NATURAL SELECTION In the mid 19th century, Charles Darwin (1809 – 1882), formulated the scientific theory of evolution by natural selection, published in his book “On the origin of species” in 1859. Darwin’s idea were inspired by the observations that he had made during a sea voyage in a sail ship called H.M.S Beagle round the world, from 1831 to 1836. Natural selection is the process whereby organisms better adapted to their environment tend to survive and produce more offspring. It is a key mechanism of evolution which involves the change in heritable traits of a population over time. The concept of fitness is central to natural selection i.e., individuals that are more “fit” have better potentials for survival. Alfred Russel Wallace (1823 – 1913) was best known for independently conceiving the theory of evolution through natural selection by working in Malay Archepelago. The concept of natural selection was published by Darwin and Wallace in a “Joint presentation of papers” (1858). Natural selection may also defined as a phenomenon that forces the species to keep improving generation after generation so that they remain in the fittest state to survive in a particular environment. Random genetic changes provide raw material that causes variations and gives natural selection a chance to operate. There are three types of selection process occurring in natural and artificial populations and they are described as stabilising, directional and disruptive. Principle of Natural Selection There is an incredible variety of selective forces in the natural world, ranging from interspecies competition, to predator-prey dynamics, to sexual selection between the different genders. The defining characteristic of natural selection is that it is a force that allows some organisms to reproduce more than others. Natural selection does not always lead to the “right” answer, as some people tend to think. Natural selection is an imperfect process. It cannot create new DNA spontaneously, or change the DNA it is given in meaningful ways. It can only slow or stop the reproduction of some DNA while allowing other DNA to persist. Every population has the opportunity to adapt, migrate to different conditions, or go extinct in the face of natural selection. The process of natural selection screens the DNA it is given, with the minor mutations and recombination that occurs during replication, and simply does not let some DNA pass. Sometimes, the screen is random, as in a lighting strike killing a single tree. Other times, the screen is biased towards certain types of organisms, causing a selection to happen. This can be seen in the pine beetle invasion in North America. The pine beetles are being selected for because they are exploiting a rich food source. The pine trees, on the other hand, are being selected against for not having adequate defenses against the beetles. Types of Natural Selection 1. Stabilizing Selection (Balancing Selection): Stabilizing selection is a form of natural selection that screens against the outliers, or exceptions to the trait. This type of selection favours average sized individuals while eliminates small sized individuals. It reduces variation and hence does not promote evolutionary change. However, it maintains the mean value in a population from generation to generation. If we draw a graphical curve of population, it is bell-shaped. Features of stabilizing selection • Operate in constant or unchanging environment • Keeps a population genetically constant • Favours the average or normal phenotypes • Introduce homozygocity in the population • Check accumulation of mutation Example: The coats of a species of mice in a forest will all be the best color to act as camouflage in their environment. Human birth weight, the number of eggs a bird lays, and the density of cactus spines Selection against homozygous sickle-cell sufferers, and the selection against the standard HgbA homozygotes by malaria. In the human populations, inhabiting in tropical and subtropical Africa the sickle cell gene produces a variant form of the protein haemoglobin, which differs from the normal haemoglobin by a single amino acid. It is caused by the substitution of glutamic acid by valine at sixth position of beta chain of haemoglobin. In people, homozygous for this abnormal haemoglobin, the red blood cells (RBCs) become sickle- shaped, and this condition is described as sickle cell anaemia. The people affected by this disease usually die before reproductive age, due to a severe haemolytic anaemia. In-spite of its disadvantageous nature, the gene has a high frequency in some parts of Africa, where malaria is also in high frequency. Subsequently, it has been discovered that the heterozygotes for the sickle cell trait are exceptionally resistant to malaria. Thus in some parts of Africa, people homozygous for the normal gene tend to die of malaria, and those homozygous for sickle cell anaemia tend to die of severe anaemia; while the heterozygous individuals survive and have the selective advantage over either of homozygotes. There is an optimum wing length for a hawk of a particular size with a certain mode of life in a given environment. Stabilising selection, operating through differences in breeding potential, will eliminate those hawks with wing spans larger or smaller than this optimum length. 2. Directional Selection (Progressive Selection): In this selection, the population changes towards one particular direction. It means this type of selection favours small or large-sized individuals and more individuals of that type will be present in next generation. The mean size of the population changes. Features of directional selection • Due to change in the environment in particular direction • Favours the phenotype which is non-average or extreme • Alters the mean value of the trait in the population in one direction • Favours accumulation of those mutations that increases fitness in the changing environment • Eliminate normal or average individuals Examples: Evolution of DDT resistant mosquitoes, industrial melanism in peppered moth, evolution of giraffe and Antibiotic Resistance in Bacteria. Resistance in mosquitoes and houseflies: The DDT, which came to use in later 1945, was thought to be an effective insecticide against household pests, such as mosquitoes, houseflies, body lice, etc. It was used extensively, sometimes by airplanes over large areas. Initially it killed 99% of mosquito population but at the same time put a lot of pressure on the surviving individuals to mutate. Mutant resistant strains survived DDT application and became the parents of the next generation. Natural selection preserved the resistant populations and eliminated the susceptible ones. This can be called an artificial selection by man, due to which today not only mosquito and housefly but also many agricultural pests have become resistant to most of the available insecticides. The industrial melanic moth: The occurrence of industrial melanism is closely associated with the progress of the industrial revolution in Great Britain, during the nineteenth century. It has occurred in several species of mothes. Of these, peppered moth (Biston betularia) is the most intensely studied. The peppered moth existed in two strains (forms): light coloured (white) and melanic (black). Biston betularia, the industrial melanic moth, is a gray colored moth that perfectly camouflages on tree trunks covered with lichen in England and escapes predation by birds. With industrial revolution in England in the middle of 19th century, lichens on tree trunks got killed due to smoke belching out of factories. Tree trunks were now bare and dark and made the light gray moth prominent to the predatory birds. Now natural selection favoured dark coloured moths, which could camouflage on bare tree trunks. Since the moth has only one generation in a year, in less than 50 generations, the natural selection replaced gray population with black population. Giraffe necks: There was a selection pressure against short necks, since individuals with short necks could not reach as many leaves on which to feed. As a result, the distribution of neck length shifted to favor individuals with long necks. Antibiotic Resistance in Bacteria: When a bacterial population encounters a particular antibiotic, those sensitive to it die. But some bacteria having mutations become resistant to the antibiotic. Such resistant bacteria survive and multiply quickly as the competing bacteria have died. Soon the resistance providing genes become widespread and entire bacterial population becomes resistant. Genetic Basis of Adaptation the Lederberg Replica Plating Experiment to illustrate Role of Natural Selection By an Experiment, Joshua Lederberg and Esther Lederberg was able to show that there are mutations which are actually pre-adaptive. Generally bacteria are cultivated by plating dilute suspensions bacterial cells on semi-solid agar plates containing complete medium with antibiotic like Penicillin. After some period colonies appear on the agar plates. Each of these colonies develops from a single bacterial cell by mitotic cell divisions. Lederberg inoculated bacteria on an agar plate and obtained a plate with several bacterial colonies. This plate is called as ‘master plate’. They then formed several replicas from this master plate. For this, they took a sterile velvet disc, mounted on a wooden block, which was gently pressed on the master plate. Some of the bacteria cells from each colony sticked to the velvet cloth. By pressing this velvet on new agar plates of minimal medium, they were able to obtain exact replicas of master plate. This was due to the fact that the bacterial cells were transferred from one plate to the other by the velvet. After that they tried to make replicas on the agar plates of minimal medium containing an antibiotic penicillin, the replica colonies were not formed. The new colonies that did grow were naturally resistant to streptomycin/penicillin. The new colonies that did not grow were sensitive colonies. Therefore, there was an adaptation in some bacterial cells to grow on a medium containing the antibiotic (penicillin).