All that, at first glance, different men and women, male and female, is the result of anisogamy.
If you conduct a survey on the subject, most likely, is not difficult to answer. As once confessed to his misfortune woodpecker tale of Saltykov-Shchedrin, to distinguish one from the other half is easy even at night. And there is nothing strange, because we learn it from an early age.
Respondents indicated would largely on anatomical differences (primarily, of course, the genitals), and some, I suppose, would recall the distinction of roles in procreation (only one floor of the two gives birth, nurtures, and also looking after the babies are usually All the same floor — the most beautiful in the world).
These signs is sufficient to distinguish the sexes in humans, but in the animal kingdom is much more complicated. For example, the majority of the birds has no obvious differences in terms of the genitals. Can you, dear reader, to determine the sex of sea urchin (see below) or bivalve picture?
What sex is a lovely creation? (Photo alessandro_eye.)
Moreover, the males of many animals and all denied that anything remotely resembling fertilizing organ. There are numerous species, males and females who do not subscribe to what seems to us the traditional role of the sexes. For example, some fish for caviar look after the males, while the males seahorses literally hatched offspring.
It turns out that the anatomical differences are not important. What then? Chromosomes? Indeed, XY-system is found not only in humans but also in many other vertebrates, and insect males are present two kinds of sex chromosome (X and Y), and the females have two copies of the same sex chromosome X. But in birds, some reptiles and butterflies all the way around: ZW females, ZZ males.
Sex chromosomes play the first fiddle in the control of hormones that contribute to the fundamental changes of the body, a prominent, usually with the naked eye. There is no doubt that these indicators help us every day to distinguish women from men, especially in humans (for example, in men because of these hormones more facial hair).
However, this factor can not be considered decisive. Many reptiles sexual identity depends on the temperature in which the developing embryos. Moreover, there are species whose representatives at one point, change sex. Do not forget about hermaphrodites, that is, individuals of embodying both sexes (eg, garden snails, which every spring harm our dachas).
So what really separates males and females? The size of gametes — sex cells, the merger of which leads to the birth of a new generation. Individuals that produce relatively small cages, males tend to be called, and all the rest — females. The emergence of the sexes was a shift from isogamy when gametes are all individuals have the same size, to anisogamy at which reproduction involves fusion of cells of various sizes.
Attention! Hermaphrodite! (Photo by Charles & Clint.)
But what about all these external signs by which we distinguish between men and women? That they do not play any role? Of course, the play, but compared to the above factors are secondary. Let's see what emerges from the asymmetry of germ cells.
First. From this amount of flour you can bake some cupcakes or large number of small. That's the nature of "decided" that it makes sense to make an astronomical number of tiny gametes (sperm cells) at a time.
On the other hand, it is so small that the cell can be done to progeny — half to ensure that its genetic material. Therefore, we need more cells — with all the "Machinery" and nutrients needed for fetal development. Think of a chicken egg. The continuation of this asymmetry can be regarded as pregnancy and lactation in mammals.
Third, the asymmetry has led to the fact that the horde of small gametes have to fight for the right to merge with large and small. This led, in particular, to widespread in nature polyandry i.e. mating females with males several for one reproductive cycle. In such cases, compete with each other rather than one of the male gametes, how many sperm are several potential fathers.
And the last. Production of small cell requires less energy and it flows continuously, whereas the female are born with a fixed number of oocytes. Obviously, the males are in this sense, in a privileged position: if someone is not able to fertilize now, you can try again a few days (hours, minutes) later. Females also have to carefully nurture offspring and take care of it after birth.
At the same time males are paying increasing competition for access to females. This leads to sexual selection, that is, to the development of the male traits that allow you to either impregnate as many females, or enchant them (remember the peacock), or win more often other males (hence the branching horns and fast sperm).
Therefore, the males of many animals develop or extravagant appearance, or the large size, or particular forms of behavior. And that's why in many species taken to females choose males, and not vice versa.
It has long been known and is called the principle of Bateman: females invest more resources in procreation, and therefore themselves become a limited resource, for the possession of which is a fierce war among the members of the opposite sex.
But if you talk very very strict, the gamete and asymmetry is a consequence, not a cause. At the heart of all — that environmental theater, the stage which unfolds the story of evolution.
Adapted from The Conversation.