CSET Practice Test Subtest II Science

Darwin's Theory of Evolution

Scientists at the beginning of the 1800s know of some
kinds of fossils, and they were very aware of homologous
and vestigial structures. Many scientists 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, his 
collections from the voyage were praised by the scientific 
community. 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 
experiments and also did experiments on seed dispersal. 

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, according to Darwin, 
evolution by natural selection would occur. 

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 to future generations. 

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.

Patterns of Evolution
Natural selection can ultimately lead to the formation of 
new species. Sometimes many species evolve from a 
single ancestral species. Similarities in skeletal and 
muscular structure of Hawaiian honeycreepers led 
scientists to conclude that the 23 species of 
honeycreepers evolved from one ancestral species. Such 
an evolutionary pattern, in which many related species 
evolved from a single ancestral species, is called adaptive 
radiation. Adaptive radiation most commonly occurs when 
a species of organisms successfully invades an isolated 
region where few competing species exist. If new habitats 
are available, new species will evolve. 

Divergent and Convergent Evolution
Adaptive radiation is one example of divergent evolution. 
Divergent evolution is the process of two or more related 
species becoming more and more dissimilar. The red fox 
and the kit fox provide and example of two species that 
have undergone divergent evolution. The red fox lives in 
mixed farmlands and forests, where its red color helps it 
blend in with surrounding trees. The kit fox lives on the 
plains and in the deserts, where its sandy color helps 
conceal it from prey and predators. The ears of the kit 
fox are larger than those of the red fox. The kit fox's large 
ears are an adaptation to its desert environment. The 
enlarged surface area of its ears helps the fox get rid of 
excess body heat. Similarities in structure indicate that the 
red fox and the kit fox had a common ancestor. As they 
adapted to different environments, the appearance of the 
two species diverged. 

In convergent evolution, on the other hand, unrelated 
species become more and more similar in appearance as 
they adapt to the same kind of environment. The two 
unrelated types of plants in the picture above have 
adapted to desert environments. Notice the resemblance 
of the cactus, which grows in the American desert, to the 
euphorbia, which grows in the African deserts. Both have 
fleshy stems armed with spines. These adaptations help 
the plants store water and ward off predators. 

Coevolution
Coevolution is the joint change of two or more species in 
close interaction. Predators and their prey sometimes 
coevolve; parasites and their hosts often coevolve; 
plant-eating animals and the plants upon which they feed 
also coevolve. One example of coevolution is between 
plants and the animals that pollinate them. 

In tropical regions bats visiting flowers to eat nectar. The 
fur on the bat's face and neck picks up pollen, which the 
bat transfers to the next flower it visits. Bats that feed at 
flowers have a slender muzzle and a long tongue with a 
brushed tip. These adaptations aid the bat in feeding. 
Flowers that have coevolved with bats are light in color. 
Therefore, bats, which are active at night, can easily 
locate them. The flowers also have a fruity odor attractive 
to bats. 

Divergent and convergent evolution and coevolution are 
different ways organisms adapt to the environment. These 
are examples of how the diversity of life on earth is due to 
the ever-changing interaction between a species and its 
environment.

Sources:
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