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Evolutionary Theory Primer
Darwinism
Evolution
The term evolution is used in several disciplines to
describe a cumulative change that occurs over long
epochs of time. Astronomers study the evolution of stars
and geologists describe the evolution of mountain ranges.
Disciplines that emphasize biological evolution (e.g.,
Biology & Psychology) use the term evolution to describe
transformation of organisms in time. Biological evolution
explains the diversity of species on earth. The concept of
biological evolution was around long before Charles
Darwin published his book On the Origin of the Species.
Darwin’s main contribution was to provide evidence for at
least one mechanism by which this transformation occurs
-- natural selection.
 In On the Origin of the Species by Natural
Selection (1859), Darwin validates three facts:
 More offspring are produced than can possibly survive.
 Offspring vary among themselves- they are not clones.
 Some of this variation is passed down by biological
inheritance to future offspring.
 Next, from these facts Darwin inferred the
principle of natural selection.
The Principle of Natural Selection
 “If many offspring must die, and individuals in all
species vary among themselves, then on average,
survivors will tend to be those individuals with
variations that are fortuitously best suited to
changing local environments. Since heredity
exists, the offspring of survivors will tend to
resemble their successful parents. The
accumulation of these favorable events over time
will produce evolutionary change.” p. 138
Gould, S.J. (1996) Full House: The Spread of Excellence
from
Plato to Darwin. New York: Three Rivers Press.
Darwin’s Theory of Evolution
Darwin’s”Theory of Evolution”is more than just the
principle of natural selection. Darwin had many ideas
concerning biological evolution, not all of which are
accepted today. For example, in his book The Expression
of the Emotions in Man and Animals (1872) Darwin
relied heavily on Lamarckian evolution to explain animal
and human behavior (yes, Darwin was a behavioral
scientist too). Jean Baptiste de Lamarck proposed a
different mechanism of evolutionary change known as
inheritance of acquired characteristics.
Inheritance of acquired
characteristics.
This principle states that when the environment acts on
an individual causing a change in a physical
characteristic, this change is passed on to the offspring.
Very often psychology students in the Learning course
express belief in this principle. Some students believe
that if an individual develops a habit because of
experiences in the environment, this habit can be inherited
by the individual’s children. A habit or any other form of
learned performance in parents cannot be biologically
inherited by their children. If you had a similar belief,
don’t fret. Darwin made the same mistake.
Evolutionary theory as a unifying
theory in the natural sciences
Although Darwin suggested principles that are known to
be incorrect today, several of his principles (e.g., natural
selection) are supported by an enormous body of evidence
from many disciplines of the natural sciences. Many of
Darwin’s principles are accepted by the majority of
natural scientists today. Unfortunately most students are
unaware of these principles or have misconceptions.
We already looked at the principle of natural selection.
Let’s look at two other principles that are especially
relevant to the study of Learning
The Principle of Continuity
Natural selection does not start from scratch every
time it evolves a new species. Natural selection
tinkers with what is available to yield a new species.
What changes occur depends on what improves an
animal’s chances of survival and reproduction.
Changes won’t occur to structures that are already
important for survival and work well. That is, as
species evolve certain characteristics are conserved
(kept; not changed). Evolutionary change is not
inevitable. Hence, species that share common
ancestors (no matter how different they appear to be)
most likely share common physical and behavioral
characteristics-- there is continuity in evolution.
The Principle of
Species-Specific Adaptations
Although species that share common evolutionary history
have common physical characteristics, they also have
differences. There are debates concerning what
constitutes a species and disagreements concerning the
factors that cause species to emerge. Nevertheless, an
important component is no doubt the unique pressures
that species experience in their unique environments.
Nature selects those individuals with specific
characteristics that allow them to survive and reproduce
in their environment. Hence, all living things are
presumed to have species-specific adaptations.
Analogy versus Homology
Similar structures that exist in two different species
because of continuity (i.e., because they evolved from a
common ancestor) are said to be homologous. Sometimes
two very different species with no recent common ancestors
may have very similar structures. In this case the structures
are said to be analogous. It is assumed that different species
evolve analogous structures when they encounter similar
environmental pressures in their respective habitats. An
example is color vision- the ability to differentiate between
objects that reflect different wavelengths of light. Nervous
systems that allow animals to perceive different colors
evolved independently in birds, fish, and primates..
Humans are Primates
Humans belong to the order of mammals known as
primates and the family known as Hominidae. A
Hominid is a member of the family Hominidae and
includes us humans and our extinct fossil relatives.
The next slide shows the evolution of hominidae up to
modern day humans.( Adapted from the web site: Teaching about
Evolution and the Nature of Science: Chapter 2.). Although the
information in the slide is accurate, it easily leads to
two very common misconceptions of the evolutionary
process.
Hominid Evolution
A. afarensis
A. africanus
early Homo
Homo erectus
modern
Homo sapiens
Two common misconceptions
•evolution is a simple succession of species, as
new species emerge the older species that gave
rise to it die out.
•evolution is a progressive process, newer
species are more complex and therefore
superior to older species.
These two misconceptions leads to the view of evolution
as a ladder or scale of progressive evolution of species
from “lowest” to “highest” (scala naturae).
Striving toward perfection?
People tend to interpret the data shown in the previous
slide as support for the idea that evolution is in the
process of creating a better and better human. You may
have noticed that the size of the skull increased which
reflects an increasing brain size. Some students have
suggested that humans will be much more intelligent in
the distant future with much larger light bulb-shaped
heads. The assumption that the evolutionary process is
striving for superior intelligence in humans is a result of
the two misconceptions described earlier.
Let’s look at the evolution of another modern mammal.
Whale Evolution
Mesonychid
Ambulocetus
Rodhocetus
Basilosaurus
Modern
Whale
Also striving toward perfection?
If the modern whale was able to study its own
evolution and discovered the sequence shown in the
previous slide, a likely misconception would be that
evolution as a progressive process is striving for the
largest possible body size and superior swimming
ability. If this assumption is correct then the modern
whale is indeed far superior to any of its extinct
ancestors (and all extant [living] species).
However evolution through natural selection does
not work in this manner. Natural selection is not a
progressive perfection-seeking process.
Correcting the two common
misconceptions
•evolution is not a simple succession of species; it
is an expanding process leading to increased
diversity of species over time. Despite this
biodiversity most species eventually go extinct.
•evolution is not a progressive process toward
perfection; it is a steady process of change
through natural selection that improves a species’
adaptation to its habitat (species-specific
adaptations). But remember that there is also
continuity in evolution.
A more accurate view
of evolution
These corrections lead us to view evolution not as a
ladder or scale of progressive change but as a continually
branching tree of species with specific adaptations to
their respective environments. While whales evolved a
streamlined body and flipper-shaped limbs to better
survive in the ocean environment, humans evolved
complex cognitive ability, language, upright posture,
sophisticated manual dexterity and many other
characteristics to better survive in the wide range of
environments that we occupy on earth and to function
more successfully within our complex social structure.
Increasing biodiversity yields
increasing complexity
As the number of species increase with time (increasing
biodiversity) the range of complexity also increases.
Humans are one of the most recent and most complex
species in the tree of life. What do I mean by “complex”?
For our purposes complexity is determined by the number
of components that exist-- the greater the number of
components, the greater the complexity. The human nervous
system is very complex because of the large number of
neural elements and neural pathways that control
physiological and psychological processes. The nervous
system of a fly is much less complex. But because there is
continuity between humans and flies these two species
share common components (e.g., the neuron).
Increasing complexity by
addition of components
Thus, one way of creating a more complex organism is
to add additional components. A reasonable analogy
is the building of a house. The initial structure may be
a simple 5 room house. To create a more complex
house rather than razing it and building from scratch,
rooms can be added on the existing structure. This can
go on indefinitely to yield a very complex structure
with very many rooms. In evolution added
components appear because they have a function -they increase the animal’s chance of survival and
reproduction.
Review of evolutionary concepts
In the following exercise you will review and reinforce
the concepts covered in this PowerPoint presentation by
studying the evolutionary history of artificial creatures
called snugglemorphs. Imagine that snugglemorphs are
simple creatures that live in a very simple world. To
survive they need to absorb nutrients. The only way they
can absorb these nutrients is by fitting snuggly into
crevices that exist on the surface of the land. Thus the
body of the first snugglemorphs matched the shape and
size of the crevices in the area of their origin.
Review of evolutionary concepts
However, the shape and size of the crevices vary
throughout their world. As the reproductively successful
snugglemorphs increased in number over time they
inhabited an increasing percentage of their world. The
process of natural selection transformed some of these
snugglemorphs into new species by adding components
to take advantage of different shaped crevices. As a
result the snugglemorphs evolved into a diverse number
of species of varying complexity. The next two animated
slides show the evolutionary history of the
snugglemorphs. Study the slides and answer the
questions in the Microsoft Word document that you were
asked to print.
The Evolutionary History of
Snugglemorphs
Penta-morph
Quadra-morph
Tri-morph
Di-morph
1
2
3
4
5
6
Uni-morph
Epoch
Another branch of the snugglemorph
evolutionary tree
The next slide shows another
branch of the snugglemorph
evolutionary tree. Note that he Unimorphs and Di-morphs are shown
again.
The Evolutionary History of
Snugglemorphs- another branch
Di-morph
Uni-morph
Bi-morph
Bi-med-morph
Bi-long-morph
Tri-long-morph
1
2
3
4
5
6
Epoch
For Further Reading
 Darwin, C. (1998) The Expression of the Emotions in Man
and Animals, Third Edition. New York: Oxford University
Press.
 Dawkins, R. (1987). The Blind Watchmaker. New York:
W.W. Norton & Co.
 Gould, S.J. (1996) Full House: The Spread of Excellence
from Plato to Darwin. New York: Three Rivers Press.
 Web Site: Teaching about Evolution and the Nature of
Science: Chapter 2.