Understanding Evolution: Natural Selection and Its Role in Survival

1. Higher Biology Selection Mr G R Davidson G Davidson

2. Selection • Evolution depends on variation in a population of living things. • Variation is called survival of the fittest, a process by which, environmental pressures result in some varieties surviving better than others and is called selection. G Davidson

3. Biotic and Abiotic Factors G Davidson

4. Biotic and Abiotic Factors G Davidson

5. Selection • When studying the effect of different characteristics in a population, we mustn’t forget the interplay of the environment and inheritance. • e.g. a farmer wants to study a new pure breeding variety of wheat. • The growth of the wheat can be affected by both biotic and abiotic factors G Davidson

6. Selection • To do this, he planted the seeds in a carefully tended section of a field where growing conditions were controlled and as uniform as possible. • When he harvested the crop, he recorded the weights of the seeds and plants and the height of the plants. G Davidson

7. Selection • Each individual plant was genetically identical but the seed weights and plant heights showed continuous variation in each gene. • These differences in phenotype must be due to slight differences in the environmental conditions that the farmer was not able to control . G Davidson

8. Selection • Although the traditional view that evolution is a very slow, gradual process and difficult to study and prove, there are many examples of evolution in action. • ‘Natural Selection’ and ‘survival of the fittest’ are phrases often used in the evolution theory. • It is important to remember that ‘fittest’ does not necessarily mean strongest, healthiest or fastest. G Davidson

9. Selection • An organism’s fitness in evolutionary terms is measured by its reproductive success. • In other words, by how well its genes survive into the next generation. • So natural selection is selection by survival to reproduce. • The individuals not selected, do not survive long enough to reproduce or are unsuccessful in attracting a mate or are infertile. G Davidson

10. Resistance • In 1910, a Chicago doctor discovered an unusual blood condition in one his patients. • Many of the patient’s red blood cells were crescent-shaped, since then, this condition, known as sickle cell trait, has been found in populations in central Africa, around the Mediterranean, the Near East and India. G Davidson

11. Resistance • Sickle Cell anaemia is caused by a single substitution gene mutation, leading to abnormal haemoglobin molecules and can be transmitted from generation to generation in the genes. • Hybrid individuals for the sickle cell allele are resistant to malaria but are anaemic and so can be listless and unenergetic. G Davidson

12. Resistance • If malaria were to be eliminated, the frequency of the sickle cell allele would be expected to fall through natural selection. • This does seem to be the case in immigrant populations living in non-malarial areas of the United States. G Davidson

13. Resistance • Natural selection is often thought of in terms of fighting or survival against predators. • However, resistance to disease is important and there can be little doubt that in humans, disease plays probably the most important role in natural selection. G Davidson

14. Resistance • As diseases come to be controlled by antibiotics and other therapies, selection pressures on human populations are changing. • However, the disease organisms themselves are subject to natural selection. G Davidson

15. Resistance • Since the late 1950s, the problem of antibiotic resistance in bacteria has resulted in a continual need for the development of new antibiotics. • The problem is not just one of spontaneous mutations which are antibiotic resistant. G Davidson

16. Resistance • Bacteria can ‘catch’ multiple resistance through sharing of genetic material during a kind of sexual reproduction in some strains. • Some scientists have argued that irresponsible use of antibiotics in farming has contributed to the increase of antibiotic resistance. • There have been recorded cases of outbreaks of drug resistant food poisoning, which have been traced to infected calves. G Davidson

17. Industrial Melanism • One of the most well researched examples of evidence for natural selection is that of Biston betularia – the peppered moth. • This moth, which is common in England, normally rests on trunks and branches of trees. G Davidson

18. Industrial Melanism • One form of it is a light colour, but since 1848, melanic (darker, with more melanin pigment) forms have been found, the first near Manchester. • As the moths are predated on by birds, the dark form survives better in polluted areas, while the light form survives better in non-polluted areas. • This is an example of polymorphism i.e. the existence of more than one form of the same species. G Davidson

19. Industrial Revolution 1900 less white moths – more dark moths Clean air act 1950 more white moths – less dark moths Industrial Melanism 1844 mainly white moths G Davidson

20. Industrial Melanism • The reason white moths survive better in non-polluted areas is the presence of a light coloured lichen growing on trees providing the white moths with camouflage whereas dark moths are easily seen and eaten by the birds. • In heavily polluted areas, the soot kills the lichens and darkens the trees, giving camouflage to the dark moths and making the white moths susceptible to predation. G Davidson

21. Polymorphism • Other examples of polymorphism include sickle cell anaemia and normal cells as well as calcifuges and calcioles. • These are two forms of the same species of violet – the calcifuges growing better in acid soils, the calcioles in alkaline soils. G Davidson