independently thought of natural selection as an evolutionary mechanism. But to Darwin goes the immense merit of actually giving body to the idea by laboriously building a monumental set of evidence in its favor.
Fig. 7.1.
Natural selection.
This diagram illustrates the obligatory consequences of replication. As long as replication occurs faultlessly, information is faithfully transmitted from generation to generation. It is the source of genetic continuity. When, as must inevitably happen, replication is not faultless, the imperfect copies produced generate diversity, leading to competition among the variant forms for available resources. As a result of this competition, the variant form or forms best able to survive and, especially, produce progeny under prevailing conditions necessarily emerge. This is natural selection. A fact of capital importance is that copying imperfections (mutations) are chance events or, more precisely, occur in a manner in no way related to any sort of adaptive anticipation of a future environmental challenge (see intelligent design, chapter 8 ).
Fig. 7.2.
Natural selection illustrated.
The Galápagos finches. One of the main findings made by Darwin during his historic voyage on the Beagle (1831â1836) was that the finches on each of the Gálapagos Islands had different beak shapes, adapted to the available food. He reasoned that these adaptations had emerged by natural selection during the long period of isolation of the birds. From Charles Darwinâs On the Origin of Species
.
Natural selection acts under our very eyes
Today, the reality of natural selection leaves no room for doubt. Among countless examples arrayed in its support one can cite, as having taken place under our very eyes, the many cases of antibiotic resistance that developed in only a few decades afterpenicillin was first used during the last war, opening the way to the discovery of a string of other, similar drugs. In each case, introduction of a new antibiotic for therapeutic use was rapidly followed by the development of pathogens resistant to the antibiotic, clearly originating from rare, naturally resistant varieties that prevailed when exposed to doses of the antibiotic that killed the more sensitive bacteria. Paradoxically, hospitals have become sites where some of the most dangerous infections may be caught, as they provide an environment particularly enriched in antibiotics and, therefore, particu-larly conducive to the selection of the most antibiotic-resistant pathogen varieties. Similar cases of resistance have been observed with herbicides, insecticides, and other pesticides, even with chemotherapeutic agents, to which resistance may build in a matter of months in the cancer cells of treated patients.
Another often-quoted example of natural selection is industrial melanism, a phenomenon that affected some English peppered moths (
Biston betularia
) that exist in two different varieties, one white, the other black. In the nineteenth century, when smoke and soot produced by the Industrial Revolution covered all surfaces with a black coating, the white moths almost completely disappeared, while the black ones flourished. The situation has reversed since the end of the Second World War, when laws were enacted to clean the air. The explanation is simple. Predators that feed on moths more readily detect the white ones on a dark background and the black ones on a light background.
The mutations subjected to natural selection are accidental events devoid of finality
A key trait of natural selection, already suspected by Darwin and now confirmed by all that has been learned since, is thatthe mutations on which natural selection operates are due to
chance
(see fig. 7.3 ). Thus, one cause of mutations, inevitable, as well as unpredictable, is faulty replication. The molecular mechanism that drives this process is of astonishing fidelity, one wrongly inserted base in about one billion, the equivalent of copying
Fig. 7.1.
Natural selection.
This diagram illustrates the obligatory consequences of replication. As long as replication occurs faultlessly, information is faithfully transmitted from generation to generation. It is the source of genetic continuity. When, as must inevitably happen, replication is not faultless, the imperfect copies produced generate diversity, leading to competition among the variant forms for available resources. As a result of this competition, the variant form or forms best able to survive and, especially, produce progeny under prevailing conditions necessarily emerge. This is natural selection. A fact of capital importance is that copying imperfections (mutations) are chance events or, more precisely, occur in a manner in no way related to any sort of adaptive anticipation of a future environmental challenge (see intelligent design, chapter 8 ).
Fig. 7.2.
Natural selection illustrated.
The Galápagos finches. One of the main findings made by Darwin during his historic voyage on the Beagle (1831â1836) was that the finches on each of the Gálapagos Islands had different beak shapes, adapted to the available food. He reasoned that these adaptations had emerged by natural selection during the long period of isolation of the birds. From Charles Darwinâs On the Origin of Species
.
Natural selection acts under our very eyes
Today, the reality of natural selection leaves no room for doubt. Among countless examples arrayed in its support one can cite, as having taken place under our very eyes, the many cases of antibiotic resistance that developed in only a few decades afterpenicillin was first used during the last war, opening the way to the discovery of a string of other, similar drugs. In each case, introduction of a new antibiotic for therapeutic use was rapidly followed by the development of pathogens resistant to the antibiotic, clearly originating from rare, naturally resistant varieties that prevailed when exposed to doses of the antibiotic that killed the more sensitive bacteria. Paradoxically, hospitals have become sites where some of the most dangerous infections may be caught, as they provide an environment particularly enriched in antibiotics and, therefore, particu-larly conducive to the selection of the most antibiotic-resistant pathogen varieties. Similar cases of resistance have been observed with herbicides, insecticides, and other pesticides, even with chemotherapeutic agents, to which resistance may build in a matter of months in the cancer cells of treated patients.
Another often-quoted example of natural selection is industrial melanism, a phenomenon that affected some English peppered moths (
Biston betularia
) that exist in two different varieties, one white, the other black. In the nineteenth century, when smoke and soot produced by the Industrial Revolution covered all surfaces with a black coating, the white moths almost completely disappeared, while the black ones flourished. The situation has reversed since the end of the Second World War, when laws were enacted to clean the air. The explanation is simple. Predators that feed on moths more readily detect the white ones on a dark background and the black ones on a light background.
The mutations subjected to natural selection are accidental events devoid of finality
A key trait of natural selection, already suspected by Darwin and now confirmed by all that has been learned since, is thatthe mutations on which natural selection operates are due to
chance
(see fig. 7.3 ). Thus, one cause of mutations, inevitable, as well as unpredictable, is faulty replication. The molecular mechanism that drives this process is of astonishing fidelity, one wrongly inserted base in about one billion, the equivalent of copying
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