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In this section, we shall discuss a classic example of natulal selection in action. In the preceding unit, it was stated that natural selection always aims at eliminating alleles, which are less adapted to the environment. Conversely selection indirectly aims to promote those alleles or individuals that are well adapted to a given environment.
The promotion and establishment of an allele and the resultant phenotype may take several hundreds of years in natural populations. But there are indeed instances where the selective forces have brought changes in the character of individuals within historical times.
The peppered moths Biston betillaria occur in two forms, the melanic and non-melanic. As shown in Fig. the melanic forms are black in colour and non-mel?nic ones are mottled grey. Until the early part of the 19th century, the melanic forms were regarded as rare and prize collections by insect collectors, and the light non-melanic forms were abundantly present in natural populations. The rarit.? of black forrns was essentially due to the higher rate of predation by birds. The black forms resting on light coloured and lichen encrusted trees were the easy targets for the birds. The light coloured non-melanics blend with their background, and therefore, are not easily visible to the predators. With the onset of the industrial revolution the distribution of melanic forms particularly in the industrial centres underwent a great change. An examination of museum collection of insects over the past 100 years showed that the melanic forms increased in numbers and the non-melanics were becoming rarer and rarer. What could be the possible reason for such an occurrence? In industrial regions, the sooty smoke emanating from factories blackened the bark of the trees and prevented the growth of lichens. This resulted in the black coloured forms resting on such trees becoming invisible to the p~edators and the light coloured ones being increasingly susceptible to the predator attack because of their contrast with the background. In industrial areas the birds predated upon non-melanics in extremely large numbers whereas in other areas melanics were preferentially caten. Since the proportion of melanics increased due to industrial activity, the examplc is often referred to as industrial melanism.
What else is carried in the plasma? In addition to proteins, plasma having salts (ions), glucose, lipids and amino acids, hormones, carbon dioxide and urea
EYELID S - 3 in number. Dermis is thinnest in it. Upper and lower eye lid with eye lashes. Upper eye lid is more motile In reptiles lower eye lid is more motile. In cycl
Define Procedure for Detection of Number of Bacteria in Milk? 1. Place a clean, non-greasy slide over the graph paper. 2. Spread 0.01 ml of the milk over an area of 1 cm2 as
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Estimation of minerals in foods The minerals in foods are determined by ashing or incineration at temperatures in the region of 500°C following standard procedure. This destroy
what are disadvantage of protozoa
What is the function of the collagen fibers of the connective tissue? There are dissimilar collagen types. The major function of these proteins is to keep the shape and the str
What are vitamins? What are the main vitamins needed by humans? Most vitamins are coenzymes (fundamental substances for the enzyme functioning) that are not formed by the organ
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Tracheoles - Respiration The tracheal system consists of an air filled system of tubes - the tracheal tubes that branch repeatedly until they become fine as capillaries, and
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