Site author Richard Steane
The BioTopics website gives access to interactive resource material, developed to support the
learning and teaching of Biology at a variety of levels.
EVOLUTION and NATURAL SELECTION
Evolution is the process by which all the different types (species) of living and extinct organisms are said to have developed. This theory, which is not questioned by most Biologists today, depends on:
- understanding of the causes of variation
- acceptance that natural selection takes place of individuals which are by chance best suited to their environment (others - unsuited - die or fail to reproduce)
- knowledge that the offspring of these organisms will mostly broadly resemble their parents - i.e. they will inherit the characteristics (features) which made their parents more suited
- an accumulation of these factors giving rise to new species over an extremely long time scale.
Fossil evidence for evolution
Fossilised remains of dead animals and plants can be found in sedimentary rocks dating back up to 600 million years. These fossils (especially bones and teeth) give information about the dominant groups of living organisms at different stages in the development of the earth, although it is not always clear why major changes, e,g, mass extinctions, occurred. Fossils also show how the body structure of individuals within certain groups changed over time, and these changes can often be more confidently related to environmental influence to which these organisms became more efficiently adapted.
A prime example of this is shown in the development of the horse, and especially the bones of the foot. The earliest ancestors of modern horses (dated about 50 million years ago) were small, and the feet in contact with the ground had 3 toes (as well as a reduced fourth toe on the front legs). This is presumably a development from the standard pentadactyl (5-toed) limb which most mammals possess.
Later on, horses evolved a larger middle toe and reduced the outer ones.
A horse's leg is thus effectively the equivalent of the human's middle finger, and bends at the wrist!
The development of larger limbs and hooves allowed horses to spread out and become more successful in grassland habitats as they developed.
At the same time, the fossils show that horses developed teeth which are more suited to their changing diet - grazing grasses rather than browsing.
Each significant in this process can be given a scientific name, similar to classifying living organisms.
Many species within the horse family have become extinct, some quite recently.
Selection in action
The example of the evolution of the horse involved gradual changes over a long period of time, and these changes must have been brought about as a result of the combined action of many genes.
There are several examples of simple changes which are caused by single genes.
For instance, the peppered moth Biston betularia exists in two forms: the normal form is light coloured with light brown mottled patches, whereas the alternative form (carbonaria) is very dark in colour - practically black. This phenotypic change in colour is in fact caused by a single gene, and the dark allele is dominant to the light allele, so only one copy of the dark allele is necessary to prodice dark coloration.
This condition is called melanism, and darker melanic forms with extra pigmentation are known for many species - effectively the opposite to albinism. The darker form presumably arose from the normal lighter form as a result of mutation. Such events naturally occur quite rarely in Nature - perhaps a chance of 1 in a million.
The gene responsible for such a major change will only become established in a population if it results in a distinct advantage, or possibly if it gives neither an advantage or disadvantage.
The dark form of Biston betularia was first noticed in the industrial midlands in the middle of the nineteenth century, but by the beginning of the twentieth century it had become the predominant form in the area. This is presumably because it was better camouflaged when resting during daytime on trees blackened by smoke, whereas the lighter moth is more suited to hiding on tree trunks encrusted by light coloured lichens in the countryside. Incidentally, it is known that these lichens are themselves badly affected by acidic gases released from industrial activities.
In classic experiments to test this hypothesis, Dr. H.B. Kettlewell released several hundred moths of each colour in a polluted city area and in an unpolluted country area, then after a short time attempted to recapture them. In each case the form which was better adapted showed a survival rate which was twice as good as the other form. It was also proved that predation by birds was a significant factor in selecting for the variety which was more fit in each environment. Interestingly, the Clean Air Act which was brought in to reduce air pollution has had the effect of reducing the predominance of the dark coloured form!
This and other examples show how genetic changes can affect the success of living organisms, especially in a changing environment, and in time the different forms would come to be seen as distinct species.
Other examples of variation and selection leading to the establishment of a new varieties are well known in the field of medical microbiology. Antibiotic-resistant strains of bacteria have been shown to develop when antibiotics are taken inadvisedly. In this case it is known that there may be (normally unnoticed) mutant forms in the bacterial population which can survive the antibiotic treatment, and these will obviously be able to reproduce - and therefore establish a population which are able to withstand exposure to the antibiotic, so they are effectively out of control. Although this may not seem comparable to the formation of new species by evolution, the faster reproduction potential of bacteria certainly shows how changes can become established within populations.
A similar process has been used by breeders to encourage the production of types of animals and plants which are of greater use or interest to Man. According to the National Curriculum, this selective breeding has been responsible for a variety of economic benefits, such as improved yields in agriculture.
Whilst some may argue over the worth of some of this endeavour, it is certainly true that many changes have resulted from the application of the scientific principles learned in the study of heredity and reproduction, and that these changes have been comparatively quickly achieved.