Evolution/Males and females
Why is there (in all species?) approximately the same number of males and females? I have tried to understand Stephen Jay Gould's explanation from "The Panda's Thumb" but I do not. Are there not actually some species that are, or can become, all females? And some that change sex?
In species in which sex is determined by the presence of a sex chromosome (For example, in mammals, an X and a Y), the parent with the heteromorphic (i.e., two different shapes/sizes) of sex chromosome determines the sex of the offspring. In normal male meiosis in a mammal, the germline cell giving rise to sperm has two sex chromosomes, and X and a Y. When the cell divides to produce four sperm, the X and the Y travel to separate cells. End result: two sperm from each germline cell get an X chromosome, and the other two get a Y.
All things being equal (which they often are not), there should be a 50:50 chance that any given ovum is fertilized by a sperm carrying an X or a Y. This is *pretty much* the case. So, on average, there's about a 50% chance that any given offspring will be male (or female).
However, there are some mitigating factors that can interfere with this. In our global population, slightly more male humans are born than female humans. (Y-carrying sperm swim a littler faster, but X carrying sperm last a little longer; Y-linked, harmful recessive traits are more likely to be expressed in males, sometimes causing prenatal death, etc.)
Yes, there are some species that are all female (parthenogenic), and some that can change sex. But these are the exceptions, rather than the rule.
And not all species have sex determined solely by heteromorphic sex chromosomes. But it's still about numbers.
It's not exactly true to say that in *all* species, there are 50% males and 50% females. But in species in which males and females are equally represented, it's mostly a matter of simple probability.
Hope that helps.