Thursday, July 14, 2011

The White Queen and the Evolution of Sex

'Now I'll give you something to believe,' said the White Queen. 'I'm just one hundred and one, five months and a day.'

'I can't believe that!' said Alice.

'Can't you?' the Queen said in a pitying tone. 'Try again: draw a long breath, and shut your eyes.'

Alice laughed. 'There's no use trying,' she said. 'One can't believe impossible things.'

'I daresay you haven't had much practice,' said the Queen. 'When I was your age, I always did it for half-an-hour a day. Why, sometimes I've believed as many as six impossible things before breakfast.

Lewis Carroll, Through the Looking Glass

One of the vexing problems with any view of evolution that holds it to be an unguided, random process is the sheer improbability of the chance evolution of phenomena like abiogenesis, insect metamorphosis, and sexual reproduction.

Focussing on the last of these, Jonathan M. at Evolution News and Views has an interesting discussion of the intractable difficulties which confront attempts to explain the evolution of sex on purely naturalistic terms. Here's an excerpt:
There are several reasons why the origin of sex presents a problem. For starters, there is the waste of resources in producing males. Assuming a sexually-reproducing female gives birth to an equal number of male and female offspring, only half of the progeny will be able to go on to have more offspring (in contrast to the asexually reproducing species, all the offspring of which can subsequently reproduce).

Thus, it is to be expected that the asexual female will proliferate, on average, at twice the rate of the sexual species. Given the disadvantage thereby confronting the sexually-reproducing species, one would expect them to be quickly outcompeted by the asexual species. Moreover, it must be borne in mind that, in contrast to the asexual species, the females of the sexually-reproducing species perpetuate only half of their successful genotype. To transition, therefore, from a state of asexuality to sexual reproduction is, in effect, to gamble with 50% of one's successful genotype.

Given that the whole purpose of natural selection is the preservation of those organisms which pass on their successful genes, this strikes at the heart of evolutionary rationale.

The problems extend even deeper than this. For there is, of course, the additional conundrum related to the fact that gametes (i.e. sex cells) undergo a fundamentally different type of cell division (i.e. meiosis rather than mitosis).
Anyone who recalls their high school biology will realize how amazing it would be for meiosis to develop solely by undirected chance and random mutation.
Jonathan continues:
Meiosis entails the copying of only half of the chromosomal material. In similar fashion to mitosis (which occurs in somatic cells), each chromosome is duplicated to yield two chromatids. the start of meiosis, each visible 'chromosome' possesses four chromatids. At the first division, these homologous chromosomes are separated such that each daughter nucleus has exactly half the chromosome number. At this stage, each is present as two copies (chromatids).

These chromatids are hence separated at the second division such that each new nucleus only has a single copy. In order for sexual reproduction to work, it is essential that the process of meiosis evolve to halve the chromosome number. And this ability must also only occur in the gametes and not in the somatic cells. This difficulty is accentuated by the multitude of novel elements which are found in meiosis, rendering it unlikely to be explicable in terms of single mutational steps.

And then there is the added problem of male and female complementarity -- a seemingly remarkable incidence of co-evolution.
Complementarity involves the simultaneous evolution not only of physiological and morphological processes and structures but also simultaneous evolution of behavioral patterns that synchronize the sexual rhythms of the two sexes of a particular species. It wouldn't do, for example, to have the male blossoms of a plant coming into flower in the spring and the female blossoms appearing in the autumn.

At any rate, one can be an evolutionist without sacrificing one's skepticism, but it's awfully hard to see how one can be a darwinian evolutionist - i.e. one who believes that the process proceeded merrily along without benefit of any intelligent guidance - without, like the White Queen, having to believe six impossible things before breakfast every morning. Yet somehow the darwinians manage to pull it off even as they scoff at theists for being skeptical and "superstitious".