BIOLOGY Volume 3 - A Guide to General Biology - 2004

26. EVOLUTION, OR THE HISTORY OF LIFE ON EARTH

26.5. Natural Selection

According to Darwin and Wallace, natural Selection is the mechanism by which new species arise from pre-existing ones. This hypothesis, or theory, is based on three observations and two deductions, which can be formulated as follows:

Observation 1. Individuals making up a given population produce more offspring than is necessary to maintain population size.

Observation 2. The number of individuals in each given population remains approximately constant.

Deduction 1. Many individuals fail to survive and leave offspring. A "Struggle for Existence" takes place within the population.

Deduction 2. In the "struggle for existence," individuals whose traits are best adapted to the environment possess a reproductive advantage and produce more offspring than less adapted individuals. Deduction 2 contains the hypothesis of natural selection, which can serve as The Mechanism of evolution.

Class="center">26.5.1. Evidence Supporting the Existence of Natural Selection

Observation 1. It was Malthus who drew attention to human reproductive potential and noted that population size increases exponentially (Section 10.7). The capacity for reproduction is characteristic of all living things and represents the fundamental force ensuring the preservation of a species. This applies to A wide variety of organisms (Table 26.2). If every female gamete were fertilized and developed into a sexually mature individual, the Earth would become overpopulated within a matter of days.

Table 26.2. Reproductive potential of certain species

Observation 2. The size of all populations is limited by various environmental factors, such as food resources, space, and light. Population sizes increase as long as the environment is still able to support their further growth, after which a certain equilibrium is reached. Population size fluctuates around this level (Section 10.7.3). Thus, population size generally remains relatively constant over a period of time whose duration depends on The life cycle of the Organism in question.

Deduction 1. Continuous competition among individuals for environmental factors leads to the "struggle for existence." Whether competition occurs within a single species (intraspecific competition) or between representatives of different species (interspecific competition) makes no difference in terms of its effect on population size (see Section 10.7), but in either case, some organisms will fail to survive or leave offspring.

Observation 3. The Study of beetles during his student years in Cambridge, observations made during the voyage of the Beagle, and knowledge gained from breeding and selecting pigeons for specific traits convinced Darwin of Structure/19.html">The Importance of intraspecific variation. Meanwhile, the adaptive significance of interspecific variation discovered in Galápagos finches provided Darwin with the key to his second deduction. Material collected by Wallace in Malaysia confirmed the existence of interpopulation differences. However, Darwin and Wallace were unable to identify the sources of all forms of variation. The question remained unclear until Mendel discovered the particulate nature of inheritance.

Deduction 2. Since all individuals within a given population are subject to variation, and since the existence of a "struggle for existence" has been established, individuals possessing certain traits will prove better adapted to survive and leave offspring. The decisive factor determining survival is adaptation to the environment. Any physical, physiological, or behavioral change, however slight, that gives one organism an advantage over another will act in the struggle for survival as a selective advantage. The term "selective advantage" lacks the emotional coloring of the phrase "survival of the fittest," coined by the philosopher and sociologist Herbert Spencer. The expression "survival of the fittest" is frequently misunderstood. No physical combat takes place between members of a given species, contrary to what Victorians envisioned as "nature, red in tooth and claw." Therefore, it is probably best to avoid using the phrase "survival of the fittest."

Favorable variations will be inherited by the next generation. Unfavorable variations are eliminated by selection, as their presence is detrimental to the organism. Operating in this manner, natural selection leads to an increase in the "vigor" of a species and ensures its survival. The entire essence of the Darwin-Wallace theory of natural selection is expressed most concisely by Darwin himself:

"As many more individuals of each species are born than can possibly survive; and as, consequently, There is a frequently recurring struggle for existence, it follows that any being, if it vary however slightly in any manner profitable to itself, under the complex and sometimes varying conditions of life, will have a better chance of surviving, and thus be naturally selected. From the strong principle of inheritance, any selected variety will tend to propagate its new and modified form" (Darwin, 1859).

The Theory of evolution formulated by Darwin accumulated numerous misconceptions, which can be summarized as follows:

1. Darwin did not seek to explain THE ORIGIN OF life on Earth; he was interested in how new species could arise from existing ones.

2. Natural selection is not merely a negative, destructive force; it can be the mechanism by which positive innovations are introduced into a population (see Section 27.5). In The process of popularization, unfortunate expressions such as "survival of the fittest" and "elimination of the unfit," introduced by the philosopher Herbert Spencer and picked up by the press of the time, became widespread.

3. The simplified, overly direct interpretation by the press of METABOLISM/2.html">THE CONCEPT OF "human descent from apes" deeply wounded the sensibilities of both clerical and secular segments of society. The church viewed this as an insult to its teaching that God created man in His own image, while society was outraged by the downgrading of man's "lofty" position in the animal kingdom.

4. The apparent contradiction between the six-day creation of the universe described in the Book of Genesis and the Concept of the gradual formation of ever-new species was further intensified following the meeting of the British Association for the Advancement of Science held in June 1860. At this meeting, Samuel Wilberforce, the Bishop of Oxford, fiercely attacked Darwin's Conclusions as set forth in On the Origin of Species; but since he was not a biologist, his presentation suffered from inaccuracies. Concluding his speech, the bishop asked one of the defenders of Darwinian theory, Professor Thomas Huxley, whether he believed himself to be related to apes through his grandfather or his grandmother. In response, Huxley outlined Darwin's main ideas and pointed out the distortions made by Wilberforce. He concluded by implying that he would rather have an ape for an ancestor than "be connected with a man who uses his great talents to befog and distort the truth." This unfortunate debate had a sequel and became known as "Genesis versus Evolution." Professor R. J. Berry characterized the two sharply opposing factions as follows:

1) those who worship scientists and believe that the biblical account has been refuted;

2) those who remain under The Influence of Holy Scripture and their own interpretation of it, shutting their eyes to the fact that God's creation can be studied by scientific Methods.