Download ARTIFICIAL AND NATURAL SELECTION As a human activity

ARTIFICIAL AND NATURAL SELECTION
As a human activity, artificial selection predates written history.
In artificial selection, humans control the
reproduction of other species. The artificial selection of a particular grass species has provided people with the greatest
bulk of foodstuffs throughout history and indeed is in part the basis for the high levels of agricultural productivity that
humans have achieved in the recent past. All breeders of plants or animals operate in essentially the same way. Knowing
that individuals within any population differ from each other and that offspring tend to resemble their parents, the
breeder merely selects those individuals that possess traits that are deemed desirable. In other words, the breeder
encourages reproduction among the individuals he or she values and discourages reproduction among the individuals he or
she does not value.
Thus, in the next generation the breeder's artificially selected population will contain a greater
proportion of desirable individuals than in the preceding generation. Since any pair of parents can produce more than two
offspring, the breeder is assured that, within some limits and given some time, a population can be produced in which very
nearly all of the individuals may have a particular desirable characteristic.
Charles Darwin is known as the Father of Evolution Theory because of a book he wrote that outlines a theory of
biological change based on natural selection. His 1859 publication, On the Origin of Species by Means of Natural
Selection, along with an essay by Alfred Russel Wallace, caused a revolution in biological thinking. This theory of evolution
has been accepted by almost all leading scientists today. So how did Darwin develop this theory?
In 1831, Darwin set sail on the British HMS Beagle as the ship's naturalist. The trip lasted for five years. During
those years, Darwin collected hundreds of specimens and made detailed observations of the regions through which he
traveled. He had plenty of time for thinking about what he saw. He also read the first volume of The Principles of Geology
by Charles Lyell, which had been published shortly before he left. Lyell proposed that the earth was very old, that it
had been slowly changing for millions of years and that it was still changing. His ideas led Darwin to think that perhaps
living things had also changed slowly over long periods of time.
On his trip Darwin made several types of observations that supported his idea.
He noticed that there was a
gradual change in each species as he traveled down the coast of South America. In particular, several bird species
were alike, yet slightly different from one island to the next. He also saw fossils that were different from the living
animals he saw in the same region, but similar enough to suggest that they might be related to modern animals. Darwin made
similar observations about many plants, insects and other organisms. He came to believe that these organisms originally had
reached the islands from the mainland.
Because of their isolation on the islands, the species had opportunities to
develop special adaptations to each different region.
Darwin returned to England in 1836 convinced that species evolve, that is, change over time. Although he had
recorded many observations that supported his hypothesis, he could offer no explanation of how evolution occurred.
Because he could not, he did not publish his ideas at once. Instead he continued to collect and organize his data and to
search for a reasonable theory of how evolution occurs.
Darwin realized that all organisms face the same danger of overpopulation as humans do as he read an essay by
Thomas Malthus on human population growth. He was familiar with the competition and struggle for existence that occurs in
nature. He reasoned that if a population of organisms could be radically altered by humans through artificial selection, then
there is no reason to believe that a similar process could not occur in nature without the intervention of humans. In
artificial selection humans control which animals are bread into the next generation, and in natural selection survival
and reproduction involve no conscious will. Just as the breeder prevents reproduction among those individuals that are not
valued, conditions of the environment prevent the survival and reproduction of certain individuals. In other words, organisms
with favorable variations would be better able to survive and to reproduce than organisms with unfavorable variations. He
called this process natural selection, because nature "selects" the survivors. Darwin's theory is based on two inferences
drawn from two observations:
Observation 1: Under good conditions organisms multiply geometrically/exponentially. (2, 4, 8, 16) However the size of most
populations stays the same over time.
Inference 1: Not all eggs are fertilized and not all eggs grow into adults. Not all adults survive to reproduce.
Observation 2: There is variation in all types of organisms and individuals compete for resources such as food, space and
mates.
Inference 2: Organisms with favorable variations are more likely to survive long enough to reproduce than those with
unfavorable traits. Thus, the offspring inherit the favorable traits at a higher frequency than the unfavorable traits.
The result of natural selection is evolution.
Evolution by natural selection is basically a two-stage process:
(1) the
production of variation, and (2) the selection of these variations. The six main points of Darwin's theory are summarized
below.
Overproduction. It can easily be observed that a pair of sexually reproducing parents can produce more than two
offspring. Most species produce far more offspring than are needed to maintain the population. Species populations remain
more or less constant because only a small fraction of offspring live long enough to reproduce.
Competition. Since resources are limited, offspring in each generation must compete among themselves and with
other species for the necessities of life. Only a small fraction can possibly survive long enough to reproduce.
Variation. Any population of organisms is made up of individuals that differ from one another; none are exactly
alike. They may differ in the exact size or shape of a body, in strength or running speed, in resistance to a particular
disease, and so on. These differences are called variations. Some variations may not be important. Others may affect the
individual's ability to get food, to escape enemies or to find a mate. These variations are of vital importance.
Inherited Adaptations. Because of variations, some individuals will be better adapted to survive and reproduce than
others. In the competition for existence, the individuals that have favorable adaptations to their environment will have a
greater chance of living long enough to reproduce. An adaptation is any kind of inherited trait that improves an organism's
chances of survival and reproduction in a given environment.
Offspring inherit the traits of their parents; so parents
with favorable adaptations will pass these adaptations to their young.
Natural Selection. In effect, the environment selects organisms with desirable traits to be the parents of the
next generation. Individuals with variations that make them better adapted to their environment survive and reproduce in
greater numbers than those without such advantageous adaptations.
Speciation. Over many generations, favorable adaptations gradually accumulate in the species and unfavorable ones
disappear. Eventually, the accumulated changes become so great that the net result is a new species. Species is
defined as "a group of organisms that share similar characteristics and can interbreed with one another to produce fertile
offspring." The formation of a new species is called speciation.
Darwin's theory would account for the evolution of the modern giraffe. The original giraffe population had short
necks and ate grass. Darwin's theory assumes some giraffes had longer necks than others. Those with longer necks could
eat the lower leaves of trees as well as grass. In times when grass was scarce, the longer-necked animals could obtain more
food than the others and, therefore, would be more likely to survive and reproduce. Their offspring would inherit the
favorable variation of a longer neck. The longer the neck of a giraffe, the higher it could reach for leaves on the trees and
the greater its chances for survival. As a result of natural selection, giraffe necks were slightly longer on the average in
each succeeding generation. The modern long-necked animal is the result of this gradual process of evolution.
Overall, Darwin's theory of natural selection gives a satisfactory explanation of evolution. However, there were
weaknesses in his theory. For one thing, it did not explain how variations originate and are passed on to the next generation.
(Today scientists know that variations originate in the genes. Darwin knew next to nothing about genetics. Gregor Mendel,
who will be discussed later, had not yet published his classic work on the principles of heredity.) For this reason and
because of conflicting religious beliefs, Darwin's theory of evolution was not accepted by the majority of scientists until
many years later, and continues to be controversial outside the scientific community.
QUESTIONS: Use these to guide your note taking as you prepare for your starter.
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Compare and Contrast artificial and natural selection.
What led Darwin to develop his theory of natural selection?
Give one example each of organisms produced by artificial selection from the following groups: (a) plants, and (b) animals.
Describe the six main points of Darwin's theory of evolution by natural selection?