The Theory of Evolution - Crovo.in

THE THEORY OF EVOLUTION

Section 1-Natural selection

The theory of evolution is the central, unifying principle of modern biology. Although entire books have been devoted to explaining the theory in detail, its fundamental ideas can be summarized briefly.

• 1) The individual members of any given species vary considerably from each other, and some of that variation is due to genetic factors. Furthermore, new heritable traits are constantly arising because of random mutations.
• 2) Since, in each species, there are more offspring than the environment can support, many individuals die without reproducing.
• 3) Those members of the species who have inherited traits that make them less likely to survive will, on average, have fewer offspring. Hence, in the next generation, there will be fewer individuals with those traits.
• 4) The result is the selective elimination of less useful traits and, in comparison, the natural selection of more useful traits.
• 5) This leads to a series of small changes within a species; and the gradual accumulation of many such small changes eventually results in the formation of a new species, related to the original one."

Note that two separate processes are involved: (a) random mutations (which occur entirely by chance); and (b) selection of the more useful traits (a process that is far from random). The notion that evolution is governed entirely by chance is therefore false.

At no stage in either process have the resulting organisms been deliberately designed. However, since natural selection results in organisms that are very well adapted to their environments, they usually give the appearance of having been designed.

Most frequently, selection is for traits that make it more likely that the individual organism will survive (such as greater size, speed, strength, intelligence, or resistance 

to disease). This might be called "survival selection." However, in sexually reproducing species, an individual cannot pass on its genes unless it mates with a member of the opposite sex. Consequently, traits that make an individual a more attractive mate will also be selected for, whether or not they aid in survival. This is called "sexual selection," and is an important evolutionary mechanism.?

Section 2 - Genes

Heritable traits are transmitted from parent to child by microscopically small particles called genes. All cells contain genes, but it is only the genes contained in the sperm and egg cells that are responsible for heredity. Normally, an individual receives two copies of each gene, one from each parent. However, he will pass only one copy of each gene on to his offspring.3

It is common for there to be two or more slightly different forms of a given gene. These variants are called alleles. If, for some gene, an individual receives identical alleles from his two parents he is said to be homozygous at that gene site. If instead he receives different alleles from his two parents at that gene site, he is heterozygous for that gene. In the latter case, the individual often exhibits the trait corresponding to just one of those alleles (the "dominant" one), and the other allele (the "recessive" one) will have no effect. However, the recessive allele is not destroyed, and is just as likely as the dominant one to be passed on to the person's offspring.

A gene is a fragment of a large molecule called DNA. However, for most purposes, one can consider an individual gene to be an independent molecule, a molecule which consists of a long string of simpler units called nucleotides. There are four types of nucleotides; and the various genes (and alleles of the same gene) can differ from each other by containing either:

1)      Different numbers of nucleotides; or

2      The same total number of nucleotides, but not the same number of each type; or

3      The same number of each type of nucleotide but arranged in a different order.

Mutations occur when a gene in a sperm or egg cell is altered, most commonly by some random natural occurrence such as:

• ·         cosmic rays
• ·         gamma rays emitted from radioactive materials in the Earth's crust.
• ·         solar radiation (particularly ultraviolet radiation)
• ·         certain chemicals ("mutagenic" chemicals)
• ·          ordinary thermal agitation.

(Human activities can also produce gamma rays, ultraviolet rays, X-rays, and mutagenic chemicals; but so far these have had almost no impact on our gene pool.)

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