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Evolutionary Theory Primer Darwinism Evolution

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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 emphasizebiological 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.


Origin of the Species

  • 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
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

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
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
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
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
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
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 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


Homo sapiens

A. africanus

early Homo

Homo erectus

Hominid Evolution

A. afarensis

two common misconceptions
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
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




Modern Whale

Whale Evolution


also striving toward perfection
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
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
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
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
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
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 concepts22
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
The Evolutionary History of Snugglemorphs

Press spacebar

to evolve to next

epoch of time













another branch of the snugglemorph evolutionary tree
Another branch of the snugglemorph evolutionary tree

The next slide shows another branch of the snugglemorph evolutionary tree. Note that he Uni-morphs and Di-morphs are shown again.

the evolutionary history of snugglemorphs another branch
The Evolutionary History of Snugglemorphs- another branch














for further reading
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.