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CP biology - chapter 10 Evolution
Early ideas about evolution
Early scientists proposed ideas bout evolution
Evolution is the process of biological change by which descendants come to differ
from their ancestors. Much of today’s understanding of evolution is based on
Charles Darwin’s work in the 1800s. But Darwin did not come up with the idea of
evolution himself. Many other scientists contributed important ideas to the study of
evolution. Several historical ideas – about species, geology, and the mechanisms of
evolution – are described below. Like all science, the modern understanding of
biological evolution builds on hundreds of years of study and research.
Species
In the 1700s, a botanist named Carolus Linneaus came up with a system to organize
and name all of the different types of organisms, or species. A species is a group of
organisms that can reproduce and have fertile offspring. Linneaus’ classificiation
system grouped organisms according to physical similarities. His system also shows
evolutionary relationships, and is still in use today.
In Linneaus’ time, there was a common belief that the organisms that lived on earth
were fixed, or that species did not change. Linneaus proposed that species could
change. For example, he observed in experiments that two different plant species
could cross, and make a new type of plant.
In the 1700s, it was also a common belief that Earth was only 6000 years old. But
several geologists began to challenge this idea. The geologists believed that they
had evidence that Earth was much older. The evidence of the old age of Earth was
important to Darwin’s development of his idea.
Mechanisms of evolution
Many scientists in the 1700s did not believe that species could go extinct. But many
scientists thought species could change, or evolve,. There were many different
ideas, however, about the mechanism of evolution, or how evolution happens. For
example, different scientists had different ideas about how environmental changes
affect evolution, how changes get passed on to offspring, and what causes biological
variation.
Theories of geologic change set the stage for Darwin’s theory
The study of fossils led some scientists to conclude that species do go extinct.
Fossils are traces of organisms that existed in the past. The locations of fossils in
different rock layers provide clues about Earth’s past. But how did those rock layers
form? Geologists held different ideas about geologic change.
 Catastrophism is the idea that past natural disasters – like floods and
volcanic eruptions – shaped landforms, and caused species to become extinct
in the process.
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Gradualism is the idea that landforms were shaped by very slow changes
over a long period of time, and not by natural disasters.
Uniformitarianism is the idea that the same processes that shaped
landforms in the past also shape landforms today. In other words, the
geologic processes that shape Earth are uniform, or the same, through time.
Uniformitarianism became the favored theory of geologic change and played
an important role in the development of Darwin’s theory.
10.2 Darwin’s observations
Darwin observed differences among island species
Darwin spent over 20 years researching biological evolution. He made important
observations during his travels on a ship that sailed the coast of South America and
the Pacific Islands. Darwin was amazed by the variation of traits among similar
species that he saw in his travels. The word variation has many common uses, but
in biology variation means the difference in physical traits among individuals in a
group of organisms.
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Darwin noticed that there was variation between species on different islands. This
was especially noticeable in the Galapagos Islands, a chain of islands off the coast of
Ecuador in South America. For example, he noticed that birds called finches had
different kinds of beaks in areas with different food sources. He found finches with
thick beaks in areas with large hard-shelled nuts, and finches with smaller beaks in
areas where fruits were available.
These differences seemed to match the environment and the diet of the finches.
Darwin realized that species may somehow be able to adapt to their surroundings.
An adaptation is a feature that allows an organism to better survive in its
environment.
Darwin observed fossil and geologic evidence supporting an ancient Earth
During his travels, Darwin found that some fossils looked similar to living species. T
his suggested that the living species might have some relationship to the fossil
forms. In order for such changes to occur, Darwin figured that Earth must be much
older than 2000 years.
Darwin found much evidence supporting the ideas of uniformitarianism. For
example, he found fossil shells of marine organisms high up in the Andes mountains.
Later he experienced an earthquake and saw land that had been underwater get
shifted up above the sea level. He put together observations like these, and
concluded that daily geologic processes can add up to much bigger changes over a
long period of time.
10.3 Theory of Natural Selection
Several key insights led to Darwin’s idea for natural selection
The variation of similar species among islands, fossil evidence, and geologic events
convinced Darwin that evolution occurs But he still wondered how evolution
occurs. Here, you will read about some of Darwin’s reasoning that led him to his
idea for natural selection.
Artificial selection Darwin noticed that plants and animals that are raised by
humans had variations in traits that were not seen in their wild relatives. Think of
all the different breeds of dogs that you have seen. In a process called artificial
selection, humans select individuals with the traits they desire, and then breed
them to produce more individuals with those traits.
Heritability In order for artificial selection to occur, traits must be heritable.
Heritable is the ability of a trait to be passed down from one generation to the next.
Things that are acquired in an organism’s life, like a broken bone, are not heritable.
Natural selection Darwin reasoned that a process similar to artificial selection
could happen in nature. In artificial selection, humans are the source of selection. In
natural selection, the environment is the source of selection.
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Natural selection is a process in which individuals that have inherited beneficial
adaptations produce more offspring than to other individuals.
Struggle for survival Darwin was influenced by the work of an economist named
Thomas Malthus. Malthus proposed that resources like food, water, and shelter
were limits to human population growth. Darwin reasoned that a similar struggle
happened in nature.
Darwin saw great variation within populations of organisms. A population is all the
individuals of a species that live in an area. He saw individuals with adaptations
that matched their environment. Darwin proposed that these adaptations arose
over many generations in a process he called “descent with modification.”
Natural selection explains how evolution can occur
Darwin was not the only scientist studying evolution during this time. Another
scientist named Alfred Wallace independently developed an explanation of how
evolution occurs. Wallace’s explanation was very similar to Darwin’s. In the late
1850’s, the idea of Darwin and Wallace were presented to the scientific community.
There are four main principles to the theory of natural selection: variation,
overproduction, adaptation, and descent with modification.
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Variation Individuals of a species differ due to genetic variation. Heritable
differences are the basis for natural selection.
Overproduction Organisms have more offspring than can survive. This
results in competition among offspring for resources.
Adaptation Some individuals have certain variations that allow them to
survive better than other individuals in their environment. These individuals
are “naturally selected” to live longer and produce more offspring that also
have those adaptations.
Descent with modification Over time, natural selection will result in
species with adaptations that are beneficial for survival and reproduction in
a particular environment. More individuals will have the trait in every
following generation, as long as the environmental conditions stay the same.
Let’s apply these four principles to an example of natural selection.
About 11,000 years ago, jaguars faced a shortage of food due to a changing climate.
There were fewer mammals to eat, and jaguars had to eat reptiles to survive.
Variations in jaw and tooth size allowed some individuals to more easily eat shelled
reptiles.
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Natural selection acts on existing variation
Natural selection cannot make new alleles. It can only work with variation that
already exists. In other words, natural selection acts on phenotypes, or physical
traits, and not on genetic material itself.
Changing environments As an environment changes, different traits become
beneficial. Think about the jaguars. When mammals were their main food source,
small teeth and jaws were beneficial. But when the environment changed, larger
teeth and jaws became beneficial to better eat reptiles. Because the environment
constantly changes, a trait that is an advantage today may be a disadvantage in the
future.
Adaptations as compromises Adaptations can sometimes be thought of as
compromises. One example of an adaptive compromise is the panda bear’s ‘thumb”.
Modern pandas have five digits that are like your fingers and a sixth digit that
functions like a thumb. This digit is actually a wrist bone. Ancestral pandas with
bigger wrist bones had an advantage in holding and eating their food. Over time,
this adaptation became typical of the species.
10.4 Evidence of evolution
Evidence for evolution in Darwin’s time came from several sources
Genetic inheritance was not known in Darwin’s time. But Darwin supported his
ideas with evidence from many other sources – fossils, geography, embryology, and
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anatomy. His evidence was very strong, and left no doubt in the minds of scientists
that all organisms share a common ancestor. Today, the concept of evolution ties
together all fields of biology.
Fossils
Before Darwin, scientists studying fossils knew that organisms changed over time.
Fossilized organisms were different in different layers of rock. The bottom layers of
rock are the oldest, and contain fossils of more ancient organisms. The upper layers
of rock are the youngest, and contain fossils of more recent organism. Findings in
the fossil record support Darwin’s idea of descent with modification.
Geography During Darwin’s travels, he noticed that plants and animals on islands
looked similar to species on the mainland, but not exactly the same. He
hypothesized that organisms from the mainland had migrated to the islands.
Different islands had different food sources, climates, and predators. The different
environmental conditions favored different traits in these migrant organisms. For
example, the finches on the different Galapagos islands have different shapes of
beaks that evolved in response to the different island habitats. This is an important
part of biogeography, the study of the distribution of organisms around the world.
Embryology The study of embryos and their development. Darwin noticed that the
embryos of different species may look very similar, although the adult species look
very different. For example, fish, birds, reptiles, and mammals all have gill slits as
embryos. These slits develop into gills in fish, but develop into ears and throats in
mammals. The similar features of embryos in very different organisms suggest
evolution from a distant common ancestor.
Anatomy Some of Darwin’s best evidence came from comparing the body parts of
different species. He found that some organisms have body parts that are similar in
structure, but might be used differently.
 Homologous structures are features that are similar in structure, but
appear in different organisms and may have different functions. For
example, think of the front limbs of humans, bats, and moles. The front limb
of these organisms share similar bone structure, but each organism uses
these limbs differently. Homologous structure appear across man different
species, and offer strong evidence for common descent
 Analogous structures are structures that perform a similar function, but are
not similar in origin. For example, both birds and butterflies have wings.
They both use their wings to fly, but their wings do not share a common
origin. Bird wings have bones, and butterfly winds do not.
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analogous structures
vestigial structures
Just because two organisms share similar structures does not mean they are closely
related. Homologous structures show common ancestry, while analogous structures
do not.
Structural patterns are clues to the history of a species
Vestigial structures are small leftover organs or structures that had a function in
an early ancestor. For example, snakes and whales have tiny pelvic bones. Even
though neither organism walks, their ancestors were four-legged animals. These
pelvic bones are vestigial structures.
The appendix is an example of a vestigial structure in humans. The appendix is a
remnant of an organ that helped to digest certain plant material eaten by human
ancestors. The human appendix has lost the ability to digest this material, and
actually has no known function.
Vestigial structures do not become smaller in one individual organism. These
structures became smaller over many generations. Today, biologists consider
vestigial structures among the most important examples demonstrating how
evolution works.
1`0.5 Evolutionary biology today
Fossils provide a record of evolution
Paleontology is the study of fossils or extinct organisms. The fossil record is not
complete. One reason for this is because most organisms do not form fossils after
they die. Fossils form only in particular environmental conditions. However, fossil
evidence that does not support evolution has never been found.
Darwin wondered why he did not find fossils that showed transitions between
different groups of organisms. Since Darwin’s time, many of these transitional
fossils have been found. For example, fossils have been found of a transitional
species in the evolution of whales. This organism had a whalelike body, but still had
the limbs of land animals. Fossils continue to provide new information and
evidence for current ideas about evolution.
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whale evolution
Molecular and genetic evidence support fossil and anatomical evidence
The fields of genetics and molecular biology have added strong support to Darwin’s
theory of natural selection. All living tings have DNA, share the same genetic code,
and make most of the same proteins from the same 20 amino acids. Comparisons of
DNA and protein sequences can be used to show evolutionary relationships
between different organisms. The more related two organisms are, the more similar
the sequences will be. Because there are thousands of genes in even simple
organisms, DNA contains a huge amount of information on evolutionary history.
Some particular genes are found in many organisms – from fruit flies to humans –
and therefore give evidence of a very different common ancestor. For example,
homeobox genes, which control development, are found in a wide range of
organisms. These genes are even found in organisms that lived 600 million years
ago.
Evolution unites all fields of biology
Scientists continue to actively study evolution through natural selection. The theory
of natural selection combined with genetics is sometimes called the modern
synthesis of evolutionary theory.
New tools are providing more data than ever before. Modern tools from different
fields of study add to what has been discovered through fossil evidence. For
example, as shown below, the comparisons of milk protein genes support the fossil
evidence for the relationship between whales and hippopotamuses. New
discoveries are limited only by the time and resources of scientists.
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molecular evidence
The basic principles of evolution are used in all fields of science, including medicine,
geology, geography, chemistry, and ecology. For example, the idea of common
descent helps biologists understand where new diseases come from and how they
might be best treated.
As much as we know about life on Earth, there is so much more waiting to be
discovered. As the great geneticist Theodosius Dobzhansky once noted, “Nothing in
biology makes sense except in the light of evolution.”
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