Saturday, June 9, 2012

31. Biological Evolution

In 1859, Charles Darwin announced one of the greatest ideas ever to occur to a human mind: cumulative evolution by natural selection (Richard Dawkins).

In the previous post (Part 30) I used the terms 'biological evolution' and 'natural selection', without realizing that I have not yet explained them. I do that in this post.

The name most associated with evolution is that of Charles Darwin. The year 2009 marked his second birth centenary, as also 150 years of the publication of his celebrated book On the Origin of Species by Means of Natural Selection. The basic idea of biological evolution by natural selection is remarkably simple, yet of fundamental importance:

Consider the mother-child relationship. Mothers go through very substantial pain, hardship, risk to health and life, deprivation, sacrifices etc., and yet most of them are happy to bear a child and rear it. Why is that so?

In a population of females, there would be some variation regarding attitude to motherhood. There are bound to be some who avoid undergoing all the hardships and sacrifices I mentioned above. As a result, they do not get pregnant and bear children. Similarly, there are some females in the population who are happy with the very thought of motherhood; pain and sacrifices notwithstanding. Such females would not only contribute their progeny to the population, the progeny is likely to be like them, favourably disposed to the idea of motherhood.

Over many generations, the result would be that the percentage of females not inclined to become mothers decreases; no progeny means no representation of the tendency against motherhood in the population. This is 'natural selection': Nature selects in favour of those females who are happy being mothers, and weeds out those who are not. There is a gradual 'evolution' of this trait in the population, so much so that, in due course, there are hardly any females left who do not wish to bear children.

Such reasoning can be generalized to other forms of biological evolution. Living organisms are thermodynamically open systems, i.e. they are constantly exchanging matter and energy with the environment. There is a fair amount of dynamic equilibrium between a living organism and its surroundings. The organism cannot survive if this equilibrium is disturbed too much, or for too long. The fact that an organism survives implies that, in its present form, it has been able to adapt itself to the environment. If the environment changes slowly enough, organisms can evolve (over a long-enough time period) a new set of capabilities or features which enable them to survive even under the changed conditions. Over long periods of such evolutionary change, creatures may even develop into new species. This was the message of Darwin’s (1859) bold theory of evolution through cumulative natural selection.

A consequence of Darwin's theory was that all living organisms are the descendents of only one or a few simple ancestral forms.

Darwin started with the observation that, given enough time, food, space, and safety from predators and disease etc., the size of the population of any species can increase in each generation. But this indefinite (exponential) increase does not actually occur. In fact, usually only a small minority of the offspring reach maturity to produce the next generation of offspring; the rest die prematurely. Thus, there must be limiting factors in operation. Influenced by Malthusian ideas, Darwin imagined that if, for example, available food is limited, only a fraction of the population can survive and propagate itself. But what decides who will survive and who will not?

Darwin’s answer was that, since not all individuals in a species are exactly alike (i.e. since there is variation in the population), those better suited to cope with the prevailing conditions stand a better chance of survival ('survival of the fittest'). Moreover, the fittest individuals not only have a better chance of survival, they are also more likely to procreate. Thus, attributes conducive to survival and propagation have a better chance of getting ‘naturally selected’ at the expense of less conducive attributes. And the effects of this natural selection accumulate over time, i.e. over several generations. This is the process of cumulative natural selection recognized by Darwin.

Children tend to resemble their parents to a substantial extent. The reason is that the progeny of better-adapted individuals in each generation, which survive and leave behind more offspring than others, acquire more and more of those features which are conducive for good adaptation to the existing or moderately-changing environment. A species perfects itself, or adjusts itself, for the environment in which it must survive, through the processes of both cumulative natural selection and inheritance.

Thus there are four basic features of Darwinian evolution:
  • Variability and variety in members of a population in the matter of coping with a given environment.
  • Inheritance of this variation by the next generation, with a few random modifications.
  • Differential survival and reproductive success of individual members of this new generation in the given environment.
  • Establishment of a new population more adapted to the environment, possessing new variations for passing on to the next generation.
Darwin changed the way we humans perceive ourselves. And the basic idea of evolution by natural selection has gone far beyond the precincts of biology. Apart from biological Darwinism, we speak of chemical Darwinism, quantum Darwinism, neural Darwinism, and what not. What evolves in any open system of interacting entities is complexity.

Biological entities embody enormous amounts of order and organization. Can Darwinian evolution alone explain it? No. As Stuart Kauffman has emphasized, there is, in fact, an underlying complexity and order on which Darwinian evolution operates. Evolution of biological complexity is determined by two factors: self-organization (to be discussed in the next post), and natural selection. Self-organization or spontaneous ordering can occur in any open dynamical system, Darwinism or no Darwinism. Darwinian natural selection acts on this existing order and hones it further.