Chapter 15
Evolution: Evidence and Theory
Theories of Evolution
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Ideas of Darwin’s Time
o In the 18th century scientists thought that species were permanent and
unchanging
o They also believed the Earth was only thousands of years old
Biological evolution- the change of populations of organisms over generations
Lamarck’s Explanation
o 1st to propose species modification
o Similar species descended from a common ancestor
o Acquired traits were passed on to offspring. An acquired trait is initially not
genetic. The organism acquires it through necessity. (Not true)
Darwin’s Theory
o Proposed that species were modified by natural selection
o He was a naturalist on a ship called the Beagle. His job was to collect samples of
organisms and keep records
o He theorized that because environments change over millions of years organisms
that lived there changed just as slowly.
o Darwin was famous for his studies of finches, turtles, and iguanas on the
Galapagos Islands
Darwin’s Two Theories
o Descent with Modification (evolution)
 All species descended from one or a few original types of life
 Modern organisms had evolved from organisms that lived millions even
billions of years ago
o Modification by Natural selection
 States how evolution occurs
 Darwin observed that the environment limits the growth of populations
by increasing the rate of death or decreasing the rate of reproduction, or
both
 A population of organisms adapt to their environment as their proportion
of genes for favorable traits increases
 Fitness- a single organisms genetic contribution to the next generation
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An individual with high fitness is well adapted to its environment
and reproduces more successfully than a low fitness members
Darwin proposed the following forces could affect natural selection
o Overproduction- Organisms produce several offspring in a
lifetime because some will be lost to predation, disease, and other
factors. This ensures the stability of the species
o Genetic Variation- Within a population, individuals have different
traits. This ensures adaptability buy the population
o Struggle to Survive- Individuals must compete with each other
and other species for food, space, and a mate
o Differential reproduction- organisms with the best adaptations
are most likely to survive and reproduce. Populations will become
different as they adapt to different environments
Darwin stated that adaptations are those traits that increase the fitness
of individuals, and populations tend to be well adapted to survive the
conditions in which they live.
Evidence of Evolution
o Fossil Records
o Biogeography
 The study of the locations of organisms around the world
 Both Darwin and Alfred Russell Wallace saw evidence of evolution in the
distribution of organisms
 They observed animals that seemed closely related yet were adapted to
different environments in nearby regions
 They also observed animals that seemed unrelated but that had similar
adaptations to similar environments in regions that were far apart
 Example: There are native Australian animals that resemble
wolves, cats, mice, moles, or anteaters…however most Australian
mammals are marsupials, mammals that have pouches for
carrying their young
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o Anatomy and Embryology
 Homologous structures- anatomical structures in one species that,
compared to other anatomical structures in another species, originated
from a single ancestor of the two species
 Example: fore limb of humans, penguins, alligator, bat
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Analagous structures- anatomical structures in one species that is similar
in function and appearance, but not in evolutionary origin to another
anatomical structure in another species.
 Example: Insect wing, bird wing, bat wing
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Vestigial structures- features that serve no useful function
 Example: human tailbone, human appendix, snakes with pelvic
bones
o They allow us to see how an organism evolved over the
years and what other organism they could have been
related to
o Biological Molecules
 Biologists study DNA, RNA, proteins and other molecules to see how
closely related different organisms are.
 The more closely related they are the closer their DNA and other
biological molecules are
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Evolution in Action
o Coevolution- the mutual evolution of two different species interacting with each
other.
 Example: Insects and flowers help each other and evolve together
o Convergent evolution- the process by which unrelated species become more
similar as they adapt to the same kind of environment
 Example: Porpoise and shark; mammal and fish that have similar body
design and fins
o Divergent evolution- the process of two or more related species becoming more
and more dissimilar because of differing habitats. Can lead to new species
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It occurs when descendants of a single ancestor diversify into species that
each fit different parts of the environment
Adaptive radiation is when a new population in a new environment, such
as an island, will undergo divergent evolution until the population fills
many parts of the environment
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