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Saturday, August 20, 2005

on the origin of sexes

A close friend of mine put forth this question recently:

"In the begining there was one cell life (no concept of the gender) then multi cell life (no concept of gender still) and we are here (male and female). so at some point 2 sexes (male and female) evolved instead of one. why?"


Heres my attempt at an answer:

Evolution works using two complementary processes: introducing random changes in the genone, and then propogating any changes which may be found beneficial (in the "survival of the fittest" sense).

Now the primary way of introducing changes is by mutation. And the primary way of propogating these changes is from parent to child.

With asexual reproduction, once organism "A" has uncovered a beneficial mutation "z", that mutation will be passed on to all its offspring, but will never be seen by any other members of the same species. slowly and steadily, as mutations accumulate, the "family" of organism "A" will diverge from its own species sufficiently to qualify as a seperate species. But this is a very inefficient way of evolving. No doubt, other members of the species would have also developed useful mutations, and if by some way members of single species could "exchange notes" so to say, they could evolve much faster and much more efficiently, without each mutation fragmenting a species into two.

So now we agree that exchange of some genetic material is required. Even though we think of unicellulars as "asexual", there still are instances when two unicellular organisms exchange genetic material using what are called "vectors", small strands of DNA/RNA, which are physically passed from one cell to another. This allows for exchange of beneficial traits between members of a species. So technically, sexes are still not required.

But surprisingly, what is found is that even in this simple case, among the pair of organisms exchanging a "vector", one is found to have the tendency to always "recieve" the vector, and the other is found to have a tendncy to always have the tendency to always "transmit" a vector. So even here, we find a very low degree of differentiation in the roles.

Now I am firmly in the domain of pure speculation:

As species developed, and became more complex, it became clear that special "support structures" would be needed in the organisms' body to permit bringing-forth of offsprings, hence the development of ovaries and the like. As this function became quite complex, it became essential to further differentiate members of a species into two distinct groups:

1. One group, which has the necessary physiological support for nurturing a foetus in its early stages of development, when it needs to be protected from the external world. This support unfortunately led to a condition where, from conception till birth, the members of this group tended to be very vulnerable aginst predators and other dangers. Enter the second group:

2. The second group, which developed the physiological support to provide DNA for the "mixing" of genes which is, as discussed, curcial to effective evolution. This group also developed so as to be able to provide protection to group one during the vulnerable period.

So though it can be argued that instead of bifurcating into sexes, all memebers of a species should have had the capabilites of acting both as a "male" and a "female" at times. And i believe this behaviour is found in some rare species still. But due the highly specialized nature of organs required for the "female" and the high degree of vulnerability introduced during pregnancy, i believe over time, it was inevitable that two specialized groups would emerge within a species.

What do you think?


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