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Natural Selection Allele and Phenotype Frequency

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It is evident that some organisms have some peculiar features or characteristics that increase their probability of survival in a particular situation, compared to others of the same species. Species is a group of organisms that are similar and can reproduce successfully among themselves to produce fertile offspring. There is also the existence of variations in a species which can be caused as a result of meiosis, random fertilization, mutation, and the environment.

Natural selection sometimes known as survival of the fittest can be used to explain why such trend exists in nature. Against this background, organisms that adapt better to their surrounding will survive and live long enough to breed and pass on their useful alleles, while those less adapted will not. As this natural phenomenon continues, there will be an increase in the population of individual organisms with these favored alleles, resulting in a higher proportion of the population with the phenotype determined by these organisms that have been adapted well and have acquired these favoured or useful alleles. Allele is the existence of the same gene in different forms like T and t, while the features resulting from expression of the genes and their interaction with the environment explains the existence of phenotypes.

However, if eventually part of this increasing divergent population becomes isolated from the major group which nature has favoured to adapt better and breed more, changes in allele frequency and mutation will develop in the two groups independently, until such an extent that both groups can no longer interbreed, resulting in both becoming different species. It is important to note that the development of new species from the existing one is the basis of evolution.

According to Darwin's book "The Origin of Species" published in 1859, he based his theory on natural selection on two major observations. These include among others:

 More offspring are produced than actually survive and


 Within a species, there is variation, and some individuals are suited to the existing conditions better than others.

Accordingly, he made under mentioned deductions based on his earlier observations.

 There is always competition between organisms for resources like food, shelter and a mate, in order words a struggle for existence.

 Organisms that are better adapted will live longer and have more offspring than those that are less well adapted.

 The offspring will inherit the favored features that enabled their parents to survive, making it better for the new offspring to further better survive.

 The less adapted ones may not reach maturity before they die, in order words may not live long enough to reproduce. This will result in the decrease in population of individuals with less favorable features.

 This is 'survival of the fittest' or Natural Selection.

 There will be change in the individuals of a species.

 A new species may develop after a time.

In this contest 'fittest' explains that those organisms which are best suited to adapt to a particular environment. Indeed, they are not the strongest rather they are the individuals most likely to find food and shelter, attract mates, breed successfully and avoid predators. This unbalanced ability of individuals to survive and reproduce in a population, will ultimately lead to gradual change in the organisms with favorable features which may result in the formation of a new species.

Natural selection can be used to explain why Giraffes have got long necks. The existence of variation in their neck length means that some are better adapted to their conditions as they can easily reach the higher leaves at times of food shortage. The consequence shows that with time only the long-necked ones can live longer and breed and pass on these favorable features to their young. The short-necked ones will be selected against as they are unable to reach the leaves and are not likely to survive long enough to breed. Eventually, only the long-necked giraffes will


remain as nature has endured them to be the fittest, while the short-necked ones will be reduced and with time may extinct as shown in figure 1a and 1b.

Figures 1a and 1b: Variation in Giraffe population.

Types of Natural selection

Natural selection can be stabilising, disruptive or directional depending on the circumstance, however, they are all affected by environmental factors which can influence the survival rates of different phenotypes present in a population.The mode of selection adopted depends on the relationship between phenotype and fitness. The type of selection can also be described by its on average trait values and trait variance, and when environmental factors or selection pressure acts on a trait, the distribution mode can be altered as shown in figure 2.


Figure 2: The standard distribution of trait across a population (courtesy www.sparknotes.com

Directional Selection

This occurs when the environmental change produces selection pressures that favour organisms with extreme form of peculiarity. Here, a single phenotype is prefered causing allele freqency to continuosly shift in one particular direction as examplified in figure 2. The continuos distribution phenotype curve shift can either be on the right or the left.

An example was the introduction of pesticde Warfrin ( poison used to kill rat) to reduce the effect of rats in an environment. While the poison was able to kill most of the rats, however some of the rats already had chance mutation that they became resistance to the poison. The new environmental factor (Warfrin) killed the normal rats, the resistant rats survived, reproduced and passed on the resistance gene which eventually produced a new population of resistant rats.

Another good demontration of directional selection was the introduction of antibiotic Penicillin in the 1940s. Although the antibiotic was introduced as a way of killing bacteria that were wasting energy producing unnecessary enzymes, however some were able to undergo chance mutation of their genes, thereby making it



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