12.1Adaptation to the Environment


Various Environmental Factors

On the Earth, organisms are distributed across a vast array of environments ranging from the abyssal ocean to the inside of bedrock deep underground to the stratosphere of the atmosphere, as well as from arid deserts to sulfurous springs. The zone in which organisms reside on the Earth is collectively referred to as the biosphere. Although organisms are influenced by inorganic factors such as light, temperature, moisture, soil, and the atmosphere, a variety of organisms can also exert influence on the environment. Their ability to alter environmental conditions through their living is called an environment-forming function. For instance, even in places where scoria (a sort of volcanic ejecta) is accumulated as a result of flowing lava, the luxuriance of lichens leads to the seeds of perennial plants flying to the dimples of rocks. After the seeds take root, dead leaves pile up around the area to promote soil formation. Using these spots as scaffolding, herbaceous plants and trees will eventually flourish to form bright woods. As this process progresses, illuminance and temperature conditions in the woods change to prevent drying and increase organic compounds in the soil with the help of plentiful deciduous leaves, thereby enabling new species of organisms to enter the woods. In this manner, the very presence of living organisms alters environmental conditions by the minute.
Interactions between living organisms include competition for the same foodstuff and predation in which predators feed on prey (see Section 2 in this Chapter). As described above, organisms live in diverse inorganic and organic environments, where they have to survive in a functional manner. In other words, they need to adapt as stated below.

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Adaptation to Environment—The Effect of Natural Selection

Organisms are endowed with structures and properties to optimize their functions in their living environments from the perspective of morphology, physiology, and ecology, in other words, adaptation. Although their adapted traits fulfill functions that seemingly comply with the objectives of their living, it is of course not because the organisms respond immediately to correspond to it in accordance with circumstances. Through many years of living in certain environments, mutations have caused new genes to emerge and vanish. This has contributed to natural selection, whereby individuals with more advantageous genetic traits on average produce more offspring than others. An extended period of this process has resulted in imparting the adaptive traits to the organisms we can observe today.
Let us take the example of Bar-headed geese, which migrate from India to Tibet, flying over the Himalayan peaks at an altitude of almost 8000 meters. Bar-headed geese have a hemoglobin α-chain in which an amino acid is substituted; thus, the primary structure is altered to make the affinity for oxygen molecules exceptionally higher in regions surrounding a ferric iron in these geese, compared to other closely related geese. Owing to this, the conditions allowing the birds to fly at a very high altitude for migration have been met. Such a mutant gene must have been generated in the ancestors of Bar-headed geese and spread among small groups. One such group might have flown over the mountains and bred in summer in the Tibetan Plateau, thus depriving them of the chances of genetic exchange with closely related geese. This is conceivably how the reproductive isolation was established.
Adaptation therefore can be observed particularly evidently among organisms living in peculiar environments such as high mountains, abyssal oceans, and high/low temperatures. Nevertheless, it can certainly be seen among those living in temperate environments as well.

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