Download Chapter 15 - Holden R

The Theory of Evolution

Darwin developed the first theory on
evolution, which is the basis for modern
evolutionary theory
◦ Darwin spent 5 years sailing around the world
collecting organisms and fossils
◦ His idea that species can change over time is based
on his observations on the Galapagos Islands where
many species were similar to species in other parts
of the world, yet still unique

Darwin bred pigeons and studied how traits
were inherited from one generation to the
next
◦ He was able to breed for specific characteristics
using artificial selection
◦ He then hypothesized that desirable traits were
bred into the population through natural selection
 The organisms without the desirable traits were less
likely to survive and pass on their traits
 As a result, only the desirable traits got passed on

An adaptation is any variation that aid’s an
organisms chances of survival in its environment
◦ Structural adaptations include any change to the physical
appearance or body parts of an organism
 These develop very slowly (over millions of years)
 Examples include beak shape, neck length (giraffes),
camouflage and mimicry
 Mimicry is when one organism looks similar to another
organism
◦ Physiological adaptations are changes in the metabolic
processes of an organism
 They develop over fairly short time periods (a few
generations)
 Examples include antibiotic and pesticide resistance

Fossils are used to examine physical similarities
in organisms
◦ Homologous structures are similar in function,
arrangement or both between organisms with a common
evolutionary origin
 An example would be forelimbs of whales, crocodiles and
bird wings
◦ Analogous structures are similar in function between
organisms that do not have a common evolutionary
origin
 An example would be bird and bat wings
◦ Vestigial structures are those which probably used to
serve a purpose but are no longer needed, yet still
inherited
 In humans, your appendix is a vestigial structure
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An embryo is the earliest stage of growth and
development for both plants and animals
◦ Even though they develop into very different
organisms, the embryos of fish, reptiles, birds and
mammals are very similar, indicating that they may
have had a common ancestor

Scientists often compare amino acid or DNA
sequences of organisms
◦ Organisms that have more in DNA or amino acid
sequences in common are said to be more closely
related in terms of biochemistry

Genetic equilibrium is a state where the
genes in a population are the same over
several generations
◦ The combination of all the alleles of a population’s
genes is called the “gene pool”
 The percentage of any specific allele in the gene pool
is called the allelic frequency
 Any factor that changes the allelic frequency results in
the process of evolution
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There are several factors that can cause
changes within a population, some of which
impact small, isolated populations such as
islands, and others than impact larger
populations
◦ Mutations, genetic drift and gene flow can have a
big impact on small populations
 Genetic drift is the alteration of allelic frequencies by
chance events
 Gene flow is the movement of genes in and out of a
population through migration

There are 3 types of natural selection:
◦ Stabilizing selection is natural selection that favors
average individuals in a population
◦ Directional selection is natural selection that favors
one extreme of a trait
◦ Disruptive selection is natural selection that favors
both extremes of a trait
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Speciation occurs when new species are
produced because members of similar
populations can no longer breed to produce
fertile offspring
◦ Geographic isolation occurs when a physical
barrier divides a population and can lead to a new
species
◦ Reproductive isolation occurs when populations
who used to interbreed no longer mate and
produce fertile offspring
 This can be because of genetic changes, such as
polyploidy, or behavioral changes

Depending on the mechanism involved,
speciation can occur quickly or gradually
◦ Gradualism is the idea that new species originate
through a gradual change of adaptations
◦ Punctuated Equilibrium is the idea that speciation
occurs relatively quickly, in short rapid bursts with
long periods of genetic equilibrium in between
 Both mechanisms are supported by the fossil record
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There are 2 main patterns of evolution:
◦ Convergent evolution occurs when unrelated
species occupy similar environments in different
parts of the world and develop similar traits due
to natural selection
◦ Divergent evolution occurs when populations that
were once similar become increasingly distinct as
they adapt to different environments
 Adaptive radiation is an example of divergent evolution
 In adaptive radiation, one ancestral species evolves into
many separate species to occupy different habitats