Darwin's Theory of Evolution
Scientists at the beginning of the 1800s suspected that some kind
of evolution had given rise to living things around them. However,
they had no unifying theory to explain how evolution might have
occurred. Two scientists led the way in the search for a mechanism
of evolution. The first was Jean Lamarck. The second was one of
the greatest figures in biology, Charles Darwin.
Evolutionary Theory Before Darwin
The first systematic presentation of evolution was put forth by the
French scientist Jean Baptiste de Lamarck (1774-1829) in 1809.
Lamarck described a mechanism by which he believed evolution
could occur. This mechanism was known as "the
inheritance of acquired characteristics."
Assume that there were salamanders living in
some grasslands. Suppose, Lamarck argued, that these
salamanders had a hard time walking because their short legs
couldn't trample the tall grasses or reach the ground. Suppose that
these salamanders began to slither on their bellies to move from
place to place. Because they didn't use their legs, the leg muscles
wasted away from disuse and the legs thus became small.
Lamarck's theory said that the salamanders passed this acquired
trait to their offspring. In time the salamander's legs were used so
rarely that they disappeared. Thus, Lamarck argued, legless
salamanders evolved from salamanders by inheriting the acquired
characteristic of having no legs. Lamarck presented no
experimental evidence or observation and his theory fell out of
scientific favor. The next significant idea came from the British
scientist Charles Darwin.
Darwin's Background
Charles Darwin (1809-1882), like many people of genius,
did not at first appear to have extraordinary talents. From
a young age Darwin disliked school and preferred
observing birds and collecting insects to study. He was sent to
medical school in Scotland when he was 16. Young Darwin found
medicine "intolerably dull." He was much more interested in
attending natural history lectures. Seeing that Darwin lacked
enthusiasm for becoming a doctor, his father suggested he study
for the clergy. Darwin was agreeable to the idea and enrolled in the
university at Cambridge, England, in 1827. Here again, Darwin
admitted, "My time was wasted, as far as the academic studies
were concerned." However, Darwin found that his friendship with
John S. Henslow, professor of botany, made life in Cambridge
extremely worthwhile. Through long talks with Henslow, Darwin's
knowledge of the natural world increased. Henslow encouraged
Darwin in his studies of natural history. In 1831 Henslow
recommended that Darwin be chosen for the position of naturalist
on the ship the HMS Beagle.
The Voyage of the Beagle
The Beagle was chartered for a five-year mapping and
collecting expedition to South America and the South
Pacific. Darwin's job as ship naturalist was to collect specimens,
make observations, and keep careful records of anything he
observed that he thought significant. At the beginning of the
voyage Darwin read a geology book given to him by Henslow.
This book, Principles of Geology by Charles Lyell, spurred his
interest in the study of land forms. In Chile Darwin observed the
results of an earthquake: the land had been lifted by several feet. In
the Andes he observed fossil shells of marine organisms in rock
beds at about 4,300 m. He came to agree with Lyell that over
millions of years earthquakes and other geologic processes could
change the geology of the land. Because the land changed, new
habitats would form. Darwin realized that animals would have to
adapt to these changes. During the Beagle's five-year trip the
captain often dropped Darwin off at one port and picked him up
months later at another. One reason that Darwin was so eager to
study life on land was that he suffered from terrible seasickness
and couldn't wait to get off the Beagle. During his time on land
Darwin trekked hundreds of miles through unmapped region. He
observed thousands of species of organisms and collected many
different types of fossils. On the long sea voyages he used his time
to catalog his specimens and write his notes.
Darwin in England
When Darwin returned to England in October 1836, the scientific
community praised his collections from the voyage. Darwin sent
many specimens to experts for study. A bird specialist, or
ornithologist, studied Darwin's bird collections from the Galapagos
Islands, located about 1,000 km west of South America. He
reported that Darwin had collected 13 similar but separate species
of finches. Each finch species had a distinctive bill specialized for
a particular food source. Other experts studied Darwin's fossils and
classified them as remains of extinct mammals. The fossils
included rodents the size of hippopotamuses. The similarities of
the Galapagos finches led Darwin to infer that the finches shared a
common ancestor. The similarities between the fossil mammals
Darwin collected and modern mammals led him to believe that
species change over time.
In 1837 Darwin began his first notebook on evolution. For several
years Darwin filled his notebooks with facts that could be used to
support the theory of evolution. He found evidence from his study
of the fossil record: he observed that fossils of similar relative ages
are more closely related than those of widely different relative
ages. Comparing homologous structures, vestigial organs, and
embryological development of living species gave him additional
evidence of evolution. He consulted animal and plant breeders
about changes in domestic species. He ran his own breeding and on
seed dispersal experiments.
Evolution by Natural Selection
The central question still remained: if evolution occurred, by what
means did it occur? In 1838 Darwin read a book called Essay on
the Principle of Population by a British economist, Thomas
Malthus (1776-1834). Malthus stated that a human population
growing unchecked would double every 25 years. Resources such
as food, air and water cannot increase at the same rate, Malthus
argued. Thus human beings are involved in an intense "struggle for
existence," competing for the limited resources. This idea helped
Darwin uncover the mechanism he needed.
Combining the idea of competition with his other observations,
Darwin explained how evolution could occur. First, he stated that
variation exists among individuals of a species. Second, he stated
that scarcity of resources in a burgeoning population would lead to
competition between individuals of the same species because all
use the same limited resources. Such competition would lead to the
death of some individuals, while others would survive. From this
reasoning Darwin concluded that individuals having advantageous
variations are more likely to survive and reproduce than those
without the advantageous variations.
Darwin coined the term natural selection to describe the process by
which organisms with favorable variations survive and reproduce
at a higher rate. An inherited variation that increases an organism's
chance of survival in a particular environment is called an
adaptation. Over many generations, an adaptation could spread
throughout the entire species. In this way, Darwin believed
evolution by natural selection occured.
As an example Darwin noted that the ptarmigan turns white in
winter. This color change, he inferred, helped protect it from
predators, which would have a hard time spotting the bird in snow.
Ptarmigans that didn't change color in winter would be spotted
easily and eaten. In this way, Darwin implied, ptarmigans that
turned white in winter would be more likely to survive, reproduce,
and pass this adaptation on.
The Origin of Species
Darwin compiled evidence for evolution by natural selection for
about 20 years. Between 1842 and 1844 he wrote a 230-page essay
summarizing his theory and the evidence for it. In the 1850s he
began working on a detailed, multivolume book to present his
theory to the scientific community. Darwin might never have
completed the book if another British scientist, Alfred Russel
Wallace (1823-1913), had not come up with the same idea in 1858.
While living in the Malay Archipelago in the Pacific Ocean,
Wallace formulated his theory and wrote it in an essay, which he
sent to Darwin. Darwin's fellow scientists persuaded him to let
them present his theory and Wallace's essay jointly at a scientific
meeting. The presentation excited very little attention, according to
the modest Darwin.
However, the publication of Darwin's book The Origin of Species
in 1859 changed biology forever. The first printing of the book
sold out in one day. Darwin clearly and logically presented the idea
that natural selection is the mechanism of evolution. In Darwin's
own lifetime many scientists became convinced that evolution
occurs by means of natural selection. Today this theory is the
unifying one for all biology.
Darwin's Theory of Evolution
Scientists at the beginning of the 1800s suspected that some kind of evolution had given rise to living things around them. However,
they had no unifying theory to explain how evolution might have occurred. Two scientists led the way in the search for a mechanism
of evolution. The first was Jean Lamarck. The second was one of the greatest figures in biology, Charles Darwin.
Evolutionary Theory Before Darwin
The first systematic presentation of _______________ was put forth by the French scientist Jean Baptiste de Lamarck (1774-1829) in
1809. Lamarck described a mechanism by which he believed evolution could occur. This mechanism was known
as "the inheritance of __________________________________."
Assume that there were salamanders living in some ____________________. Suppose, Lamarck argued, that
these salamanders had a hard time walking because their ______________couldn't trample the tall grasses or
reach the ground. Suppose that these salamanders began to slither on their bellies to move from place to place.
Because they didn't use their legs, the leg muscles _______________________ from disuse and the legs thus
became small. Lamarck's theory said that the salamanders passed this ________________________ to their offspring. In time the
salamander's legs were used so rarely that they _____________. Thus, Lamarck argued, legless salamanders evolved from
salamanders by inheriting the acquired characteristic of having no legs. Lamarck presented no experimental evidence or observation
and his theory fell out of scientific favor. The next significant idea came from the ____________________________Charles Darwin.
Darwin's Background
Charles Darwin (1809-1882), like many people of genius, did not at first appear to have extraordinary talents. From
a young age Darwin _______________ school and preferred observing birds and ____________ insects to study.
He was sent to medical school in _________when he was 16. Young Darwin found medicine "intolerably dull." He
was much more interested in attending ______________________ lectures. Seeing that Darwin lacked enthusiasm
for becoming a doctor, his father suggested he study for the clergy. Darwin was agreeable to the idea and enrolled
in the university at Cambridge, England, in 1827. Here again, Darwin admitted, "My time was wasted, as far as the academic studies
were concerned." However, Darwin found that his friendship with John S. Henslow, professor of botany, made life in Cambridge
extremely worthwhile. Through long talks with Henslow, Darwin's knowledge of the natural world ______________. Henslow
encouraged Darwin in his studies of natural history. In 1831 Henslow recommended that Darwin be chosen for the position of
naturalist on the ship the ___________________________.
The Voyage of the Beagle
The Beagle was chartered for a five-year mapping and collecting expedition to South America and the South Pacific.
Darwin's job as ship naturalist was to collect specimens, make observations, and keep careful records of anything he
observed that he thought significant. At the beginning of the voyage Darwin read a geology book given to him by
Henslow. This book, Principles of _________________ by Charles Lyell, spurred his interest in the study of land
forms. In Chile Darwin observed the results of an earthquake: the land had been lifted by several feet. In the Andes he observed fossil
shells of marine organisms in rock beds at about 4,300 m. He came to agree with Lyell that over millions of years earthquakes and
other geologic processes could change the ___________ of the land. Because the land changed, new habitats would form. Darwin
realized that animals would have to adapt to these changes. During the Beagle's five-year trip the ___________ often dropped Darwin
off at one port and picked him up months later at another. One reason that Darwin was so eager to study life on land was that he
suffered from ______________ seasickness and couldn't wait to get off the __________. During his time on land Darwin trekked
___________________________ through unmapped region. He observed thousands of species of organisms and collected many
different types of _________________. On the long sea ___________ he used his time to catalog his specimens and write his notes.
Darwin in England
When Darwin returned to England in October 1836, the scientific community ______________ his collections from the voyage.
Darwin sent many specimens to experts for _________. A bird specialist, or ______________________, studied Darwin's bird
collections from the ___________________________, located about 1,000 km west of South America. He reported that Darwin had
collected 13 similar but separate species of ____________. Each finch species had a distinctive bill specialized for a particular food
source. Other experts studied Darwin's fossils and classified them as remains of ________________ mammals. The fossils included
rodents the size of __________________________. The similarities of the Galapagos finches led Darwin to infer that the finches
shared a common ______________. The similarities between the fossil mammals Darwin collected and modern mammals led him to
believe that species _____________________________________.
In 1837 Darwin began his first notebook on evolution. For several years Darwin filled his _______________________ with facts that
could be used to support the theory of evolution. He found ________________ from his study of the fossil record: he observed that
fossils of ________________ relative ages are more closely related than those of widely different relative ages. Comparing
___________________ structures, vestigial organs, and embryological development of living species gave him additional evidence of
evolution. He consulted animal and plant breeders about changes in domestic species. He ran his own breeding and on seed dispersal
experiments.
Evolution by Natural Selection
The central question still remained: if evolution occurred, by what means did it occur? In 1838 Darwin read a book called Essay on the
Principle of Population by a British economist, Thomas Malthus (1776-1834). Malthus stated that a human population growing
unchecked would ______________ every 25 years. Resources such as food, air and water cannot __________________ at the same
rate, Malthus argued. Thus human beings are involved in an intense "struggle for existence," competing for the limited resources. This
idea helped Darwin uncover the _____________________ he needed.
Combining the idea of competition with his other _______________________, Darwin explained how evolution could occur. First, he
stated that variation exists among individuals of a species. Second, he stated that _______________ of resources in a burgeoning
population would lead to competition between individuals of the same _______________ because all use the same limited resources.
Such ___________________________ would lead to the death of some individuals, while others would survive. From this reasoning
Darwin concluded that individuals having _________________________________ are more likely to survive and reproduce than
those without the advantageous variations.
Darwin coined the term ____________________________ to describe the process by which organisms with favorable variations
survive and __________________________________________. An inherited variation that increases an organism's chance of
survival in a ____________________________ is called an adaptation. Over many generations, an adaptation could spread
throughout the entire species. In this way, Darwin believed evolution by natural selection occured.
As an example Darwin noted that the ptarmigan turns white in winter. This color change, he inferred, helped
__________________________________________, which would have a hard time spo-tting the bird in snow. Ptarmigans that didn't
change color in winter would be spotted easily and eaten. In this way, Darwin implied, ptarmigans that turned white in winter would
be more likely to survive, reproduce, and pass this _____________________________ on.
The Origin of Species
Darwin compiled evidence for evolution by natural selection for about 20 years. Between 1842 and 1844 he wrote a 230-page essay
summarizing his theory and the evidence for it. In the 1850s he began working on a detailed, multivolume book to present his theory
to the scientific community. Darwin might never have completed the book if another British scientist, Alfred Russel Wallace (18231913), had not come up with the same idea in 1858. While living in the Malay Archipelago in the Pacific Ocean, Wallace formulated
his theory and wrote it in an essay, which he sent to Darwin. Darwin's fellow scientists persuaded him to let them present his theory
and Wallace's essay jointly at a scientific meeting. The presentation excited very little attention, according to the modest Darwin.
However, the publication of Darwin's book The Origin of Species in 1859 changed biology forever. The first printing of the book sold
out in one day. Darwin clearly and logically presented the idea that natural selection is the mechanism of evolution. In Darwin's own
lifetime many scientists became convinced that evolution occurs by means of natural selection. Today this theory is the unifying one
for all biology.
http://bioweb.cs.earlham.edu/9-12/evolution/HTML/natural.html