X. Read and translate the short text without any dictionary:

Fact of life: Highly sensitive dating techniques tell us that the Earth is between 4.5 and 5.0 thousand million years old. It is generally agreed by scientists that the Earth was originally devoid of life, and that the first living organisms arose by biochemical evolution from complex organic chemicals formed in the atmosphere and seas of early Earth. These first forms of life gave rise to countless millions of species. Most have become extinct, but some have evolved into organisms found today. According to the latest estimates, 20-30 million species share our planet.

XI. Food for thought:

In 1809 Jean-Baptiste de Lamark suggested that the driving force behind evolution was the need for organisms to adapt to changing environmental conditions. His theory became known as the theory of evolution by the inheritance of acquired characteristics. He believed that adaptations developed by an organism during its lifetime could be passed on to its offspring. According to Lamark, modern giraffes might have evolved from a short-necked ancestors in the following way. Giraffes feed on leaves ripped off the branches of trees. When leaves on the lower branches were removed, or when the trees became taller, the ancestral giraffe needed to stretch to reach leaves on higher branches. By continually stretching, their necks lengthened and the ability to grow a slightly longer neck was inherited by the next generation which carried on stretching, and so on.

We know that this explanation of the evolution of the giraffe’s neck is untrue because activities such as stretching to feed do not affect the gametes. Therefore, this type of characteristic acquired during the life of an organism is not inherited by its offspring. Expressed in modern terms, Lamarckism would mean that changes in phenotype could determine the genotype of future generations. This does not agree with modern genetics, and there are no generally accepted examples of acquired characteristics being inherited. Suggest a neo-Darwinian explanation for the evolution of the modern long-necked giraffe from a short-necked ancestor.

20. Text 6.2. Natural Selection

■ Essential targets:

By the end of this text you should be able to:

• explain what is meant by “ survival of the fittest”;

• distinguish between directional selection, stabilising selection, disruptive selection.

Pre-reading

■ With a partner consider the following questions and try to answer them. Then scan the text to check your answers.

1. What is natural selection?

2. What environmental factors effect on surviving and producing offspring?

■ Read the given text and make your essential assignments:

Survival of the fittest

Darwin had the idea that natural selection is the mechanism that drives evolution after reading An Essay on the Principal of Population by Thomas Malthus, a clergyman and political economist. Malthus argued that, in time, the growth of human populations will outstrip the food supply, and that this will lead to “famine, pestilence, and war”. Darwin applied this idea to populations of other animals and of plants. In his book on the origin of species, Darwin wrote: “There is no exception to the rule that every organic being naturally increases at so high a rate that if not destroyed, the Earth would soon be covered by the progeny of a single pair”. In spite of reproducing quickly, no single species has completely over-run the planet, although the populations of some species may be increasing at any one particular time. Darwin concluded that populations are kept in check by a “struggle for existence” as they compete for limited resources and are exposed to disease. Environmental factors that keep populations in check are called selection pressures or environmental resistances. These include:

• disease

• competition for resources such as food and a place in which to live

• predation

• lack of light, water, or oxygen

• changes in temperature.

Those organisms best suited to the environmental conditions, with

characteristics that give them an advantage in the “struggle for existence”, will have the best chance of surviving and producing offspring. Their high natality (birth rate) gives them a selective advantage. On the other hand, those will unfavourable characteristics are more likely to die. Their high mortality (death rate) gives them a selective disadvantage. Darwin argued that this difference in natality and mortality results in natural selection. As environmental conditions change certain characteristics within a randomly varying population are favoured, and natural selection occurs. This has become known as the “survival of the fittest”.

In evolution, fitness is defined as the ability of an organism to pass on its alleles to subsequent generations, compared with other individuals of the same species. The “fittest” individual in a population is the one that produces the largest number of offspring that survive to reproduce themselves. Natural selection by “survival of the fittest” means that the genetic characteristics of a population gradually change from generation to generation in response to changes in the environment. As we shall see in the following spreads, natural selection affects a gene pool by increasing the frequency of alleles that give an advantage, and reducing the frequency of alleles that give a disadvantage. (A gene pool is all the genes and their different alleles present in an interbreeding population.)

Three types of natural selection

Natural selection is not always a mechanism for change. There three different types: stabilising selection, directional selection, and disruptive selection. These are three different ways in which natural selection acts on the phenotypes in a population (the observable characteristics such as colour or height). Typically, the frequency in the population of each phenotype has a normal distribution, described by a bell-shaped curve.

Stabilising selection happens in an unchanging environment. Extremes of the phenotype range are selected against, leading to a reduction in variation (more individuals tend to conform to the mean). Stabilising selection occurs in the natural selection of birth mass in humans.

Directional selection favours one extreme of the phenotype range and results in a shift of the mean either to the right or to the left. This type of selection usually follows some kind of environmental change. The long neck of the giraffe is thought to have evolved in this way. Probably, when food was in short supply, only the tallest individuals could reach enough food to survive. They passed on their genes to the next generation.

Disruptive selection selects against intermediate phenotypes and favours those at the extremes. This leads to a bimodal distribution (the distribution curve has two peaks or modes) and two overlapping groups of phenotypes. If the two groups become unable to interbreed, then each population may give rise to a new species. Disruptive selection may have contributed to the evolution of Darwin’s finches. Because there were few other birds to compete, finches with short strong beaks had exclusive use of nuts as a food source, while those with long slender beaks had almost exclusive use of insects. Those finches with an average, unspecialised beak were more likely to have been in completion with other species of bird and would have reproduced less successfully.

■ Glossary of essential terms for you to know

№	English term	Russian equivalent
1	natural	естественный
2	selection	отбор
3	growth	Рост
4	famine	голод
5	pestilence	Чума
6	progeny	потомство
7	origin	происхождение
8	to increase	увеличиваться
9	disease	болезнь
10	lack of sth	недостаток ч.-л.
11	condition	условие
12	to suit	подходить
13	natality	рождаемость
14	mortality	смертность
15	favourable	благоприятный
16	fitness	приспособленность
17	frequency	частота
18	disruptive	разрушительный
19	curve	изгиб
20	either….or	Или…..или, либо……либо
21	in response to	в ответ на ……
22	to occur	происходить
23	to give rise to	давать начало чему-либо
24	finch	зяблик
25	beak	Клюв
26	insect	насекомое

■ Your Essential Assignments