Selection and Evolution • Selection is the sorting of individuals in a natural population on the basis of differences in their characteristics. • Tool for selection is variation in a population. • Variation means differences in the phenotypic characteristics among the individuals of the same species. • There are five tools for variations. • Independent assortment • Crossing over • Random fertilization • Random Mating • Mutation • Independent assortment and Crossing over takes place at the time of gamete formation so these tools are responsible to cause variation in the gametes. • Random fertilization occurs at the time of fertilization and causes variation in the zygotes. • Random mating also occurs at the time of fertilization however it usually involves to bring two different alleles pools (alleles from different varieties of the same specie) in a zygote. • Mutation causes the insertion of totally new alleles in the pre-existing allele pool, so variation in the gamete take place which are entirely different from other individual of the same species. • All tools except mutation are responsible to cause variation just by reshuffling of the existing alleles in different combination with bringing new alleles. • Mutation is the only tool that insert new allele in the pre-existing gene pool and hence play important role in evolution. Independent assortment: The random alignment of homologous chromosomes on the metaphase plate during metaphase I of meiosis allow the alleles of one homologous pair to appear with the alleles of other homologous pairs in any combination in their respective gametes. Each resulting gamete have probability of different allelic combination that may result in variant alleles. Crossing over: The exchange of segments of DNA in the homologous chromosomes during prophase I of meiosis causes reshuffling of the alleles on the homologous chromosomes. This may lead to new combination of alleles (different from both the parental cells) in their respective gametes and lead the development of gametes which are not only differet from one another but also from both the parental cells. Random fertilization: Since all the gametes are formed as a result of meiosis therefore they are genetically different. When anyone of male gamete randomly fertilize anyone of the female gamete then each time the resulting zygote will have different combination of both parental alleles. Random mating: Each individual of the same species has different allelic pool, therefore a cross between individual of the same specie with certain allele combination may produce offspring that would be different from the offspring produced from two other individuals of the same specie as their parents. Mutation: Since it is a change in the structure of a gene therefore it may cause the arrival of a new allele which may be segregated in the gametes and the offspring which is formed from a zygote having a mutated allele (subject to survival) will be quite different in the entire population. Mutation is the only tool for variation which brings new alleles in the pre- existing pool of alleles. Evolution and natural selection The arrival of a new specie from the pre- existing specie is called evolution. Tool for evolution is natural selection of new allele frequencies. Insertion of new alleles and arrival of new environmental selection pressure over many generations in a population brings about evolution. The mechanism of evolution can be explained well on the basis of Darwin's theory of evolution. Darwin’s theory of evolution is based on the mechanism of natural selection. Mechanism of natural selection. Nature select organism on the basis of their fitness to survival in a natural population. It means that the better adapted organisms are more likely to survive and breed in a population with respect to those who are less adapted. The better adapted organism not only survive but also have the potential to reproduce passing on the successful characteristics to the following generation increasing their allele frequency in the population. The less adapted organisms will fail to survive so their number will slowly decline and hence their allele frequency greatly decrease. All organisms produce a large number of offspring in order to increase the population size. This is to replace the population and ensure the continuity of the species. Individual variation exists between offspring produced from sexual reproduction. The most suited variation will provide organisms a selective advantage over others. Individuals in a population compete for environmental resources in order to survive, therefore there is a struggle for existence among the individuals. The organisms that have adapted with more suitable characteristics will be able to survive better and reproduce in the struggle for existence. They will also have a reproductive advantage and will produce more offspring compared to those organisms that are not so well adapted. The increase in population size therefore reaches to equilibrium due to environmental constraints such as living space and food supply. The more suitable characteristics that are possessed by surviving and breeding organisms are more likely to have offspring who are look like with their parents. As the better adapted organisms succeed in surviving and reproducing over many generation, more variations will be developed with in the population and this will gradually lead to the arrival of a new specie (evolution). To sum up, variation produce different and new characteristics. These characteristics are selected by the natural selection process based on “struggle for existence” and “fittest to the survival” which gradually causes evolution, Types of variations Discontinuous Variations: • Type of variations that involve in the qualitative differences like blood group, gender etc. is called discontinuous variation. • Qualitative differences fall into easily distinguishable categories with limited intermediates. • Such variations are usually not affected by environmental features. • Only limited number of alleles are responsible while only two alleles are present at a time to express the phenotype. • Usually shown by a bar chart when represented graphically. Genetic basis of Discontinuous variation Different Alleles of the same gene at the single gene locus have large effect on the phenotype. Different genes have quite different effect on phenotype and do not affect the same phenotype. Continuous variation Type of variation that involves in the quantitative differences like height or body mass of people in a population is called continuous variations. Quantitative difference is generally very difficult to calculate due to a large range of intermediates in the population. Continuous variation is generally as a result of polygenic characters or because of interaction between genes and environment. Environment have a large effect on quantitative differences. Genetic basis of continuous variation Different alleles at a single gene locus have very small effect on the phenotype. Different genes have the same and additive effect on the phenotype. A large number of genes may have a combined effect on a particular genotype (polygenes) Types of natural selections Natural selection can occur with or without environmental change. In a constant environment, natural selection will keep a population stable and essentially maintain the status quo. In a constant environment, however if a new variation arises that is more advantageous, the new trait will be more perpetuated and the species will evolve. In a changing environment, natural selection will favour variation that result in a better fitness in the new environment, resulting in adaptation and evolution. Stabilizing Selection: When the organisms in a population are already well adapted to such an environment which is fairly stable for a long period of time then natural selection is unlikely to bring about change in allele frequency of the population. The most common alleles in the population will be those that favour an advantage on the organism’s survival and therefore these alleles will continue to pass to successive generation. In this case the environmental pressure often against the two extremes selecting the alleles of average traits. This result in a population with a narrow range of a quantitative characteristic where most of the individuals in the population are near to average. Example: A plant that is too short may not be able to compete with other plants for sunlight. However, extremely tall plants may be more susceptible to wind damage. Combined, these two selection pressures select to maintain plants of medium height. The number of plants of medium height will increase while the numbers of short and tall plants will decrease. Directional Selection: When the environment changes, it usually selects phenotypes at one end of the spectrum of existing variation. The selection pressure is applied on only one of the extreme of the range while the average and other extreme of the traits are declined. The range of variation shifts toward one of the extreme. This is because a change in the environment select the allele that were previously less adaptable now may become more adaptable in the new environment. Example: Prior to the Industrial Revolution, the moths were predominately light in color, which allowed them to blend in with the light-colored trees and lichens in their environment. As a result of pollution, the trees became darkened, and the lightcolored moths became easier for predatory birds to spot. Over time, the frequency of the melanic form of the moth increased because they had a higher survival rate in habitats affected by air pollution because their darker coloration blended with the sooty trees. Disruptive Selection: Natural selection that favors phenotypes of both extremes of the existing variants in a population is called Disruptive selection. Selection pressures act against individuals in the middle of the trait distribution. The result is a two-peaked curve in which the two extremes of the curve create their own smaller curves. Example: Imagine a plant of extremely variable height that is pollinated by three different pollinators, one that was attracted to short plants, another that preferred plants of medium height and a third that visited only the tallest plants. If the pollinator that preferred plants of medium height disappeared from an area, medium height plants would be selected against and the population would tend toward both short and tall, but not medium height plants. Such a population, in which multiple distinct forms or morphs exist is said to be polymorphic. Example of natural selection - antibiotic resistance In case of infection, the multiplying pathogenic bacteria inside of our body are not all identical. Most of them are the same, but few of them will be slightly different genetically. If they were all genetically identical, they would react to their environment the same way and all be harmed by the same things. One harmful thing could wipe out the entire species. But when there are slight genetic differences, harmful conditions, like the presence of an antibiotic, might kill most of the pathogenic bacteria but some may be better able to survive and then multiply. These characteristics are favored in this environment so these bacteria can continue to thrive, and make you ill. Different environments favor different traits and so natural selection has taken place!
