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PinkMonkey Online Study Guide-Biology

11.1 (B) Recombination as a Source of Variability

In organisms which reproduce asexually, the offsprings are identical as they are formed by mitosis. However, in sexually reproducing organisms the offsprings differ from their parents in many respects. This is due to genetic recombinations. During gametogenesis, the homologous chromosomes exchange genes by the process of crossing over. This produces a new combination of genes. The process of fertilization brings together genetic material from two different cells, the sperm and the ovum, which differ in their characters. Thus genetic recombinations take place firstly by bringing together paternal and maternal chromosomes and secondly the by crossing-over mechanism. This process of genetic recombination results in new variations. If during such recombination process any advantageous characters develop, they are passed on to the offspring generation after generation and thus contribute to the evolution of better and strong organisms.

11.1 (C) Role of Selection

The process of natural selection plays important role in the following processes.

Figure 11.3. Industrial melanism, Biston betularia and its black form

B carbonaria on tree trunk in (A) unpolluted area, and (B) polluted area

(i) Variations. Variations result due to generic recombinations. If the variations are favorable, (i.e. if they are of some help or advantage), they are passed to the next generation. This can be best studied with reference to certain characters. Human complexion colors vary from almost black, through brown to white. These shades result because of difference in the amount of melanin developed in the skin. In tropical areas such as Africa, sunlight is intense, and melanin develops in the skin and screens out sunís rays, yet adequate amounts of vitamin D can still be synthesizedin the skin. In the northern areas there is less sunlight. Less melanin is developed so that light can penetrate the skin and synthesize vitamin D. Therefore, humans in the north of the tropic countries tend to be lighter complected.

Industrial melanism (Figure 11.3) is another example of favorable variations developed and selected. In the industrial areas of Britain, the soot released from numerous factories is deposited on the tree trunks which made them dark. The normal peppered moth, Biston betularia, has a grey body which more closely matches with the color of lichens on the bark of trees. Some moths have developed melanin. When such dark moths settled on dark tree trunks, they are not noticed by predators. But the original light-colored moths could be easily noticed and killed by their predators and thus diminished in number gradually.

The darker variety of moth Biston carbonaria is at advantage in this environment. They escapes predators and survive and reproduce. Similar changes have been observed in nearly 100 other species of insects and spiders. Thus, selection plays an important role in choosing favorable variations.

(ii) Adaptations. Any characteristic that is advantageous to an organism is referred to as an adaptation. Adaptations are genetically controlled traits that help an individual to survive and produce offspring in a particular environment. Evolution implies that the surviving species are more adaptive than the those that do not survive. Adaptations can be classified under three categories, structural, physiological and behavioral adaptations. The adaptations may also be described under the following heads when different environmental conditions are taken into consideration. They are described as cursorial adaptations (terrestrial adaptations especially for fast running), fossorial adaptations (subterranean adaptations shown by diggers), aquatic adaptations (shown by animals living in water), scansorial or arboreal adaptations (shown by animals living in or on trees), volant adaptations (shown by animals that can fly), cave adaptations and desert adaptations.

Depending upon environmental pressures, animals adjust to the environmental changes. Adaptive features may result by the process of genetic recombinations or mutations. Organisms which show favorable variations can survive and reproduce their kind. Thus, the adaptive features are passed on to the offspring down through generations and in this manner new varieties develop. Selection plays important role in allowing the new variety to survive and thus, a new species evolves from the pre-existing simple species. The paleontological record proves how organisms must have adapted and evolved into more fit and successful individuals.

Figure 11.4 Adaptive radiation in mammals Adaptive radiation produces bush-like pattern of evolution

Table of Contents

11.0 - Introduction
11.1 Darwin's Theory of Natural Selection

Chapter 12


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