Evolution – Ideas and Concepts
Approximately 3.5 billion years ago all that existed was single celled organisms, this is how life began1. For the following 3 billion years there was no change. Evidence suggests that 700 million years ago there was a planetary surge of glaciers which could have stretched from pole to pole2. It was the thawing of this which is thought to have been the catalyst for the development of more complex forms3. These were the building blocks of the multicellular organisms we have today; the changes in them between then and now have been brought about by the process of evolution.
ReptileX is a computer simulation of evolution. Evolution is the process of living organisms changing over time and adapting to their surrounding environment whilst competing with others for survival. This is driven by natural selection, a term which was used by Charles Darwin in his book On The Origin Of Species:
“How do those groups of species, which constitute what are called distinct genera, and which differ from each other more than do the species of the same genus, arise? All these results, as we shall more fully see in the next chapter follow inevitably from the struggle for life. Owing to this struggle for life, any variation, however slight and from whatever cause proceeding, if it be in any degree profitable to an individual of any species, in its infinitely complex relations to other organic beings and to external nature, will tend to the preservation of that individual, and will generally be inherited by its offspring. The offspring, also, will thus have a better chance of surviving, for, of the many individuals of any species which are periodically born, but a small number can survive. I have called this principle, by which each slight variation, if useful, is preserved, by the term of Natural Selection, in order to mark its relation to man’s power of selection.”3
Natural selection is the process by which organisms which develop characteristics that make them better adapted to their environment are more likely to survive and reproduce therefore passing on these characteristics.
Genetics and Selection
An organism’s characteristics are determined by genes. When Charles Darwin published On The Origin of Species genes had not yet been discovered. People knew that specific traits e.g. blond hair or blue eyes, were passed on from parent to child but how this process worked was unknown.
A gene is a portion of a DNA (deoxyribonucleic acid). DNA is a molecule containing the genetic code which carries the instructions for how an organism is built. A gene codes for specific characteristics, genes can also work together to code for characteristics. Here is a definition from G.C. Williams in his book Adaptation and Natural Selection (Princeton University Press, Princeton, 1966)
So evolution can also be defined as the change in frequencies of genes over time. The frequency of a particular gene is the percentage of a population that feature that gene. Figure 7.1 explains this.
Given three genes named A, B and C:
If these frequencies were measured again after the passing of some generations to find the frequencies had changed then evolution has occurred.
The following quote from Richard Dawkins explains that genes which are benificial to the bodies they reside in have a high frequency.
“A gene which tends, on average, to have a good effect on the survival prospects of the bodies in which it finds itself will tend to increase in frequency in the gene pool” 5
Evolution can be split into two main categories, these categories are defined by the timescales over which they take place:
- Microevolution–this describes refers to small changes over short time period, like bacteria developing an immunity.
- Macroevolution–which refers to large changes over long periods of time, like the growing of a trunk or speciation (when members of a species change so much that they can no longer be called members of that species).
When an organism reproduces it’s DNA sequences (genes) are copied and passed onto offspring. This process is very accurate, which is a necessity for the survival of life, but it is not perfect. The imperfections of this process are what’s known as mutations. The mistakes in the copying of genes can go to two extremes, one is for a gene to completely disappear or become extinct and the other is for a new gene to appear.
The new gene can have any number of affects:
- It can be harmful to the organism.
- It can be beneficial to the organism.
- It can have no real affect on the organism.
Natural selection (the driving force of evolution) is how the environmental conditions affect which mutations are beneficial and which mutations are not (some mutations will never be beneficial). Organisms which feature have beneficial mutations which aid their survival will pass these on to their offspring. This will result in a change in frequencies of the beneficial genes over time.
5 R.D. Dawkins, The Greatest Show on Earth: The Evidence for Evolution, Black Swan, London 2010, p.249