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All of life shares a common evolutionary history

All of life shares a common evolutionary history

Evolution—change in the genetic makeup of biological populations through time—is the major unifying principle of biology. Charles Darwin compiled factual evidence for evolution in
his 1859 book On the Origin of Species. Since then, biologists have
gathered massive amounts of data supporting Darwin’s theory
that all living organisms are descended from a common ancestor. Darwin also proposed one of the most important processes
that produce evolutionary change. He argued that differential
survival and reproduction among individuals in a population,
which he termed natural selection, could account for much of
the evolution of life.
Although Darwin proposed that living organisms are descended from common ancestors and are therefore related to
one another, he did not have the advantage of understanding
the mechanisms of genetic inheritance. Even so, he observed
that offspring resembled their parents; therefore, he surmised,
such mechanisms had to exist. That simple fact is the basis for
the concept of a species. Although the precise definition of a
species is complicated, in its most widespread usage it refers to
a group of organisms that can produce viable and fertile offspring with one another.
But offspring do differ from their parents. Any population
of a plant or animal species displays variation, and if you select
breeding pairs on the basis of some particular trait, that trait is
more likely to be present in their offspring than in the general
population. Darwin himself bred pigeons, and was well aware
of how pigeon fanciers selected breeding pairs to produce offspring with unusual feather patterns, beak shapes, or body sizes
(see Figure 21.2).
Charles Darwin raised pigeons as a hobby, and he noted similar forces at work in artificial and natural selection. The “fancy” pigeons shown here represent three of the more than 300 varieties derived from the wild rock dove Columba livia (at left) by artificial selection for character traits such as color and feather distribution.
Figure 21.2

 He realized that if humans could select for specific traits in domesticated animals, the same process could operate in nature; hence the term natural selection as opposed to artificial (human-imposed) selection.
How would natural selection function? Darwin postulated
that different probabilities of survival and reproductive success
would do the job. He reasoned that the reproductive capacity
of plants and animals, if unchecked, would result in unlimited
growth of populations, but we do not observe such growth in
nature; in most species, only a small percentage of offspring survive to reproduce. Thus any trait that confers even a small increase in the probability that its possessor will survive and reproduce would be spread in the population.
The leaves of all plants are specialized for photosynthesis—the sunlight-powered transformation of water and carbon dioxide into larger structural molecules called carbohydrates. The leaves of different plants, however, display many different adaptations to their individual environments.
(Figure 1.3)

Because organisms with certain traits survive and reproduce
best under specific sets of conditions, natural selection leads toadaptations: structural, physiological, or behavioral traits that enhance an organism’s chances of survival and reproduction in its
environment (Figure 1.3). In addition to natural selection, evolutionary processes such as sexual selection (selection due to mate
choice) and genetic drift (the random fluctuation of gene frequencies in a population due to chance events) contribute to the rise
of diverse adaptations. These processes operating over evolutionary history have led to the remarkable array of life on Earth.
If all cells come from preexisting cells, and if all the diverse
species of organisms on Earth are related by descent with modification from a common ancestor, then what is the source of information that is passed from parent to daughter cells and from
parental organisms to their offspring?
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