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natural selection

posted 27 Feb 2012, 02:27 by John Brown   [ updated 13 Mar 2012, 19:47 ]
Brains, and the greater central nervous system, are the end-result of a long evolutionary process of natural selection. Natural selection offers a perspective on how brains came about, and Richard Dawkins provides a most insightful explanation of natural selection (Dawkins 1987). Appreciating why brains came about, is another matter again.

Dawkins explains that “The fundamental unit, the prime mover of all life, is the replicator. A replicator is anything in the universe of which copies are made. Replicators come into existence, in the first place, by chance, by the random jostling of smaller particles. Once a replicator has come into existence it is capable of generating an indefinitely large set of copies of itself.” However, errors in the copying process introduce new alternate replicators which subsequently compete for resources. Replication success is dependent both on the replicator's characteristics and on the current environment, which may include other replicators. The presence of other replicators can at times be mutually beneficial and at other times detrimental to replication success. Natural selection describes the increase in population of one replicator over another due to an extended lifespan, faster replication, and/or more accurate replication.  As the population becomes increasingly dominated by the most successful replicators, then they are increasingly competing with copies of themselves, or with close variants.

The replicators responsible for life in our corner of the universe are known as genes (not animals or plants). Genes are encoded in molecules of DNA. Some of the initial minute fragments of DNA interacted with the environment in such a way that promoted their own replication. The interaction with the environment of a gene, or group of genes is known as the phenotype. There were DNA copy errors which resulted in different variations of these genes. Different variations of a gene, or group of genes, are known as different genotypes. Each gene variant, or genotype, sported a different interaction with the environment, a different phenotype. The successful phenotypes (physical characteristics and behaviors) promoted superior replication of the underlying genes. This success of one phenotype over another  is the natural selection of one genotype over another. The genes are selected based on their phenotypes. 

Some groups of mutually beneficial genes (a genotype) resulted in the construction of an enclosing cell (a phenotype). Further evolution resulted in the development of multicellular organisms. Still further evolution saw the development of a recognizable Central Nervous System (CNS) and eventually brains, along with a host of other systems and capabilities. For many species, the advent of sexual reproduction provided an important source of gene variants to participate in the natural selection process. 

Each step along this very long path, was an adjustment to the then current genotype, which was entirely reliant on all of the prior adjustments. The success of the adjustment was dependent on the environment, and the competition, at the time. Success is measured by an increase in population.

Dawkins refers to the phenotype as the replicator's survival machine. “A survival machine is a vehicle containing not just one gene but many thousands. The manufacture of a body is a cooperative venture of such intricacy that it is almost impossible to disentangle the contribution of one gene from that of another.”

Dawkins goes to some length to dispel the myth that genes completely determine the development and behavior of each individual organism (Dawkins 1999). There are simply not enough genes in the human genome to specify the entire structural connectivity of the brain (Damasio 1994). Genes are not like a computer program which results in the same output every time. Genes are not like a set of engineering drawings that completely specify each component and assembly of the whole. Rather, genes interact with the environment during development of the organism and represent a tendency for a particular development path. If the environment changes, then the development path will change. And it must be noted: a significant source of environmental change is the actions of genes. Neither genes or the environment are static: both are in a continuous state of change; each directly and indirectly influencing the other.

In sum, brains are just one intricate part of an organism which evolved during a very long natural selection process. Each step in the evolution built on the pre-existing structure from the bottom up. Each step was more successful in creating replicas of the genetic material than the alternatives which existed in that environment, at that time. Genes have a tendency, rather than destiny, to develop in particular ways, and the tendency interacts with the environment. 

Intelligence looks a bit like a grab bag of capabilities; because it is!

Damasio,A.R (1995), Descartes' Error : Emotion, Reason, and the Human Brain. Harper Perennial. 
Dawkins, R. (1987), The Selfish Gene. Oxford University Press. 
Dawkins,R. (1999), The Extended Phenotype. Oxford University Press.