Chapter 14
• Principles of
Evolution
Evolutionary Thought
• Evolution by natural selection is a unifying
theme for all of biology
• The foundation of evolutionary thought
developed gradually over centuries
• Main ideas of evolution were not widely
accepted until after Charles Darwin
published On the Origin of Species in 1859
Explanations for Diversity of Living
Organisms
• Early Biological Thought
– Heavily influenced by theology
– All organisms were created
simultaneously
– Each distinct life-form was
permanently fixed and did not change
over time
• Expressed by ancient Greek
philosophers
– Plato (427-347 B.C.)
– Aristotle (384-322 B.C.) arranged all
organisms on a linear scale of
increasing complexity (“ladder of
Nature”)
Exploration of New Lands
• Number of species was greater than
expected
• Some species closely resembled one
another
• Some naturalists concluded that similar
species may have developed from a
common ancestor
Where are these organisms?
Fossils
• Fossils showed that life had changed over time
• Fossils are the remains or impressions of
organisms that lived in the past
• Most are found in sedimentary rock
Fossils
• Certain fossils were always found in the
same layers of rock
• Organization of fossils and rock layers was
consistent
• The deeper (older) the layer, the more
dissimilar the fossils from modern
organisms
• Many fossils were of extinct organisms
Explanations Based on
Unchanging Concept of Species
• Catastrophism (Cuvier 1769-1832): modern
organisms survived
–
–
–
–
Creation: a vast supply of species
Catastrophes (floods)  rock  fossils
Modern species: survivors
Problem: There are no fossils of present species
together with ancient fossils.
• Louis Agassiz (1807-1873)
– new creations after each catastrophe
– Problem: too many catastrophes?
Speculation That Life Had Evolved
• Comte de Buffon (1707-1788) proposed
that some modern species had evolved
through natural processes
• Idea was not accepted
– Did not provide a mechanism for evolution
– Earth was not old enough to allow time for the
process of evolution
Explanation: Life Might Evolve
• Uniformitarianism James Hutton
(1726-1797) and Charles Lyell (17971875) studied geologic processes
(wind, water, earthquakes, volcanism)
– Rock formations reflect repeated cycles of
geologic change
– Geologic change resulted from slow,
continuous actions similar to those at
work today: Sedimentation.
– Earth must be older than the proposed
6,000 years; thus there is enough time
for evolution to occur
–  modern geologist: 4.5 billion years old.
– Problem: mechanism of change?
Possible Mechanism of Change:
Lamarck
• Lamarck (1744-1829):
– Fossil progression: Older rocks simpler forms
– Mechanism: Inheritance of Acquired
characteristics (1801).
• Body parts modified by use and disuse  inherited
– not fully correct
Mechanism of
Change:
Natural Selection
• Darwin and Wallace (1858)
– Descent with modification 
life variety
– In populations, not all survive
to reproductive age, only
those ‘selected’ by nature.
– Darwin’s finches, residents of
the Galapagos Islands
– Darwin published On the Origin
of Species by Means of Natural
Selection in 1859
Evolution by Natural Selection
Potential for
rapid reproduction
Relatively constant resources
and population size over time
Competition for survival
and reproduction
Variability in
structures and behaviors
NATURAL SELECTION:
On the average, the fittest
organisms leave the most offspring
Observation
Conclusion based on observation
Some variability
is inherited
EVOLUTION:
The genetic makeup of the population
changes over time,
driven by natural selection
A flowchart of evolutionary reasoning
Evolution by Natural Selection
• Proposed process of
evolution based on four
postulates regarding
populations
• 1: Individual members of a
population differ from one
another in many respects
– Variations arise purely by
chance resulting from
random mutations in DNA
– Differences are obvious in
many physical characteristics
and extend to molecular level
Evolution by Natural Selection
• 2: At least some of the differences among
members of a population are due to
characteristics that may be passed from
parent to offspring
– However, the mechanism of inheritance was
not understood at this point in time
Evolution by Natural Selection
• 3: In each generation, some individuals in
a population survive and reproduce
successfully but others do not
– Darwin observed that many more individuals
are born than survive
– Some individuals have more offspring than
others
Evolution by Natural Selection
• 4: Individuals with advantageous traits
survive longest and leave the most
offspring, a process known as natural
selection
How Does Natural Selection Work?
• Modern genetics (Mendel, 1865) confirmed
Darwin’s Assumption of Inheritance
– New variations arise as a result of random mutations
in DNA (chance).
– New variations may be beneficial, harmful, or neutral
• Natural selection acts on individuals within a
population; however, it is the population that
changes over time
– The evolution of the horse
Pliohippus
5
Hipparion
Archaeohippus
Anchitherium
Merychippus
Mesohippus
forefoot
tooth
25
35
Hyracotherium
Paleotheres
50
browsing
millions of years ago
1
grazing
Equus
0
Evidence of Evolution:
• 1. Fossils:
– Change Over Time:
Progressive series of
fossils that show
• Evolution of body
structures (i.e. modern
whales evolved from
land-dwelling
ancestors)
• Ancient species tend to
be simpler in form than
modern species
Evidence of Evolution:
2. Comparative Anatomy:
• Some structures provide evidence of
common ancestry
• Homologous structures are structures that
have the same evolutionary origin despite
their current appearance or function
– Bird and mammalian forelimbs are homologous
structures
Evidence of Evolution:
…2. Comparative Anatomy:
• Vestigial structures are remnants of
structures that are inherited from ancestors
• Had important functions in ancestors
• Serve no obvious purpose in present day
organism
– i.e. :
» Molar teeth in vampire bats
» Pelvic bones in whales and certain snakes
Evidence of Evolution:
…2. Comparative Anatomy:
• Analogous structures are structures that are outwardly
similar in appearance, but differ in their evolutionary origin
• Result through the process of Convergent Evolution:
Exposure to similar environmental pressures produces
similar structures in unrelated organisms.
– Wings of insects and birds
Comparative Anatomy
• Analogous structures include
– Streamlined shapes of seals and penguins
Evidence of Evolution:
• 3. Embryology
• All vertebrate embryos, resemble one another
in their early development
– Similar developmental stages: genes direct
development of gill slits and a tail; these genes
were inherited from a common ancestor
Embryology
• Adult fish retain gills and tail because the
genes are active throughout their
embryonic development
• Humans are born without gills and a tail
because the genes are active only during
early embryonic development
Evidence of Evolution:
• 4. Modern Biochemical and Molecular
Biology.
– DNA analysis reflects the degree of
similarity between organisms; similar DNA,
RNA, genetic code for translation, 20 aa,
ATP.
• e.g. the DNA nucleotide sequence of the
human and mouse cytochrome c gene is very
similar, suggesting shared ancestry
Evidence of Effects of Selection
• 1. Artificial selection (selective breeding)
resembles natural selection
– It modifies organisms:
• In only a few thousand years, humans artificially selected
for all breeds of modern dogs. Modern dogs descended
from wolves
Evidence of Effects of Selection
• 2. Natural Selection
Today: Examples:
• Coloration in Trinidadian
guppies:
– In areas with predators,
males are dull in color
– In areas without predators,
males are brightly colored. If
these are transferred to
areas with predators, they
will be eaten easily.
Evidence of Effects of Selection
2. Natural Selection Today: Examples:
• Pesticide resistance
– Roaches developed resistance to Combat®, an
insecticide bait that acted as an agent of natural
selection
– Resistant roaches possessed a rare mutation that
caused them to dislike glucose, the main attractant
in Combat®
– At least one insect species is resistant to every
pesticide in existence
Evidence of Effects of Selection
• 2. Natural Selection Today:
Examples:
• Experimental introductions of
Anolis sagrei lizards
• Lizards with long legs from
environment with trees in
Staniel Cay were introduced
into some small Bahaman
islands with thinly-branched
bushes and no trees
• After 14 years, lizards were
found to have evolved shorter
legs.