Mechanisms of Evolution
The Role of Natural Selection
For more than 20 years Darwin collected vast amounts of scientific data and pondered the issue of how animals and plants changed their morphology over long periods of time. In addition to establishing the science of evolutionary biology, Darwin was an accomplished geologist and had collected many fossils from various strata of rocks during his five-year voyage on the Beagle; as a result of this activity, he was aware of the vast age of the earth. He also studied mutations resulting from breeding experiments with domestic animals and plants. He was able to consider all this information when he was trying to determine the ‘specific mechanism’ that permitted animals and plants to change over time. Eventually he realized that the mechanism underlying the process of evolution was that of ‘natural selection’. This idea led him to publish On The Origin Of Species, arguably the most significant book of the last two centuries, on November the 24, 1859.
To explain the mechanism of natural selection, the first two paragraphs of Chapter 4, on natural selection, taken from his original publication are presented below so that you can read it, in Darwin’s own words. To read the entire Chapter or the entire publication, go to On the Origin of Species.
Natural Selection — its power compared with man’s selection — its power on characters of trifling importance — its Power at all ages and on both sexes — Sexual Selection — On the generality of intercrosses between individuals of the same species — Circumstances favourable and unfavourable to Natural Selection, namely, intercrossing, isolation, number of individuals — Slow action — Extinction caused by Natural Selection — Divergence of Character, related to the diversity of inhabitants of any small area, and to naturalisation — Action of Natural Selection, through Divergence of Character and Extinction, on the descendants from a common parent — Explains the Grouping of all organic beings
How will the struggle for existence, discussed too briefly in the last chapter, act in regard to variation? Can the principle of selection, which we have seen is so potent in the hands of man, apply in nature? I think we shall see that it can act most effectually. Let it be borne in mind in what an endless number of strange peculiarities our domestic productions, and, in a lesser degree, those under nature, vary; and how strong the hereditary tendency is. Under domestication, it may be truly said that the, whole organisation becomes in some degree plastic. Let it be borne in mind how infinitely complex and close-fitting are the mutual relations of all organic beings to each other and to their physical conditions of life. Can it, then, be thought improbable, seeing that variations useful to man have undoubtedly occurred, that other variations useful in some way to each being in the great and complex battle of life, should sometimes occur in the course of thousands of generations? If such do occur, can we doubt (remembering that many more individuals are born than can possibly survive) that individuals having any advantage, however slight, over others, would have the best chance of surviving and of Procreating their kind? On the other hand, we may feel sure that any variation in the least degree injurious would be rigidly destroyed. This preservation of favourable variations and the rejection of injurious variations, I call Natural Selection. Variations neither useful nor injurious would not be affected by natural selection, and would be left a fluctuating element, as perhaps we see in the species called polymorphic.
We shall best understand the probable course of natural selection by taking the case of a country undergoing some physical change, for instance, of climate. The proportional numbers of its inhabitants would almost immediately undergo a change, and some species might become extinct. We may conclude, from what we have seen of the intimate and complex manner in which the inhabitants of each country are bound together, that any change in the numerical proportions of some of the inhabitants, independently of the change of climate itself, would most seriously affect many of the others. If the country were open on its borders, new forms would certainly immigrate, and this also would seriously disturb the relations of some of the former inhabitants. Let it be remembered how powerful the influence of a single introduced tree or mammal has been shown to be. But in the case of an island, or of a country partly surrounded by barriers, into which new and better adapted forms could not freely enter, we should then have places in the economy of nature which would assuredly be better filled up, if some of the original inhabitants were in some manner modified; for, had the area been open to immigration, these same places would have been seized on by intruders. In such case, every slight modification, which in the course of ages chanced to arise, and which in any way favoured the individuals of any of the species, by better adapting them to their altered conditions, would tend to be preserved; and natural selection would thus have free scope for the work of improvement.
Source: Charles Darwin.; On The Origin of Species: The Easton Press Norwalk Con.
Additional reading and websites:
Larson, Edward; Evolution: The Remarkable History of a Scientific Theory, Modern Library Edition 2004.
Dawkins, Richard; The Blind Watchmaker:, W.W. Norton and Co. 1996
Diamond, Jared; The Third Chimpanzee, Harper Perenniel, 1993.
The Role of Inheritance
The genetic coding for traits is performed by the sequences of DNA nucleotides. But how do the DNA sequences get distributed to the offspring of an organism? This is the process of inheritance, and was one of the missing concepts that Darwin had to guess about when he first wrote On the Origin of Species. Part of the story of inheritance came from the work of Gregor Mendel, an Austrian monk who carefully bred kinds of pure bred pea plants, and then studied the ratios of various characteristics in the offspring. He was able to explain his results with the ideas that parent plants each contributed one copy (“allele”) of a particular “gene”, and the offspring contained a pair of alleles for each kind of gene. Depending on the nature of the two alleles in that pair, various traits could be manifest by the offspring.
Sources and Further Reading:
The Monk in the Garden : The Lost and Found Genius of Gregor Mendel; by Robin Marantz Henig
The Role of DNA and Genetic Mutations
At the top of the diagram is the original molecule. In the middle of the diagram the molecule is shown with its two strands unwinding. In the bottom portion, each of the strands is shown with a newly synthesized complementary strand, connected by the process of pairing.
The Molecule of Life!
DNA is the genetic blueprint of life. Within each cell of every organism there are a number of DNA molecules which contain all the information required to create and make an organism function. DNA consists of 2 strands and each strand is made of a sequence of 4 kinds of chemicals called nucleotides or bases. These bases are called Adenine (A); Guanine (G); Cytosine (C) and Thymine (T).
The two strands of DNA are joined together to form a ‘spiral ladder’ which is more formally known as a ‘double helix’. Each strand of DNA is an exact opposite of the other – wherever there is an A on one strand there is a T on the other, and wherever there is a G on one there is a C on the other. The links, or steps, of the ladder are made by the pairingof these complementary bases. (See the diagram).
To compare DNA between species, scientists must work out the sequence of bases along sections of the DNA molecule. This is done by artificially separating and copying the DNA from the region of interest from each species, doubling the amount of DNA at each copying cycle. The technique used to accomplish this is known as the polymerase chain reaction (PCR) is based on several unique properties of DNA.
DNA is a special molecule because it can make copies of itself. However, occasionally mistakes are made in coping the molecule and these errors are referred to as mutations. There are several different kinds of mutation that are known.
A particularly easy mutation to understand is a change in a single nucleotide (base, code letter). For example an A may be substituted for a G and in many cases these mutations don’t seem to have a noticeable effect on the organism, because some changes merely produce a different “codon” (three nucleotide sequence that tells which amino acid to use in making proteins) that codes for the SAME amino acid. Over a very long time and through many generations of organisms, mutations accumulate. Different species tend to have different and unique sets of mutations and so, these mutations can be used to measure how long ago two species diverged from one another. Species with similar DNA codes are more closely related than those with dissimilar DNA. For example, the DNA of humans is very similar to that of chimpanzees but quite different to that of a horse and very different from a gum tree DNA.
To understand the the numerous kinds of genetic mutations that do occur we suggest that you go to the following website, as the information is clearly presented there.
Sources and Further Reading:
http://www.dnalc.org – Gene Almanac
http://www.world-of-dawkins.com/ – World of Dawkins
The Genetic Code (the Codons). This shows which triplets of DNA letters (codons) indicate which amino acids are to be inserted at a location. But, demonstrating that science progresses by testing ideas and discarding false ideas, this research shows that sometimes a "silent" mutation, a mutation that makes a codon for the same amino acid that was at a location before, can cause changes in the protein that is made! This overthrows a long-held but false idea that if two codons direct that the same amino acid shall be used at a location, then the earlier and the mutated proteins will fold up to be identical.
DNA molecule, side view and end view
Understanding Evolution, a very readable introduction to evolution with interesting examples and principles explained. Useful material for teachers. From the University of California Museum of Paleontology.
AAAS – Evolution on the Front Lines. Resources for teachers and journalists, from the largest association of American scientists.
Evolution Resources From the National Academies. The National Academies are America’s official, highly respected, bodies of eminent scientists, engineers, and doctors.
A standards based curriculum, covering 1 year of high school science with evolution as the central theme.