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THEORIES OF EVOLUTION: LAMARCK AND ACQUIRED CHARACTERISTICS
One scientist intrigued by fossils was the French naturalist Jean Baptiste de Lamarck. IN 1809, Lamarck proposed that organisms evolved in response to their
environment. To evolve means to change from one form to another. Lamarck was the first biologist to suggest that organisms undergo evolution.
Lamarck’s theory explained two observed facts. The first was the fossil record, which showed that organisms in the past were different from those living today.
Second, his theory explained why each orgnaism was so well adapted to its environment. As Lamarck had observed, each organism had adaptations that suit its particular way
of life. Elephants have long trunks for gathering food; lions have powerful claws and jaws for capturing prey; and antelope have long, swift legs for escaping predators.
Lamarck believed that an organism acquired these traits during their lifetime and then passed these traits down to their offspring. No evidence has been found to
support this concept. As you know, genetic material is contained in the chromosomes. Except for rare mutations, genetic information is passed on unchanged from
generation to generation. If acquired characteristics could be inherited, then the children of concert pianists would be born already knowing how to play the piano. This is
not the case. Acquired skills must be developed anew in each generation. Geneticists today have been unable to find any evidence that acquired traits can be inherited.
THEORIES OF EVOLUTION: CHARLES DARWIN AND NATURAL SELECTION
About 50 years after Lamarck proposed his theory of evolution, the British naturalist Charles Darwin revolutionized the thinking of most biologists. In 1859,
Darwing published a book called The Origin of Species by Means of Natural Selection. Like Lamarck, Darwin stated that living things gradually evolve adaptations to the
environment. But Darwin proposed an entirely different mechanism to account for the changes in species.
Darwin thought that variations among species are inherited. He hypothesized that in the natural environment, selection takes place.
Natural selection results from the interaction of a population of organisms with its environment. Darwin realized that in nature, most organisms produce more
offspring than can survive. For example, oak trees produce thousands of acorns and frogs lay hundreds of eggs. But only some of these offspring survive to produce offspring
in the next generation. Since not all offspring can survive, Darwin reasoned that there must be competition among the offspring. This competition results in the survival of
only a few.
Darwin further reasoned that only those organisms most suited for the environment survive to produce offspring. This process is called the survival of the fittest.
Darwin’s theory of natural selection can be summarized as follows:
1. Species have the ability to produce a large number of offspring.
2. The resources of the natural world are limited.
3. Therefore, there must be competition for survival among the offspring in each generation.
4. There is great variability within population of organisms. No two individuals are the same. Much of this variety is inherited.
5. The organisms that survive and produce offspring are those that have inherited the most beneficial traits for surviving in that particular environment.
6. As this process continues through many generation, the population gradually becomes better adapted to the environment.
1.
Darwin then went back to England to do further research.
2.
At that time people believed that each species was a divine creation, unchanging and existing as it was originally created. That's why species are often uniquely adapted
to their environments. This viewpoint is called _________________________________________________.
3.
There was a problem, though. This view could not explain the kinds and distribution of fossils found.
How Darwin Formulated His
Theory
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When Darwin returns home, he thinks a lot about what he saw and pieces his observations together with
other people’s ideas. Create a graphic organizer which visually shows the following information obtained
by reading Darwin Formulates His Theory.
1. While exploring South America, Darwin noticed that the South American species of plants and animals
were different from European species and even the fossils he found were uniquely South American.
Based on these two observations, what did he conclude?
2. While exploring the Galapagos Islands, Darwin noticed that species on the islands resembled species
on the South American mainland, but they were different. In addition, organisms on different islands
were different from each other! What did he conclude based on these two observations?
3. CHARLES LYELL: What was Charles Lyell’s idea about the geology of Earth? Based on his ideas,
would Lyell agree or disagree that the Earth is less than 10,000 years old (which is what people thought
during his time)? What TWO observations did Darwin make that supported Lyell’s idea?
4. THOMAS MALTHUS: In your own words, describe the relationship of population vs. resources.
What does this lead to? List 6 possible things that could happen to offspring that would make them
unable to pass down their traits.
5. VARIATION: Define. Variations are ___.
6. ARTIFICAL SELECTION (SELECTIVE BREEDING): What is it? Breeders are to artificial
selection and the_____ is to natural selection. What did Darwin conclude from this observations of
artificial selection?
7. NATURAL SELECTION: Who was another naturalist who reached the same conclusions as Darwin?
What was the name of Darwin’s book? State and describe Darwin’s TWO main points presented in his
book.
All these lead up to the idea of natural selection as the process by which evolution occurs. All arrows should
point to this box. In the box, include a complete description of natural selection.
This is the suggested format…. See how all the boxes contribute to Darwin’s final conclusion?
DARWIN’S THEORY OF EVOLUTION BY NATURAL SELECTION
Darwin Formulates His Theory
Ideas From His Exploration Darwin noticed that the plants and animals throughout South America all
had a definite South American character meaning they were unique to that area. They were quite distinct from
the species of Europe. Even the fossils that Darwin found were uniquely South American. His observations
supported the idea that species living in South America today were descended from ancestral species on that
continent.
Darwin was intrigued by life on islands such as the
Galápagos. The Galápagos are a chain of relatively young
volcanic islands about 900 kilometers off the western coast of
South America. Darwin observed that the islands had many
unique organisms. Most of the species on the islands were
similar to, but different from the plants and animals of the
nearest mainland. He observed that even the individual
islands in the chain had some different species of plants and
animals from one another. Darwin inferred from these
observations that mainland species had changed after they
colonized the islands and adapted to their various new
environments.
Ideas From Geology The writings of the geologist
Charles Lyell had a particularly strong influence on Darwin.
Lyell proposed that gradual and observable geologic
processes such as erosion could explain the physical features
of today's Earth. For example, the gradual erosion of a riverbed over thousands or millions of years can result
in a deep, river-carved canyon. A mighty mountain range can be thrust up centimeter by centimeter by
earthquakes occurring over millions of years. All that was required for an understanding of these changes was
an Earth far older than previously thought.
Darwin personally experienced an earthquake while doing field studies in the Andes Mountains of Chile. In a
harbor, he observed a block of land that had been underwater move upward above the water level as a result
of the quake. He also collected fossils of ocean organisms high in the Andes. Applying Lyell's ideas, Darwin
reasoned that earthquakes gradually lifted the rock bearing those marine fossils from the sea floor.
The geologic evidence presented by Lyell and others pointed to two conclusions. First, the slow processes of
mountain building and erosion suggested an Earth that must be very old. Second, these slow and gradual
processes occurring over vast spans of time could cause enormous change on Earth. Darwin would eventually
apply this idea of gradual change to the evolution of Earth's life forms.
Ideas From Population Dynamics In 1838, as Darwin continued to think about the question of how
species change, he read an essay on human populations written a few decades earlier by Thomas Malthus.
Malthus contended that much of human suffering, such as disease, famine, and homelessness, was due to the
human population's potential to grow. That is, populations can grow much faster than the rate at which
supplies of food and other resources can be produced. Darwin recognized that Malthus's ideas applied to all
species. The production of more individuals than the environment can support leads to a struggle for existence.
In most cases, only a small percentage of offspring will survive in each generation. Many eggs are laid, many
young are born, and many seeds are spread, but only a tiny fraction complete their development and leave
offspring of their own. The rest are starved, eaten, frozen, diseased, unmated, or unable to reproduce for
other reasons.
Ideas From Observed Variation Darwin's also observed the variation among the individuals of a
population. Variation refers to differences among members of the same species. Much of this variation is
heritable and passes from generation to generation.
Ideas From Artificial Selection In addition, Darwin looked at the results of artificial selection. Artificial
selection is the selective breeding of domesticated plants and animals to produce offspring with genetic traits
that humans value. For instance, a plant breeder might seek to improve traits such as grain production, disease
resistance, or protein content. An animal breeder might select for
growth rate or temperament. Darwin observed that breeders selected
individuals with the desired traits as breeding stock. Breeders play
the role of the environment, allowing only those plants or animals
with desired traits to reproduce. In fact, humans have been
modifying species for thousands of years. You can see evidence of
Darwin's point in the enormous diversity that dog breeders have
produced within this single species in just the last 500 years.
Darwin observed that artificial selection could produce a great deal of change in a species in a short time. He
reasoned that over thousands of generations, natural selection could also cause major change. Of course,
there are important differences between artificial and natural selection. The traits that become more common
in a population through artificial selection are those that humans choose. In contrast, natural selection favors
traits that benefit the organisms in their particular environment—environmental conditions do the "selective
breeding." The result is the evolutionary adaptation to the environment.
Darwin Publishes His Theory
As Darwin contemplated a mechanism for evolutionary change, he began to construct a scientific theory built
on observations, inferences, and ideas from his own work and the work of others.
From his observations Darwin developed his theory of natural selection. In 1844, Darwin wrote a 200-page
essay that outlined his idea, but he didn't release it to the public. Instead, for the next several years he
continued to accumulate more evidence to support his idea. He told only a few of his closest colleagues, who
encouraged him to publish his work before someone else came to the same conclusions. In 1858, another
British naturalist, Alfred Wallace, did come to the same conclusion. Darwin was shocked to receive a letter from
Wallace that described the same basic mechanism for evolutionary change that Darwin had proposed. Within a
month, some of Wallace's and Darwin's writings were jointly presented in public. Darwin published his book
The Origin of Species about a year later.
Darwin had two main goals in his book. First, he wanted to present the large amount of evidence that
evolution occurs; and second, he wanted to explain the variety and distribution of organisms on Earth in terms
of natural processes that are observable every day.
Darwin made two main points in his book. First, he argued from evidence that the species of organisms living
on Earth today descended from ancestral species. In other words, life has a history of change. Darwin
proposed that the descendants of the earliest organisms spread into various habitats over millions of years. In
these habitats, they accumulated different modifications, or adaptations, to diverse ways of life. Darwin called
this process descent with modification. He saw descent with modification as a way to account for the diversity
of life.
Darwin's second main point was his argument for natural selection as the mechanism for evolution. Natural
selection is the process by which individuals with inherited characteristics well-suited to the environment leave
more offspring on average than do other individuals. In other words, the individuals that function best tend to
leave the most offspring. When this process repeats over many generations, each new generation has a higher
proportion of individuals with the advantageous traits. This process can cause a population to change over
time.