II. Read the following text paying attention to the highlighted words. Explain or interpret the contextual meaning of the underlined phrases

The theory of evolution is one of the fundamental keystones of modern biological theory. It postulates that the various types of animals and plants have their origin in other pre-existing types and that the distinguishable differences are due to modifications in successive generations.

The diversity of the living world is staggering. More than 2,000,000 existing species of plants and animals have been named and described; many more remain to be discovered — from 10,000,000 to30,000,000 according to some estimates. What is impressive is not just the numbers but also the incredible heterogeneity in size, shape, and way of life: from lowly bacteria, measuring less than one-thousandth of a millimetre in diameter, to the stately sequoias of California, rising 100 metres above the ground and weighing several thousand tons; from bacteria living in the hot springs of Yellowstone National Park at temperatures near the boiling point of water to fungi and algae thriving on the ice masses of Antarctica and in saline pools at −23° C; and from the strange wormlike creatures discovered in dark ocean depths at thousands of feet below the surface to spiders and larkspur plants existing on Mt. Everest more than 19,868 feet above sea level.

The virtually infinite variations on life are the fruit of the evolutionary process. All living creatures are related by descent from common ancestors. Humans and other mammals are descended from shrew like creatures that lived more than 150,000,000 years ago; mammals, birds, reptiles, amphibians, and fishes share as ancestors aquatic worms that lived 600,000,000 years ago; all plants and animals are derived from bacteria-like microorganisms that originated more than 3,000,000,000 years ago. Biological evolution is a process of descent with modification. Lineages of organisms change through generations; diversity arises because the lineages that descend from common ancestors diverge through time.

The 19th-century English naturalist Charles Darwin argued that organisms come about by evolution, and he provided a scientific explanation, essentially correct but incomplete, of how evolution occurs and why it is that organisms have features — such as wings, eyes, and kidneys — clearly structured to serve specific functions. Natural selection was the fundamental concept in his explanation. Genetics, a science born in the 20th century, reveals in detail how natural selection works and led to the development of the modern theory of evolution. Since the 1960s a related scientific discipline, molecular biology, has advanced enormously knowledge of biological evolution and has made it possible to investigate detailed problems that seemed completely out of reach a few years earlier — for example, how similar the genes of humans and chimpanzees might be (they differ in about 1 or 2 percent of the units that make up the genes).

Biological evolution is the process of change and diversification of living things over time, and it affects all aspects of their lives — morphology, physiology, behaviour, and ecology. Underlying these changes are changes in the hereditary materials. Hence, in genetic terms, evolution consists of changes in the organism’'s hereditary make-up.

Evolution takes place in populations and can be seen as a two-step process. First, hereditary variation takes place; second, selection is made of those genetic variants that will be passed on most effectively to the following generations. Hereditary variation also entails two mechanisms: the spontaneous mutation of one variant to another, and the sexual process that recombines those variants to form a multitude of variations. The variants that arise by mutation or recombination are not transmitted equally from one generation to another. Some may appear more frequently because they are favourable to the organism; the frequency of others may be determined by accidents of chance, called genetic drift,or else by gene migration which takes place when individuals migrate from one population to another and interbreed with its members.

Natural selection refers to any reproductive bias favouring some genes or genotypes over others. Natural selection promotes the adaptation of organisms to the environments in which they live; any hereditary variant that improves the ability to survive and reproduce in an environment will increase in frequency over the generations, precisely because the organisms carrying such a variant will leave more descendants than those lacking it. Hereditary variants, favourable or not to the organisms, arise by mutation. Unfavourable ones are eventually eliminated by natural selection; their carriers leave no descendants or leave fewer than those carrying alternative variants. Favourable mutations accumulate over the generations. The process continues indefinitely because the environments that organisms live in are forever changing. Environments change physically — in their climate, physical configuration, and so on — but also biologically, because the predators, parasites, and competitors with which an organism interacts are themselves evolving.

Mutation, migration, and drift are random processes with respect to adaptation; they change gene frequencies without regard for the consequences that such changes may have in the ability of the organisms to survive and reproduce. If these were the only processes of evolutionary change, the organization of living things would gradually disintegrate. The effects of such processes alone would be analogous to those of a mechanic who changed parts in a motorcar engine at random, with no regard for the role of the parts in the engine. Natural selection keeps the disorganizing effects of mutation and other processes in check because it multiplies beneficial mutations and eliminates harmful ones.

Natural selection accounts not only for the preservation and improvement of the organization of living beings but also for their diversity. In different localities or in different circumstances, natural selection favours different traits, precisely those that make the organisms well adapted to their particular circumstances and ways of life.

The modern theory of evolution provides a causal explanation of the similarities among living things. Organisms evolve by a process of descent with modification. Changes, and therefore differences, gradually accumulate over the generations. The more recent the last common ancestor of a group of organisms, the less their differentiation; similarities of form and function reflect phylogenetic propinquity. Accordingly, phylogenetic affinities can be inferred on the basis of relative similarity.

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