Modern concept of natural selection

Selection• In a population some individuals are more successful

than others in passing along their genes into the next generation. This may occur for many reasons.

• Some organisms die before they reach reproductive age. • Some organisms fail to find a mate. • Among those individuals that do breed, there are

differences in the number of offsprings produced.• The result of these differences means that for one

reason or the other there is a differential rate of reproduction among members of a population, we call this selection.

Selection patterns

• Both in natural selection and artificial selection , the principle is the same: some organisms breed more prolifically than others, thus, increasing the frequency of some genes and decreasing the frequency of others.

• The following three patterns of natural selection have been recognized by population geneticists:

• Directional, • Stabilizing and • Disruptive (diversifying).

1. Directional Selection

• The directional selection describes the change that occurs when a population shows a particular trend through time

• Such selection process favours individuals that are best adapted to new situations or to new ecological opportunities

Under directional selection, the advantageous allele increases as a consequence of differences in survival and reproduction among different phenotypes. The increases are independent of the dominance of the allele, and even if the allele is recessive, it will eventually become fixed.

• In fact, directional selection transforms the gene pool of a species or population toward the highest level of adaptedness that can be reached in the new environment.

• E.g Industrial melanism in moth.

2. Stabilizing Selection

• Stabilizing selection is a type of natural selection in which genetic diversity decreases.

• It describes the change when extreme individuals are eliminated from the population. So the intermediate values for a given trait are favoured over the extreme values. The result of this process is a reduced variability in the population.

• A classic example of this is human birth weight. Babies of low weight lose heat more quickly and get ill from infectious diseases more easily, whereas babies of large body weight are more difficult to deliver through the pelvis. Infants of a more medium weight survive much more often. For the larger or smaller babies, the baby mortality rate is much higher.

3. Disruptive Selection

• This form of selection occurs when the extreme values have the highest fitness and the intermediate values are relatively disadvantageous in terms of reproductive effectiveness.

• For example, shell patterns of limpets (marine molluscs) .

• Limpets typically dwell in one of two distinct habitats, attaching either to white goose-neck barnacles, or to tan-coloured rocks. As might be expected, the light-coloured limpets seek the protection of the white barnacles,whereas tan limpets live by choice almost exclusively on the dark rocks. Limpets of intermediate shell patterns are conspicuous and are intensely selected against by predatory shore birds.

Levels of selection

The genetic makeup of a population is altered through an interaction with the ecology of the organism. We refer to this interaction as the process of natural and sexual selection. The fundamental premise of Darwinian selection is that natural selection acts on the individual, or more properly, differences in phenotype among individuals within a population

• In recent years a number of authors have argued that selection might act at a number of different levels and these levels of selection are loosely structured according to heirarchies of biological organization:

• genes -> individuals -> kin -> groups -> species

Labortory and field examples regarding action of natural

selection

1. Melanism in Moths or Industrial Melanism

• The peppered moth (Biston betularia) is a moth which flies during the night and rests on trees during the day, where it is camouflaged to hide from birds. The two most common forms are called typicall, a pale speckled moth which is well-disguised on light-coloured lichens growing on trees, and carbonaria, a black (or melanic) moth which is easy to see on the same background. Both are different forms of the same species, like humans with blonde or brown hair. Before the Industrial Revolution, when there was much less pollution, many trees were covered in lichen and the typical form was well-camouflaged when resting on them. Because the carbonariaform stood out against this pale background, birds foundcarbonaria moths much more easily than typical, so carbonariawere more likely to be eaten, and so less likely to survive to pass on their genes. This meant that the carbonaria form was rare.

• As coal-burning factories were built, air pollution increased significantly, which killed off the lichens and blackened the trees with soot. On this dark background, the pale typical moths were no longer well-camouflaged and were easily caught by birds, but the carbonaria moths were harder for birds to see, so more carbonaria survived to breed and pass on their genes, and this dark type of peppered moth became more common. By 1895, 98% of moths in heavily-polluted Manchester were carbonaria. Armed with Darwin’s new theory of natural selection, J.W. Tutt, an English entomologist, hypothesised in 1896 that the change in colour was due to selection pressure based on how often birds were able to spot the moths.

2. Australian Rabbits

• In 1859, a small colony of 24 wild rabbits (Oryctolagus cuniculus) was brought from Europe to an estate in Victoria in the south eastern corner of Australia. From such modest beginnings, the rabbits multiplied enormously and by 1928 had spread over the greater part of the Australian continent.

• According to an estimate, the number of adult rabbit was over 500 million in an area of about 1 million square miles. The rabbits caused extensive damage to sheep-grazing pastures and to wheat cropfields

• For controlling the population explosion of these rabbits, the Australian government spent huge sums of money for many years. Trapping, rabbit-proof fencing, poisoning of water holes, and fumigation all proved to be largely inadequate. Then, beginning in 1950, outstanding success in reducing the rabbit population was achieved by a biological control method, i.e., inoculating rabbits with a virus that causes the fatal disease myxomatosis

• The deadly myxoma virus was implanted into the tissues of rabbits in the southern area of Australia. In a remarkably short period of time, the virus had made its way, aided by insect carriers (mosquitoes), into most of the rabbit-infested areas of the continent. By 1953, more than 95 per cent of the rabbit population in Australia had been eradicated

• But, after their drastic decline in the early 1950s, the rabbit populations began to build up again.Evolutionary changes have occurred in both the pathogen (i.e., myxoma virus) and the host (rabbit).Mutations conferring resistance to the myxoma virus have selectively accumulated in the rabbit populations. At the same time, the viruses themselves have undergone genetic changes; less virulent strains of the virus have evolved (Frank J. Fenner, 1959).

Labortory example• Gjedrem (1979) showed that selection of Atlantic salmon (Salmo salar)

led to an increase in body weight by 30% per generation. A comparative study on the performance of select Atlantic salmon with wild fish was conducted by AKVAFORSK Genetics Centre in Norway. The traits, for which the selection was done included growth rate, feed consumption, protein retention, energy retention, and feed conversion efficiency. Selected fish had a twice better growth rate, a 40% higher feed intake, and an increased protein and energy retention. This led to an overall 20% better Fed Conversion Efficiency as compared to the wild stock.Atlantic salmon have also been selected for resistance to bacterial and viral diseases. Selection was done to check resistance to Infectious Pancreatic Necrosis Virus (IPNV). The results showed 66.6% mortality for low-resistant species whereas the high-resistant species showed 29.3% mortality compared to wild species.