Evolution – replication – variation – competition – selection

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POST 7 – SELECTION PROCESS

1.0 NATURAL SELECTION

1. REPLICATION

            1.2 VARIATION

            1.3 COMPETITION

2.0 ARTIFICIAL SELECTION

            2.2 UNCONSCIOUS (UNINTENTIONAL)

            2.2 METHODICAL (INTENTIONAL)

 

In the last post I outlined the changes that resulted from the operation of the evolutionary process that led from the primitive cell to humans. This time I am discussing the critical elements of natural selection and also a number of other natural processes that share those critical elements as well as the related processes that occur when we humans interfere in those natural processes.

 

1.0 NATURAL SELECTION

           

From the first description of the process of natural selection Darwin distinguished between natural and artificial selection and also separated the latter into unconscious selection and methodical selection. More recent authors have followed this classification while providing more details of the steps of the processes. The philosopher Daniel Dennett in his article “New Replicators”  quotes Darwin as saying that, if there is “descent [i.e., replication] with modification [variation]” and “a severe struggle for life” [competition], better-equipped descendants will prosper at the expense of their competitors.

Dennett says that evolution can occur whenever and wherever three conditions are met:

•           Replication,

•           Variation (mutation), and

•           Differential fitness (competition).

 

Dennett then asks whether any other evolutionary substrates have arisen on this planet. He responds to his own question: “The best candidates are the brainchildren, planned or unplanned, of one species: Homo sapiens”. Others have suggested more candidates for evolving systems with their own replicators including the immune system, neural development, and trial and error learning”.

 

Richard Dawkins identified and coined the term “meme” in his 1976 book “The Selfish Gene” and the term has caught on. Dennett provides a definition of a meme as a “general term for a culturally based replicator”.

 

The efficacy of each component in producing change in subsequent generations depends upon the rate of replication, the frequency of variations and the pressure of competition.

 

    1. REPLICATION

      All the possible replicators have in common that they are systems where serial duplication or copying occurs (called replication), copying errors can occur (variation) and those variants that are able to survive the competitive pressures do in fact survive and replicate further. The critical element in a replicator in this process seems to be that it comprises a package of coded information or message that is copied serially. That is, it is not a simple matter of making repeated copies from an original but the copy is then copied and that one is copied and so on … and so on…

 

Replication may occur at a rate varying with the organism involved and with the situation. A virus, in favourable circumstances, reproduces frequently with outputs of over 100,000 daughter cells in a day or two. This means that there are very short generation intervals with an associated high replication rate. Human generation interval is usually taken to be 25 years and thus the opportunity for a variant to occur happens much less frequently.

            1.2 VARIATION

Variation as a result of copying errors is part of our common experience and is perhaps most clearly expressed by incidents such as the yarn which has three “older” men walking along a beach. The first guy says “It is windy today”. The second guy says “No, it’s not, it’s Thursday”. The last guy says “So am I, so let’s go and have a beer”.

Similarly a small error in copying down a friend’s favourite a recipe could lead to an error which could lead to a major difference in outcome.

The rate at which such errors occur in the natural setting varies from organism to organism and will affect the outcome for the variant. The influenza viruses are renowned for the high rate of variation resulting from mutations and re-combination, that enables then to survive challenges such as in the face of high antibody levels in a population.

In a growing and developing organism mutation may affect both the germ-line and the somatic cell line. Germ-line mutations are passed to offspring while somatic cell line mutations lead, in the individual, to various blemishes and cancer, which affect the individual but do not become part of the germ-line.

 

            1.3 COMPETITION

Most mutations are detrimental and do not survive and may be lethal for the organism. However some survive and, if it happens to produce a change in the individual that happens to suit its survival, then it may increase its incidence in the population (or sub-population).

Selection pressure is the terminology that is used to describe the influence that affects the survival of those variants that face up to environmental challenge.

2.0 SELECTION

If a replicator is subject to variation and the variants are exposed to competition for survival then evolutionary change may occur. It is important to accept that we humans can influence that process but there is a natural process that will occur in the absence of human intervention. Darwin’s great insight followed his appreciation of what was being achieved by animal and plant breeders when he made the link with the biological evidence he accumulated. Having the Darwinian insight in place, we today, must understand that the process of natural selection will go on “naturally” whenever a replicator is subject to variation and competition.

            2.1 NATURAL

Natural (free from human intervention) selection will go on in all replicators and in order to understand what we are doing when we interfere in such a process it is important to understand the “natural” process. It is also important to know whether or not a selection process is going on or is even possible at all. The knowledge that replication is not restricted to the genetic process is important.

            2.2 ARTIFICIAL

If we recognise that the process is going on anyway it behoves us to recognise its operations if we are to interfere. How to interfere effectively becomes an important issue. We already interfere in two ways. Unintentionally we have always interfered in plant and animal genetics including by altering environments and by moving them around to new environments. And in human culture we have developed world views in an attempt to explain what we see in our world. Adherents to some of those world views attempt to promulgate them and to convert others to their view. Such human activity has an ethical dimension that I will look at in later posts.

                        2.2.1 UNCONSCIOUS (UNINTENTIONAL)

Domestication of plants and animals is the major example of human unintended interference with the natural process. We changed the environment and animals adapted to it both by acclimatising and by genetic change as their traits were favoured by the new environment.

The grey winged moths which camouflaged themselves when lying on the bark of grey trees in northern parts of England evolved a black variety as the trees became coated with soot as the industrial revolution progressed in that area.

By altering farming land with measures such as tree clearing and drainage of soils we have had a side-effect of pushing some plant and animal species towards extinction while others have thrived in the new environment.

.

                        2.2.2 METHODICAL (INTENTIONAL)

The sciences of plant and animal genetics are designed to alter that genotype of the species by speeding up the reproduction rate, increasing the variation rate and increasing the selection pressure on the less fit individuals. These steps are designed to increase the prevalence of the desirable genotypes in the population. Thus meat chickens that took 12 weeks to grow to a marketable size a few decades ago now achieve that state in 42 days and use a lot less feed to do so.

Genetic engineering sets out to achieve the same ends by manipulating the genes at a fundamental level.

 

NEXT TIME:

I will look at two non-genetic replicators in my next POST. The selection process that occurs in “memes” has been much written about but the precursor to a meme is the “brainchild” mentioned by Dennett. The process by which a meme is generated in the mind of an individual has not been identified as a replicator – to my knowledge. I identify the initial nervous impulse that is established at the sensory nerve ending as the replicator which is copied at each synapse through which it passes on its journey to the central nervous system and through the processing centres in the central system then back through the motor nerves resulting in behaviour and through the brain/mind resulting in our greatest attribute, our imagination – our inner life.

Following Dawkins’ invention of the word ”meme” to identify the cultural replicator I am coining the word “neme” to identify the nerve impulse as the replicator in the nervous system.

READINGS

I have re-read Daniel C. Dennett’s article “New Replicators”.

What sets humanity apart? Stephen Cave says it is the ability to cooperate.

How did life originate is examined.

Attempts to reproduce prebiotic conditions include those made by researchers who worked on the Los Alamos bug.

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