Modern concept of natural selection

Natural selection

• Natural selection is the differential survival and/or reproduction of organisms as a function of their physical attributes. Because of their phenotypes,which are due to the amalgam of traits that make up an individual, some individuals do better than others.

Selection patterns

• Selection is defined as some sort of functional relationship between fitness and phenotype and we can easily describe fitness in terms of three kinds of curves:

1.Directional selection in which the trait is linearly related to fitness,

2.Stabilizing selection in which there is an optimal value for the trait of interest, and

3.Disruptive selection in which individuals with the smallest and largest values of the trait have the highest fitness and individuals with intermediate values are at a fintess disadvantage

1. Directional Selection

• Favors variants of one extreme.

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Size of individuals Small Large

2. Stabilizing Selection

• Acts upon extremes and favors the intermediate.

Number ofIndividuals

Size of individualsSmall Large

3. Diversifying Selection

• Favors variants of opposite extremes.

Number ofIndividuals

Size of individualsSmall Large

• Each mode of selection alters the mean or variance of the phenotypic trait in a population or species. In the long term, directional selection can have the most dramatic impact on the evolution of a species. Directional selection can lead to the formation of a new type from an existing type. This contrasts with the action of stabilizing selection which maintains the existing type without change in mean over long periods of time. Stabilizing selection eliminates the extremes in a distribution of phenotypes, and as such it leads to a refinement of the exisiting type. By eliminating individuals from the center of the distribution, disruptive selection favors the individuals in the tails or more extreme values of the phenotype. Disruptive selection can lead to the formation of two new types from a single exisiting type.

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

Laboratory 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.

Action of natural selection leading to

convergence,radiation,regression and extinction

Radiation

• Natural selection can ultimately lead to the formation of new species. Sometimes many species evolve from a single ancestral species. Fourteen species of Darwin finches evolved from one ancestral species. Such an evolutionary pattern, in which many related species evolved from a single ancestral species, is called adaptive radiation. Adaptive radiation most commonly occurs when a species of organisms successfully invades an isolated region where few competing species exist. If new habitats are available, new species will evolve.

Extinction

• Natural selection is a process through which the traits that are helpful in the survival of an organism are passed on to the next generation while those that can hinder survival are eliminated. Natural selection then only allows the very well adapted species to reproduce. This is done to ensure survival in the ever-changing environmental conditions. Due to the evolutions that have been taking place, the habitat of organism change and to cope up with these changes, the organisms develop some genetic characteristics that would enhance their survival. However, even with natural selection, studies have shown that some organisms, both plants and animals have become extinct. Dinosaurs became extinct sixty five million years ago. Another example is the thylacine tiger of Australia.

• The minority of the organisms in the environment are the ones that become extinct together with those that are unable to survive the changes in the environment. The minority are unable to compete with the majority for survival means. Moreover, when new species are being formed or modified they exert pressure on the existing ones and finally eliminate them. The introduction of new breeds of animals leads to the extinction of the inferior ones.

• The main reason why extinction still occurs is that natural selection is good at maintaining and even improving species adaptations while on the other hand it eliminates those species that are the minority and those that are not well adapted. It is then clear that natural selection does not only play a role of preventing extinction, it also supports extinction of the species that are not well adapted for survival. This being the case then, extinction is bound to prevail even if natural selection is still there.

Convergent Evolution

• Species from different evolutionary branches may come to resemble one another if they live in very similar environments.

• For example, flight has evolved in both bats and insects, and they both have wings, which are adaptations to flight. However, the wings of bats and insects have evolved from very different original structures

Regression

• Natural selection may favor Regresion.• Regression is a process of partial or complete

reduction of organs that have lost their adaptive significance. • For example, replacementof the notochord by a cartilaginous skeleton and later by a bony skeleton in the process of

vertebrate evolution,replacement of gills by lungs when vertebrates emerged onto dry land.

Co-evolution

• Evolutionary change, in which one species act as a selective force on a second species, inducing adaptations that in turn act as selective force on the first species.

• In evolutionary biology, mimicry is a similarity of one species to another which protects one or both.

Examples of coevolution: mimicry• The classical examples of mimicry illustrate nicely different types of co-

evolutionary interactions.• Müllerian mimicry describes the convergence of unpalatable models to

a similar phenotype, i.e. reciprocal evolution between species all of which are distasteful. It is thus characterised as a +/+ mutualistic interaction, i.e. all involved species benefit

• Batesian mimicry describes the convergence of a palatable species to unpalatable models. A non-toxic, edible species mimics the warning colour of a toxic, noxious model. This is a 0/+ interaction, because only one species benefits . The system works well as long as the mimic does not become too frequent. Otherwise, the noxious model may have a disadvantage (i.e. the interaction may turn into antagonism, –/+), because predators do not avoid the warning colour any more.