is Evolutionary Theory?
are there so many kinds of organisms? Why do animals, plants, fungi,
protists and bacteria look the way they do? Why do they behave the way
they do? Why do some otherwise very different organisms have similar
morphologies or behaviors? Conversely, why do otherwise very similar
organisms have remarkably different characteristics? These are some
of the major questions of biology. They explore the match between organisms
and their environments, as well as how and why populations change over
time. Evolutionary theory explains the diversity of life. It does
not explain the origin of life, but the diversity of life.
The conceptual framework of evolutionary theory is made up of
three interrelated parts:
1) Organisms have a shared history—shared ancestry
• All species come from other species, with a history of common
descent with genetic modification. This is Darwin’s “descent
with modification” (although this idea was not new to Darwin;
it was well established by the 1830’s). This idea can be visualized
as a branching tree of life (Figure 1).
In general, this is what is meant when the term evolution is used.
2) The causes (or mechanisms) of evolution
• What causes evolutionary change?
• These are known to be natural selection, genetic drift, and
gene flow acting on genetic variation (produced by mutation) within
3) The specific pathways evolution has taken
• What has been the history of life?
Modern Evolutionary Biology
are two aspects of evolution today. First, evolution is the organizing
principle for all of biology. Just as you are descended from your
parents, and your grandparents before them, and so on, all living organisms
today are descended from organisms that lived in the past. Understanding
how and why these ancestor-descendent lineages have changed through
time helps us appreciate the diversity of life we see today. Genetics,
anatomy, physiology, neurology, morphology, and behavior—all of
these aspects of living organisms have evolved through time. The study
of the evolutionary processes that produced these traits provides the
comprehensive framework for understanding them. Without evolutionary
theory as a guiding framework, biology is just a collection of facts.
As Theodosius Dobzhansky said in his 1973 paper in American Biology
Teacher (titled “Nothing in biology makes sense except in the
light of evolution”):
in the light of evolution, biology is perhaps, intellectually the
most satisfying and inspiring science. Without that light, it becomes
a pile of sundry facts—some of them interesting or curious but
making no meaningful picture as a whole.”
to understand, in detail, the causes of evolution (the field of microevolution),
2) to discover the history of life on earth—how, specifically,
are taxa related to each other (the field of macroevolution).
two aspects of modern evolutionary biology correspond to the three parts
of evolutionary theory. That all organisms have a shared ancestry (have
evolved) is the organizing principle in biology. This idea is considered
established (very well-confirmed) and is no longer the subject of scientific
debate or inquiry. The second and third parts of evolutionary theory
(causes and pathways) are the focus of the modern science of evolutionary
biology. Why is the first part of evolutionary theory (shared ancestry)
not a focus of current scientific investigation? Why is it considered
factual (very well-confirmed)? Simply because the amount of evidence
amassed in the past century and a half for a shared ancestry of all
organisms is so overwhelming that it is no longer considered an issue.
In addition, no evidence disputing this idea has ever been found.
Evidence for Evolution
collected an abundance of evidence for common descent. After Darwin
published his ideas, the scientific community accepted the idea of evolution
(shared ancestry) relatively quickly, although it was decades before
they were convinced of his proposed mechanism of evolution, natural
selection (mostly because he did not have a plausible explanation for
Just because scientists no longer debate the issue of shared ancestry,
does not mean that it is not worth understanding why
it is no longer debated. In other words, it is worthwhile to convince
yourself (and your students) of the fact of shared ancestry by investigating
Note that evidence for a historical idea like shared ancestry is historical
in nature. Scientists use a method of induction, called the method of
hypothesis, which is also called the inference to the best explanation.
In this method, one assumes a hypothesis for the sake of investigation,
and then asks what would follow if the hypothesis were true. In other
words, one makes a prediction from the hypothesis. This prediction,
which can be a prediction of a future event, but also can be a retrodiction
of past phenomena, is then tested against the empirical world.
This is done for several competing hypotheses, and the hypothesis that
is best able to explain the observed pattern of data, gathered either
through experimentation or through historical means (e.g. fossils, patterns
among current species, etc.), is accepted as the best explanation.
Since students are used to thinking about hypothesis testing only in
the form of an experiment, it is worth taking the time to instruct them
on historical types of hypothesis testing, which includes examining
patterns and the clues left behind1. Both hypotheses:
1) species are static and unchanging, and 2) species are related through
common descent, make clear predictions about patterns of data. Examining
the predictions of these two ideas against the available evidence has
led scientists to infer that common descent is the correct hypothesis.
There are several basic lines of evidence for evolution. The reason
that scientists view this evidence as so convincing (besides the sheer
vast quantity of it) is that each of these lines of evidence is
predicted by evolutionary theory. In addition, these lines
of evidence are independent of each other. One line of evidence
does not depend on another. That they all corroborate with the same
biological explanation (shared ancestry) is extremely convincing.
is the study of how species are distributed spatially across the landscape
(geographically). How species are distributed provides evidence
for evolution. The distribution of many species does not make sense,
unless they shared a common ancestor. For example, if species were static
(unchanging), then you would expect to find the same species in areas
with similar environmental conditions around the world. Evolutionary
theory, however, predicts that modern species should be found close
to where their ancestors were, regardless of the environmental conditions.
This is the major type of evidence that convinced Darwin.
Darwin collected many animal and plant specimens, as well as fossils
during his voyage on the Beagle. It turned out that the South American
fauna were quite different from the European fauna that Darwin was used
to. It also turned out that the fossils he found in South America were
very similar to the living animals he had collected. Why should a unique
set of animals be found in the same place as what appeared to be related
fossils? The best explanation for this pattern is that the extant South
American species had descended from the now extinct fossil species.
A particularly clear example of how the current distribution of species
provides evidence for shared ancestry can be seen on islands. Evolutionary
theory predicts that islands that have similar environments, but are
in different parts of the world, will not be populated with the
same species. Instead, these islands should be populated with plant
and animal species that are closely related to the species on the nearest
mainland, even if the environment there is very different from the island.
This is what is seen throughout the world. For example, the Galapagos
Islands off of the coast of Ecuador, South America, are volcanic and
barren. These islands are not populated with species from other volcanic
islands around the world; instead, they are populated with species that
are related to those found in the nearby lush tropics of South America.
This pattern is predicted by evolutionary theory: newly formed geographically
isolated islands are populated (by migration events) with plants and
animals from the nearest mainland. These populations then adapt to the
new environments on the islands, and reproductive isolation (i.e. speciation)
eventually results. Subsequent migration from island to island within
the archipelago leads to further adaptive changes and to additional
speciation events. This pattern of migration to new habitats, adaptive
evolution in response to novel environments, divergence from the ancestral
population, and ultimately the formation of distinct species, is especially
apparent on islands. For this reason, island chains like the Galapagos
provide us with convincing evidence for shared ancestry. That different
species on these islands have a shared history is the best explanation
for their geographic pattern of distribution.
line of evidence for shared ancestry includes various aspects of the
fossil record. Since rocks are laid down sequentially, with older rocks
laid down before, and thus below, younger rocks, the chronological sequence
of organisms can be inferred from where the fossils are found. The
chronological order of the major groups seen in the fossil record shows
a succession of species that is predicted by evolutionary theory.
For example, prokaryotes, according to numerous independent lines of
evidence, are thought to be the oldest group of organisms. Thus, evolutionary
theory predicts that fossil prokaryotes should appear before (and therefore
below) eukaryotes. This is what the fossil record shows: prokaryotes
are found in older rocks than are eukaryotes. Likewise, fish appear
before amphibians, which appear before reptiles, which appear before
mammals; all as predicted by evolutionary theory.
Also as predicted, the fossil record shows transitions (links) between
groups, which are evidence that these groups have a shared history.
For example, mammals are thought to have evolved from a reptilian ancestor,
and this transition is thoroughly documented with a series of fossil
skulls (reptiles ->mammal-like reptiles -> reptile-like mammals
-> mammals). There are also examples of transitions/links between
fossils and modern species (two exceptionally well-documented cases
include horses and humans). Likewise, the newly discovered whale fossil
with hind limbs is a link between modern whales and their hypothesized
theory predicts that, if all organisms have a shared ancestry, then
all living things should have certain characteristics in common.
The genetic code (genes and how their protein products are coded) is
universal—all plants, animals, fungi, bacteria and protists have
the same genetic code. There is no chemical reason for the specific
code that we have (i.e. the genetic code is not chemically constrained
to be the way it is). Another code would have worked as well. Nor is
this the only way that information can be transferred from one generation
to the next. The genetic code that all organisms now have, just so happened
to be the one that the ancestor of all living things had. The fact that
all organisms share this code reflects this historical legacy and provides
evidence that all living taxa shared a common ancestor at one point
There are two patterns of similarity in traits among species. The first
type is an analogous similarity, which is when a trait
in two different species is similar and they have the same function.
The other type of similarity is homologous similarity,
which is when two traits are similar, regardless of the function
of the trait. In this case, two traits are similar even when it
is not functionally necessary for them to be similar. The best explanation
for this pattern of homologous similarity is that the traits are similar
because of a common history of the two species. In other words, two
species have the same trait because the common ancestor of the two species
had the trait. For example, all vertebrate embryos look very similar
during the earlier stages of development, including having gill pouches
and tails. Thus, a reptilian embryo, a bird embryo and a human embryo
look very similar, even though they develop into very different adult
best explanation for this similarity in embryos is a shared history
of vertebrates—all vertebrates share a common ancestor that had
a tailed embryo with gill pouches.
Evolutionary theory predicts that different
species will evolve different forms of shared (homologous) traits.
As lineages of organisms expand their ranges to new environments, they
adapt to function in those new environments. The similar traits that
different populations inherited from their common ancestors may be modified
and diverge from each other (due to different environments). Thus, a
shared (homologous) trait may diverge in form among related taxa. For
example, all vertebrates have forelimbs made up of exactly the same
bony elements. However, these elements have been greatly modified in
different species for different functions. If organisms have evolved,
then comparative studies of the morphology of these organisms should
reveal evidence of shared ancestry. The fact that bird wings and mammal
limbs (for example) share the same basic structure and bony elements
provides convincing evidence that these animals share a common ancestor.
Thus, homology provides evidence for shared ancestry—it provides
glimpses into the shared ancestry of organisms today, and how they have
diverged from their ancestors in the past.
Along the same lines, vestigial organs are structures
that are currently of little use to the organism (i.e. they have no
known current function). They are the historical remnants of structures
that did have a function in earlier ancestors, and provide evidence
for shared ancestry. Whales, for example, do not have hind limbs,
but some whales have the vestiges of a pelvis and leg bones. The best
explanation is that these pelvic and leg bone vestiges are homologous
to all vertebrate pelvises and leg bones. This means that the ancestors
of whales had complete structures, and that they have been greatly reduced
in whale evolution.
Evolutionary theory also predicts that different
organisms will independently evolve similar solutions to the same functional
problem (analogy). For example, the wings
of bats, birds, and insects all serve the same basic function (flight)
and are similar in appearance. They are not similar because of shared
ancestry (the common ancestor of bats, birds and insects did not have
wings), but because they serve the same function. In another example,
plants in the cactus family are only found in the New World. However,
they are very similar in appearance to plants in the euphorb family,
which are only found in the Old World. These two plant families are
not closely related, yet they have very similar traits (e.g. thorny
spines and highly reduced leaves). They do not share a common history;
instead they share a common environment (hot, dry desert), to which
they each have independently adapted similar traits. Another term for
this is convergence or convergent evolution. The existence
of similar characteristics in taxonomically different taxa with similar
environments can be evidence for adaptive evolution (i.e. evolution
by natural selection), if the similar characteristics function in similar
ways to the shared environmental conditions.
theory predicts that some traits will not be “perfectly”
adapted. Natural selection (the only evolutionary mechanism to
produce adaptations) does not “start from scratch” when
a new functional challenge is presented. If it were able to start from
scratch, we should expect to see nothing but perfection in adaptation.
But since natural selection acts on the genetic variation that is currently
available in a population, the “best” solution cannot always
be found. Often, existing traits are modified (“contrived”)
to serve a new function. The giant panda’s thumb, a modified wrist
bone, is one famous example of a clumsy adaptation contrived from an
Another example of an imperfect adaptation is the vertebrate eye (including
our own). The design of the vertebrate retina is “inside-out.”
The retina is behind the nerves that form the optic nerve. Where the
optic nerve leaves the eye, there is a hole, which results in a blind
spot. There is no functional reason for our eyes to be this way; the
best explanation is historical—a “better” retina was
not available in the common ancestor of all vertebrates. In the eyes
of some mollusks (squids and octopuses) the retina is in front of the
optic nerve, and thus they have no blind spot. Their ancestors happened
to have the structures that could be modified into functional eyes without
the design compromise of a blind spot.
about the Evidence for Evolution
General lessons on evidence:
from a nature of science unit into a general lesson on evidence in science.
A general activity on weighing evidence can be found at the ENSI site:
Flat Earth.” Also check out PBS’ Evolution
website for a lesson about evidence from students’ own lives
a Trail of Evidence.”
An important point to emphasize throughout the school year is the idea
that evolution is a unifying theme in biology. One way to do this is
to continuously refer to evolution throughout your course.
For example, when you are talking about cellular structure, mention
“this cell has evolved this structure…” Constantly
use the phrases, “…has evolved” “has adapted.”
Try not to refer to “develop” (unless of course
you are teaching about development…), as in “this species
has developed this trait.”
Also check out the PBS evolution series, episode two (“Great
Transformations”) and the video “Learning
and Teaching Evolution,” which is a companion
to the series that contains short, seven-minute segments for use in
the classroom. Segment three of Learning and Teaching Evolution (“How
Do We Know Evolution Happens?”) as well as episode two
“Great Transformations” both use whales
as an example of using different lines of evidence to understand evolution.
UCMP website has a nice (but complex) activity on island biogeography
in which the students use real data for real species: “Island
Biogeography and Evolution: Solving a Phylogenetic Puzzle Using Molecular
PBS’ Teacher’s Guide also has a lesson on the effects
of saltwater on seed germination that re-creates one of Darwin’s
experiments: “Seeds at Sea.” Go to the
online Teacher’s Guide, click on “download PDF” under
Unit 2: Who Was Charles Darwin?
UCMP website has a collection of activities on various aspects of the
fossil record, called “Learning
from the Fossil Record” “Sequencing
Time” and “What Came First”
are companion activities to understand the sequence in the fossil record.
Other lessons include “Determining the Age of Rocks and
Fossils” and “Fossilization and Adaptation,”
both of which explore just what a fossil is.
ENSI also has a lesson on whale transitional fossils: “Becoming
Excellence has a comparative embryology activity in which students’
breed Japanese Medaka fish: “Comparative
Embryology Using Japanese Medaka Fish.” The students
breed Japanese Medaka fish, collect eggs, and then watch the embryos
throughout their development. Students then compare the fish embryos
with pictures of embryos from chicken, humans, etc. If you do not have
the equipment necessary for raising fish, you could order eggs (instead
of adult fish) for a relatively cheap alternative.
Teacher’s Guide also has a lesson on vertebrate forelimbs:
“Winging It.” Go to the online Teacher’s
Guide, click on “download PDF” under Unit 3: What is the
Evidence for Evolution?
ENSI website has a read-and-discuss activity based on a couple of S.J.
Gould’s essays: “Panda’s
Thumb,” a lesson on contrivances: “Blocks
and Screws,” and a new lesson design efficiency:
Don’t Whales have Legs?”