Sunday, May 22, 2011

The sexual reproduction paradox

Life at the cellular level is hard to imagine. Some might say its TOO MUCH BIOLOGY. I am of the same kind. I was looking into why there are two varieties in most of the mammalian species, but instead, stumbled into a bit of cell biology. And if you thought cell biology was tough, try applying the theory of evolution to that!

But (and this is no joke) Darwinian principles apply to cells too! There is a form of cellular reproduction which is quite successful, but in actuality, (or in straight-forward natural selection sense), should not have evolved. And because that method of reproduction was quite successful, it has caused variation of traits in our species and in others.

I’d suggest now is a good time to catch up with some preliminary biology (if you require it). Following are links to a group of videos on Mitosis & Meiosis - the two forms of cellular reproduction. I’d suggest you go to the first link (shorter videos), and then resume with reading the rest of this post:

The video simply explains the processes in brief. Each phase in the cell reproduction is actually very dense to describe. For e.g. consider the metaphase in mitosis of a cell.

The centrosomes move to two extremes of a cell from which we have spindles orginating. Some of these spindles attach themselves to the centromeres. It is still not fully understood how the centrosomes are able to exactly locate these points. Its also said that mitosis won’t happen if these connections fail. 

Some common facts of mitosis:
  • When the cell divides we have 2n number of chromosomes - this is the genetic deck of a cell.
  • The daughter cells which result have the same trait as that of the mother cell.
  • Mitosis is the method to reproduce when cells need to replace other cells which has worn out or become dysfunctional. For e.g. a small cut on your hand will heal; after a few days you notice that that spot where you cut is similar to the surrounding. What has happened here is a mitotic reproduction where the dysfunctional cells are replaced.
Some common facts about meiosis:

  • At the end of the cell division we have 4 haploid cells, each with half the number of chromosomes (n) when compared to mother cell.
  • The cells that participate in meiosis are called gametes. It could be either the ovum or the sperm cell.
  • There is recombination or crossing over of chromosomes. This shuffles the genetic deck. This is what solely responsible for genetic variability.

And now to the Natural Selection mode of arguments

The crossing over process is very complex. The chromosomes have to align themselves properly. The cross-linkages should happen successfully. The structure of the DNA should be intact. There are a host of other physical/chemical conditions (temperature, pressure, pH level, etc) which must be conducive for successful meiosis.

There are numerous factors. There are times where all these factors or some of them might not be conducive. Sometimes the meiosis fails. But sometimes these environmental factors can alter the genetic deck. There is a level of variation due cross-over of chromosomes, but there may be variations which may not be due to the cross-over alone. These are what we call mutations. 

So what are the results of these mutations (or even variations)?

It is simply what natural selection dictates it to be. Sometimes the result may be a successful offspring; sometimes it may not. But that is simply how life of any germ cell is...(it is also the case with mitosis). The idea of natural selection is to select those processes which maximize chances of survival.

It is inappropriate at this juncture to explain how meiosis (or mitosis) came about. (The easiest answer to that is natural selection). In essence mitosis creates copies of itself. Meiosis creates variations (or mutations). But some of these offspring might die. How they die? They depend upon the rules of natural selection. This is the case with mitosis too! If the rules of natural selection are unfavourable for the cell (which is the product of mitosis) even that cell dies. But that cell is a copy of another cell, and all cells have the similar characteristics. The whole species might go extinct because of this. But lets not digress...

Meiosis is a very inefficient form of reproduction. You can come to this conclusion after you simply analyze all the processes that go into meiosis. The offspring is subject to variation (due to crossing-over of the chromosomes). It is also subject to changes in the physical/chemical environment which results into mutation. So what we must profoundly ask ourselves is:

Why has this imperfect process become so successful and flourished all these millions of millions of years?

The simple answer is NATURAL SELECTION. But, it abstracts so many things about what has actually happened. It has abstracted so many things about some aspects of life on earth itself.

So I think I have provided context enough for my next post. I hope people appreciate that post better. There are other means that let you know about the (evolutionary) stuff that I’m interested in.

You can follow me on twitter (@deostroll).

I bookmark articles about evolution on delicious.

And you can subscribe to the rss of this blog.

Ps: My inspiration for this post was this.