- 1 Natural Selection According to Darwin – Cause or Effect?
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Natural Selection According to Darwin – Cause or Effect?
The principal theme of Darwinism is Natural Selection. According to Darwin, Natural Selection acts as the driving force for evolution. As per this concept, nature selects the successful individuals in competition and it rejects the unsuccessful ones. The ultimate fate of those is death. Through her selection, some individuals get a chance to survive. According to modern synthesis of the theory of Natural Selection, differential reproduction confers to the stability of species. It means that in an environment the more the reproductive rate of one species, the more it may prove itself successful with regard to survivability. The organism showing less reproductive rate will have a greater chance to be unsuccessful with respect to survivability. However, the current concept of; Darwinian theory also supports the idea of impregnation of variations in the fittest individuals in the struggle for existence, and through mutation only, these variations may be developed.
Thus, it may be stated that the differential reproductive rate is the result of the impregnation of variation in organisms. Having a high reproductive rate, a species may also increase its number in the population which in turn leads to an increase in the frequency of specific genes. Therefore, the frequency of genes may be the influx for the fitness of one type of organism in nature. Considering all these aspects, Natural selection may be stated as the consistent difference in fitness of the phenotypically variable entities of life. Natural selection may be operative on different types of entities of life, e.g., one or more genes, genotypes, individual organisms, populations, and total species.
Relation between Evolution and Natural Selection
Evolution usually occurs through two phases Origin of variation through mutation or recombination and Fixation of the variation. Natural selection establishes phenotypic variation through substitution. Behind any phenotypic expression, there remains a specific genotype. Therefore in natural selection, one genotype is replaced by another genotype and the fixation of a genotype or phenotype is dependent on its fitness. It is to be pointed out that the phenotypes which are not determined by a genotype have no importance in evolution. A specific genotype or a phenotype because of its adaptability in an environment gets fixed. This is considered fitness. The attainment of adaptability means the attainment of survival in the changed environmental condition. For this reason, the relative proportion of a genotype may also increase through reproduction.
Fitness and Selection Co-efficient
The attribute for which a phenotype gets established in nature is known as fitness. It has a special relation to environmental conditions. A phenotype that achieves fitness in one environment, may not be adaptive in another environment, and by way of adaptation, one organism becomes fit for survival. The maximum value of fitness maybe 1. When the living entity achieves maximum fitness, it becomes the fittest for survival. In this situation, the frequency of this living entity may also be increased. On the other hand, if an entity shows fitness value less than 1, it fails to get natural selection and then its frequency gets eventually decreased.
The influence of natural selection is measured by a parameter called selection co-efficient. When a living entity achieves maximum fitness and gets natural selection, the value of fitness becomes 1 and the value of selection coefficient (s) becomes 0. Fitness is expressed as w and w = 1 – s. In the case of lethal alleles s becomes 1 and therefore, w = 1 – 1 = 0. For this reason, lethal alleles can not be retained in the population. In the case of most mutant genes, fitness becomes less than 1 and the selection coefficient is more than 0. Therefore, natural selection has little effect on such mutation and the gene may be eliminated from the population quickly.
Natural Selection and Its Types
Natural selection may be categorized into three types, namely Stabilizing selection, Directional selection, and Disruptive selection. Before a discussion on these three types of natural selection, how a character remains distributed in nature under normal conditions should be realized. Let us consider a character of man, that is, the height on which natural selection has very little to do with. In some areas, people may vary in their heights, starting from very low height to sufficiently tall (say 6.5 ft) individual. But the number of too short and the number of too tall people will be very limited. On the other hand, most of the people in the area will be average in height. If the heights against the number of people are plotted in a graph paper, then a bell-shaped curve will be formed. This sort of distribution of a character in the population is called a normal distribution.
But in a character in which natural selection is operative this type of distribution will not be observed. Based on this distribution of a character types of natural selection may be designated.
1. Stabilizing or Normalizing Selection
When averages of a character get importance to natural selection and the extremes of the features are deprived of natural selection, it is known as stabilizing selection or normalizing selection. As an example, the height of plants may be mentioned. The plants which are two tall get badly affected by storms. On the other hand, the plants that are very dwarfed in the population of plants become deprived of sunlight. However, the plants with average height are not so affected as the tall plants at the same time may obtain sufficient sunlight for growth. Therefore, plants with average height are more protected and are favored by nature. Stabilizing selection may be shown by the diagram.
Salient Features of Stabilizing Selection:
- It favours average or normal individuals.
- Stabilizing selection operates in a constant or unchanging environment.
- It introduces homozygosity in the population.
2. Directional Selection
When characteristic variations selected remain distributed towards the side away from the median values of the character, it is known as directional selection. Such a type of selection shows a distribution, which is indicated in the diagram. As an example, the natural selection obtained by Giraffes with long necks may be cited. The individuals in the population who did not show an elongation of the neck could not be favoured by nature. Similarly, the adaptive efficiency shown by the dark variety Beston betularia may be an example of directional selection.
Salient Features of Directional Selection:
- Directional Selection operates when the environment is changing in one direction.
- It favours the phenotype which is non-average or extreme.
- It brings about progressive evolution.
3. Disruptive Selection
When the extreme values of a character achieve natural selection, then the average values of the character are deprived of selection. This type of natural selection is known as disruptive selection which may be presented by the following diagram.
Suppose there are three varieties of the same species of plant. One group shows short height, one group possesses medium height and another group exhibits tall nature. The dwarf variety plants suppose are pollinated by a type of insect which are not attracted by the plants with medium height or elongated height. Similarly, insects pollinating the tall plants are not attracted by the other varieties of plants, and plants having medium height are pollinated by another group of insects. If due to certain reasons the third group of insects become absent from nature, propagation of the plants having medium height cannot be possible. In this situation, plants having short height and long height will get natural selection. This may be placed as an example of disruptive selection.
Salient Features of Disruptive Selection:
- Previously homologous populations break up into several different adaptive forms.
- Extreme values have the highest fitness.
Kin Selection and Sexual Selection
Kin selection and sexual selection are two special types of natural selection. According to the modern view, natural selection is estimated on the basis of the reproductive success of an organism. The noted selections give the firm foundation of a genotype in nature and promote reproductive success in organisms. Kin selection and sexual selection cannot be explained by any of the types of natural selection as mentioned before.
1. Kin Selection:
Kin selection is observed in some social animals. It is based on a special type of behaviour called altruism when an animal protects other animals of the same clan at its own cost or sacrifice with the help of this type of sacrifice the organism may give protection to its own genome. As an example, the social life of honey bees may be presented. In a bee hive, there are three classes of honey bees namely queen, workers, and drones. Of these three types of honey bees the queen and workers are diploids and carry the same type of genome. However, the queen is fertile and the workers are sterile females. On the other hand, the males (drones) of honey bees are haploid and fertile in nature. The worker bees because of their sterility are devoid of natural selection. But by providing nourishment to the queen in the beehive they promote the development of the queen of the honey bee in such a way that it can produce many worker bees in the succeeding generation. The worker bees developed in this way are identical to the workers of the previous generation with regard to their genomic constitution. Therefore, the altruistic behaviour of the worker bees helps in promoting natural selection to their own genotype.
Kin selection is found in the case of many birds. Especially in the case of some birds where there are limitations in the construction of a nest, there the young birds help their parents to look after their chicks as their siblings. The young birds give protection to the sisters and brothers and these also provide nourishment to them. Bee-eater is a type of bird that shows kin selection. Hamilton in 1964 proposed a rule with respect to altruism which is popularly known as Hamilton’s rule. The rule states that altruism is a behaviour in which an organism gives service to its own relations (identical in the genotypic constitution) for its well-being and comfort. Through altruism, the sacrifice of one organism is very cost-effective for the future.
2. Sexual Selection:
It is a special type of natural selection and it does not act directly on the adaptive efficiency of an organism. Rather it promotes reproductive success as a result of which offspring may be born. Darwin (1871) first realized the significance of sexual selection. In sexual selection, an organism having a specific sexual character selects its partner for mating. It is also called intersexual selection. When an animal with a particular sexual identity competes for its sexual partner with other similar animals of the same-sex, it is known as intrasexual selection. Therefore, sexual selection may be of two types as Inter-sexual selection and Intra-sexual selection. It is understood from these two types of sexual selection that some animals may achieve better reproductive success than others in the same population.
A sexual partner always selects a male having more reproductive potential. During the rainy season, the male frogs come to the water of the ponds and produce vocal sounds. With this, the female frogs are attracted and come to the edge of the water in the pond. But a female frog selects for mating a male having a louder sound. In most cases, the male to attract a female shows some special attribute. For example, the ornamental gestures of peacocks, the branched antlers of deer, mane of lion are attractive to their females.