial and placental mammals (even monotreme mammals such as the echidna use the ZW system found in birds). Obviously in birds with ZZ creating males and ZW creating females, here it is the females with the 'broken' chromosome.

With regards those species which have forgon males entirely, these are 'recent' (evolutionarily speaking) events, where sexual species have reverted to an asexual state. Stacy shows that she is familiar with the arguments surrounding the twofold cost of sex, and therefore the obvious importance of sex. It must be remembered that evolution is a continuous process, and one that is without forethought. Basically what I'm trying to say is that it is entirely possible for a species to rid itself of males, and it will benefit from the short-term gain of reproducing more quickly. However it is also probable that a species which has rid itself entirely of sex, has also signed its own death warrant, gaining a short term benefit, but eventually succumbing to death as a species. It is noteworthy that there are no 'old' species of asexual animals.

I would summarise that as we define males and females solely in terms of their gametes, one cannot determine that one sex evolved before the other. We determine gender by its opposite number. Without males there is no such thing as a female. And indeed at the evolutionary origin of sex, organisms would have been BOTH male and female at the same time, with a rigid one sex system evolving much later on several separate occasions.

I think Stacy made an excellent summary of the evolutionary origins of sex and gender. I do not have the exact details of her friend's argument, but it would appear to be a flawed understanding of the facts. And any argument for one genders superiority over another would be appear to be an ultimately futile act, as the male and female sexes are not only often found within one organism at the same time. But also have a relative importance in relation to the frequency of the other sex. Males become much more important than females to a species when their frequency is low, and vice versa. Also the myriad of different mating strategies and levels of parental investment will alter the relative dynamics between the sexes.

Any chance Stacy has future aspiration as a biologist, personally I think it sounds like she would excel!