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Evolution
2.1 Darwin's Voyage on the HMS Beagle
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Darwin's voyage on the HMS Beagle allowed him the opportunity to study a large
variety of animal and plant species across the globe. Years after his voyage, he
published the book On the Origin of Species, in which he proposed natural
selection as the mechanism that underlies the process of evolution (figure 2.3)
(134.0K) .
The theory of evolution through natural selection is overwhelmingly accepted by
scientists but is viewed as controversial by some in the general public.
2.2 Darwin's Evidence
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Darwin observed fossils in South America of extinct species that resembled living
species. On the Galápagos Islands, Darwin observed finches that differed slightly
in appearance between islands but resembled finches found on the South
American mainland. These observations laid the groundwork for his proposal of
evolution through natural selection (figure 2.4) (153.0K) and (figure 2.5)
(101.0K) .
2.3 The Theory of Natural Selection
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Key to Darwin's hypothesis of evolution by natural selection was the observation
by Malthus that the food supply limits population growth. A population, be it
plants, animals, etc., grows only as large as that which can live off of the available
food (figure 2.6) (83.0K) .
Using Malthus's observations and his own observations, Darwin proposed that
individuals that are better suited to their environments survive to produce
offspring, gaining the opportunity to pass their characteristics on to future
generations. Therefore, future generations may be different from ancestral
populations, a process known as evolution.
Spurred by a similar proposal put forth years later by Alfred Wallace, Darwin
published his book in 1859.
Darwin's Finches: Evolution in Action
2.4 The Beaks of Darwin's Finches
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By observing the different sizes and shapes of beaks in the closely related finches
of the Galápagos Islands and correlating the beaks with the types of food
consumed by the different birds, Darwin concluded that the birds' beaks were
modified from an ancestral species based on the food available, each suited to its
food supply (figure 2.10) (116.0K) .
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Research has since supported Darwin's hypothesis that natural selection
influences beak size in island finches based on the available food supply
2.11) (68.0K) .
(figure
2.5 How Natural Selection Produces Diversity
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The 14 species of finches found on the islands off the coast of South America
evolved from a mainland species that adapted to different niches, a process called
adaptive radiation (figure 2.12) (60.0K) .
Ecology
2.6 What Is Ecology?
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Ecology is the study of how organisms interact with each other and with their
physical environment. There are five levels of ecological organization:
populations, communities, ecosystems, biomes, and the biosphere.
2.7 A Closer Look at Ecosystems
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An ecosystem is a physical environment that contains a community of various
organisms. These organisms interact with each other and with their physical
environment, extracting energy and raw materials from it. Similar ecosystems
found throughout the world are called biomes (figure 2.14) (106.0K) .
2.8 How Species Evolve to Occupy Different Niches Within an Ecosystem
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Resource partitioning is the process whereby two competitive species coexist by
utilizing different portions of the habitat or different resources such as food. The
varying beak sizes seen in Darwin's finches suggest that the bird species evolved
in part due to resource partitioning (figure 2.15) (109.0K) and (figure 2.16)
(118.0K) .
Populations and How They Grow
2.9 Patterns of Population Growth
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Populations grow exponentially until limited by environmental conditions. A
population will stabilize at a size, called its carrying capacity, that can be
supported by its environment (figure 2.18) (75.0K) .
Organisms exposed to an ever-changing environment tend to reproduce and grow
rapidly, exhibiting an r-selected life history. Organisms exposed to more stable
environmental conditions tend to reproduce and grow more slowly, exhibiting a kselected life history.
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2.10 Human Populations
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Technology has allowed the human population to grow exponentially for the last
300 years to a current population of over 6 billion (figure 2.21) (71.0K) .
Human populations grow at different rates, with underdeveloped countries'
populations growing more rapidly than developed countries' populations.
However, it takes more resources to support populations in developed countries
versus those in underdeveloped countries (figure 2.22) (94.0K) and (figure
2.23) (83.0K) .
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