Arrival of the Fittest: Solving Evolution's Greatest Puzzle Read Online Free Page A
Author: Andreas Wagner
collective pool of
genes
. The genes that determine a moth’s wing color, for example, have different forms—the technical term is
alleles
—responsible for light or dark wings, that occur in different proportions or frequencies in the population. Imagine that at any one time, equal numbers of both types of alleles were present in a population of organisms, and that some new factor—a new predator, or a change in pollution—allowed moths with darker wings to live longer, and so produce more offspring. Their advantage need not be huge, but even a merely 1 percent increase in the dark-winged allele, from 50 percent to 51 percent in the first generation, could accumulate over time and allow the dark-winged variants to occupy a larger and larger percentage of the population. That’s how natural selection works: It changes allele frequencies, and thus the appearance of individuals over time.
This was revolutionary. The study of life, which had largely depended on the same tools since Aristotle—close observation and dissection in the field and laboratory, recorded in sketchbooks and notes—began to embrace the mathematics of differential equations and the analysis of variance. Through the minds of intellectual giants such as Sewall Wright, J. B. S. Haldane, and the statistician R. A. Fisher, population genetics developed into a theory that could answer precise, quantitative questions about natural selection. At the same time, naturalists studied the frequencies of alleles in wild populations such as that of the peppered moth, and experimentalists created evolution in action in the laboratory, by studying laboratory populations of small, rapidly breeding animals such as fruit flies. The mathematical theory was the mortar that helped join these observations into an intellectual edifice.
The new evidence from population genetics showed that variation covered a broad spectrum, with “pure” Mendelian variation at one extreme, and continuous variation at the other. Mendelian phenotypes—wing color, pea shape—are influenced by one gene with large effects. Continuously varying phenotypes like height are influenced by multiple genes, each with a tiny effect. Population genetics showed that natural selection affects both kinds of genes. But truly surprising was how powerfully selection could affect them. If a dark-wing allele decreased a moth’s chance to be eaten by a few percent, it could wipe out the light-wing allele within a few dozen moth generations. And both naturalists and experimentalists found far more genes in their populations with small effects than with large ones. Mendel clearly had chosen his peas very carefully, because Mendelian traits that are influenced by a single gene comprise a tiny fraction of all traits. 29 Most evolution is gradual and does not make large jumps. 30
By the 1930s, the concept of natural selection, the nature of inheritance, and population thinking had been synthesized into a body of knowledge known as the
modern synthesis,
named after an eponymous book by the biologist Julian Huxley. 31 Despite its name, the synthesis will soon be a century old. But unlike most centenarians, it shows no signs of senescence. Augmented by mathematical refinements and modern data, it is unbroken, and by some measures stronger than ever, playing an increasingly important role in understanding human biology—helping to reconstruct human origins, trace human migrations, and understand genetic diseases. If this edifice of knowledge were a physical building, it would rival everything architects have conceived, from the palaces of Angkor Wat and the mausoleum of the Taj Mahal to the great Gothic cathedrals of the thirteenth century. It is a grand achievement of the human mind.
There is, however, a dirty secret behind its success. The architects of the modern synthesis focused on the genotype at the expense of the organism and its phenotype. They neglected the marvelous complexity of organisms with their
genes
. The genes that determine a moth’s wing color, for example, have different forms—the technical term is
alleles
—responsible for light or dark wings, that occur in different proportions or frequencies in the population. Imagine that at any one time, equal numbers of both types of alleles were present in a population of organisms, and that some new factor—a new predator, or a change in pollution—allowed moths with darker wings to live longer, and so produce more offspring. Their advantage need not be huge, but even a merely 1 percent increase in the dark-winged allele, from 50 percent to 51 percent in the first generation, could accumulate over time and allow the dark-winged variants to occupy a larger and larger percentage of the population. That’s how natural selection works: It changes allele frequencies, and thus the appearance of individuals over time.
This was revolutionary. The study of life, which had largely depended on the same tools since Aristotle—close observation and dissection in the field and laboratory, recorded in sketchbooks and notes—began to embrace the mathematics of differential equations and the analysis of variance. Through the minds of intellectual giants such as Sewall Wright, J. B. S. Haldane, and the statistician R. A. Fisher, population genetics developed into a theory that could answer precise, quantitative questions about natural selection. At the same time, naturalists studied the frequencies of alleles in wild populations such as that of the peppered moth, and experimentalists created evolution in action in the laboratory, by studying laboratory populations of small, rapidly breeding animals such as fruit flies. The mathematical theory was the mortar that helped join these observations into an intellectual edifice.
The new evidence from population genetics showed that variation covered a broad spectrum, with “pure” Mendelian variation at one extreme, and continuous variation at the other. Mendelian phenotypes—wing color, pea shape—are influenced by one gene with large effects. Continuously varying phenotypes like height are influenced by multiple genes, each with a tiny effect. Population genetics showed that natural selection affects both kinds of genes. But truly surprising was how powerfully selection could affect them. If a dark-wing allele decreased a moth’s chance to be eaten by a few percent, it could wipe out the light-wing allele within a few dozen moth generations. And both naturalists and experimentalists found far more genes in their populations with small effects than with large ones. Mendel clearly had chosen his peas very carefully, because Mendelian traits that are influenced by a single gene comprise a tiny fraction of all traits. 29 Most evolution is gradual and does not make large jumps. 30
By the 1930s, the concept of natural selection, the nature of inheritance, and population thinking had been synthesized into a body of knowledge known as the
modern synthesis,
named after an eponymous book by the biologist Julian Huxley. 31 Despite its name, the synthesis will soon be a century old. But unlike most centenarians, it shows no signs of senescence. Augmented by mathematical refinements and modern data, it is unbroken, and by some measures stronger than ever, playing an increasingly important role in understanding human biology—helping to reconstruct human origins, trace human migrations, and understand genetic diseases. If this edifice of knowledge were a physical building, it would rival everything architects have conceived, from the palaces of Angkor Wat and the mausoleum of the Taj Mahal to the great Gothic cathedrals of the thirteenth century. It is a grand achievement of the human mind.
There is, however, a dirty secret behind its success. The architects of the modern synthesis focused on the genotype at the expense of the organism and its phenotype. They neglected the marvelous complexity of organisms with their
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