Evidence for Evolution
Lessons about the Evidence for Evolution
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.
There 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”):
Second, evolution is also a dynamic field of research (evolutionary biology). In general, evolutionary biologists have two overarching goals:
to understand, in detail, the causes of evolution (the field of microevolution),
These 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.
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
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.
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.
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
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
Lessons about the Evidence for Evolution
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.”
The 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 Genetics.” The 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.
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.
The 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: “Why Don’t Whales have Legs?”