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Principles of Evolution
• How Did Evolutionary Thought Evolve?
– Early biological thought did not include the concept of
– Exploration of new lands revealed a staggering diversity of
– Fossil discoveries showed that life had changed over
– Some scientists devised non-evolutionary explanations for
– A few scientists speculated that life had evolved with time.
– Geology provided evidence that Earth is exceedingly old.
– Some pre-Darwin biologists proposed mechanisms for
– Darwin and Wallace proposed a mechanism of evolution.
Fossils of extinct organisms
Fossils provide strong support for the idea that today's organisms were not
created all at once, but arose over time by the process of evolution. If all
species were created simultaneously, we would not expect (a) trilobites to
be found in older rock layers than (b) seed ferns, which in turn would not be
found deeper than (c) dinosaurs, such as Allosaurus. Trilobites became
extinct about 230 million years ago, seed ferns about 150 million years ago,
and dinosaurs 65 million years ago.
Principles of Evolution
• Darwin studied a group of closely related
species of finches on the Galapagos Islands.
• Each species specializes in eating a different
type of food and has a beak of characteristic
size and shape, because natural selection has
favored the individuals best suited to exploit
each food source efficiently.
• Aside from the differences in their beaks, the
finches are quite similar.
Large ground finch, beak suited to large seeds
Small ground finch, beak suited to small seeds
Warbler finch, beak suited to insects
Vegetarian tree finch, beak suited to leaves
How Does Natural Selection Work?
• Modern Genetics confirmed Darwin’s assumption of
• Natural Selection modifies populations over time.
• Fossils provide evidence of evolutionary change over
• Comparative Anatomy gives evidence of Descent with
• Homologous structures provide evidence of Common
• Functionless structures are inherited from ancestors.
• Many organisms have vestigial structures that serve no
apparent function. The (a) salamander, (b) whale, and
(c) snake all inherited hindlimb bones from a common
ancestor; the bones remain functional in the salamander
but are vestigial in the whale and snake.
How Do We Know That Evolution Has
Some anatomical similarities result from
evolution in similar environments
Embryological similarity suggests common
Modern biochemical and genetic analyses
reveal relatedness among diverse organisms.
• Convergent evolution can produce outwardly similar
structures that differ anatomically. The wings of (a)
insects and (b) birds and the sleek, streamlined shapes
of (c) seals and (d) penguins are examples of such
analogous structures.
• The early embryonic stages of a (a) lemur, (b) pig, and
(c) human show strikingly similar anatomical features.
What Is the Evidence That Populations Evolve
by Natural Selection?
• Controlled Breeding Modifies Organisms
• Evolution by Natural Selection Occurs Today
– When fewer predators are present, brighter coloration
can evolve.
– Natural Selection can lead to pesticide resistance.
– Experiments can demonstrate Natural Selection.
– Selection acts on random variation to favor the traits
that work best in particular environments
Dog diversity illustrates artificial selection
• A comparison of (a) the ancestral dog (the gray wolf, Canis lupus)
and (b) various breeds of modern dogs. Artificial selection by
humans has caused a great divergence in size and shape of dogs in
only a few thousand years.
How Are Populations, Genes, and
Evolution Related?
• Genes and the environment interact to
determine traits.
• The gene pool is the sum of the genes in a
• Evolution is the change over time of allele
frequencies within a population.
• The equilibrium population is a hypothetical
population that does not evolve.
What Causes Evolution?
• Mutations are the source of Genetic Variability
– Mutations are rare, but important
– Mutations are not Goal-Directed
• Mutations occur spontaneously
• Gene flow between populations changes Allele
• Allele frequencies may drift in small populations.
– Population size matters
– A population bottleneck Is an example of Genetic Drift
– Population bottlenecks reduce variation
The Effect of Population Size on Genetic Drift
• Each colored line represents one computer simulation of
the change over time in the frequency of allele A in a (a)
large or (b) small population in which two alleles, A and
a, were initially present in equal proportions, and in
which randomly chosen individuals reproduced.
Population Bottlenecks Reduce Variation
• (a) A population bottleneck may drastically reduce genetic and
phenotypic variation because the few organisms that survive
may carry similar sets of alleles. Both (b) the northern
elephant seal and (c) the cheetah passed through population
bottlenecks in the recent past, resulting in an almost total loss
of genetic diversity.
What Causes Evolution?
• Isolated founding populations may produce
• Mating within a population Is almost never
• All genotypes are not equally beneficial
– Antibiotic resistance evolves by natural selection
– Penicillin resistance illustrates key points about
A Human Example of the Founder Effect
• An Amish woman with her child, who suffers from a set of genetic
defects known as Ellis–van Creveld syndrome (short arms and legs,
extra fingers, and, in some cases, heart defects). The founder effect
accounts for the prevalence of Ellis–van Creveld syndrome among
the Amish residents of Lancaster County, Pennsylvania.
A Compromise Between Opposing Pressures
• (a) A male giraffe with a long neck is at a definite advantage in combat
to establish dominance.
• (b) But a giraffe's long neck forces it to assume an extremely awkward
and vulnerable position when drinking. Thus, drinking and male–male
contests place opposing evolutionary pressures on neck length.
How Does Natural Selection Work?
• Natural selection stems from unequal reproduction
• Natural selection acts on phenotypes
• Some phenotypes reproduce more successfully than
An environment has non-living and living components
Competition acts as an agent of selection
Both predator and prey act as agents of selection
Sexual selection favors traits that help an organism mate
• Selection can influence populations in three ways
– Directional selection shifts character traits in a specific direction
– Stabilizing selection acts against individuals who deviate too far
from the average
– Disruptive selection adapts individuals within a population to
different habitats.
Three Ways that Selection Affects a Population
Over Time
• A graphical illustration of three ways natural and/or sexual
selection, acting on a normal distribution of phenotypes, can
affect a population over time. In all graphs, the beige areas
represent individuals that are selected against—that is, do not
reproduce as successfully as do the individuals in the purple