Chapter 22

Chapter 22
Descent with Modification:
A Darwinian View of Life
A. Historical context for evolutionary theory – who led up
to Darwin?
1. Two early views about life were explained by the scale
of nature and natural theology
- Scale of nature: all living things are arranged on a scale
of increasing complexity (Aristotle).  All species are
permanent with no evolution. All species are “ideal” and
perfectly adapted to their environment (400 BC).
- Natural theology: the study of nature to discover God’s
plan (1700s).  Each species was designed by the
Creator to serve a specific purpose. By discovering,
naming and classifying all species, mankind could
“divine” the Creator’s plan.
- Carolus Linnaeus (1707-1778) developed a two-part
(binomial) system of naming organisms that we still use
today that is based on a genus and species designation (e,g.
Homo sapiens).  Taxonomy is the branch of biology
concerned with naming and classifying organisms.
Note in Figure 22.2, that Darwin built his ideas about
evolution on a base that was established by others.
2. Georges Cuvier - primarily developed paleontology by
studying fossils that are relics or impressions of organisms
preserved in rock.  Noted that sedimentary strata contain
unique groups of organisms.
- Deeper strata contain species that are often very different
from those present today.
- Rather than follow evolution, Cuvier believed in
 Catastrophism is the idea that each strata boundary
corresponds to some catastrophic event, such as flood.
3. Theories of gradualism
- James Hutton (1726-1797) introduced the theory of
gradualism which is in contrast to catastrophism.
 Gradualism holds that geological changes are the
cumulative product of slow, continuous processes.
- Charles Lyell (1797-1875) took Hutton’s idea and used it to
introduce the theory of uniformitarianism.
 Uniformitarianism refers to the idea that geologic
processes have not changed throughout earth’s history. That
means that volcanoes, sedimentation, etc. that we observe
today have occurred since the beginning of time.
 Importance of these theories? Stretch earth’s history back
further and further into time.
4. Jean Baptiste Lamarck placed fossils in an
evolutionary context and published a theory of evolution
in 1809.  His theory used two common ideas of his
a. Use and disuse  parts of the body used on a regular
basis become larger and stronger; those not used
deteriorate. And,
b. Inheritance of these acquired characteristics (the
modifications) are passed onto the offspring.
- Lamarck’s theory was based on observations of fossils,
but gave no mechanism for evolution. However, he set
the stage for Darwin.
B. The Darwinian revolution
1. Field research framed Darwin’s view of life. One of his
primary observational sites was the Galapagos Islands
- He learned that most species there occur nowhere else in
the world, but many resemble species in South America.
- Species on the islands are related to each other but, like
finches, were adapted to different lifestyles.
Figure 22.6 (p. 443) – Galapagos finches.
2. Darwin focused on adaptation 
a. How species adapt to succeed in changing or different
environments, for example, new birds land on the Galapagos
Islands… how do all the different species come about?
3. His theory was: Descent with modification  the phrase
used by Darwin to describe what he had observed during his
This concept was later worked into Darwin’s theory on
Figure 22.7 (p. 444) – Descent with modification.
- Think of descent with modification in terms of a branching
tree. Over time, organisms enter/experience new
environments which lead to changes in the population of
organisms (ADAPTATION). These changes cause
divergence, i.e. the branches of the tree. Closely related
species are closest together in the tree. The trunk represent
a common ancestor.  Evolutionary Tree
- The Linnaean classification scheme fits the concept of an
evolutionary tree  genus and species may differ, but still
belong in the same family.
4. What drives adaptation?  Natural selection. Darwin’s
theory on natural selection was based on these observations:
i. All species have the potential to overproduce offspring. If
they continued to do so, they would use up all of the world’s
ii. However, if we look at populations we see that they are
stable, except for seasonal fluctuations, or unless changed
by man altering the environment.
iii. Thus, environmental resources limit their production.
Inference from these observations is that:
1. Conditions lead to a struggle for existence among
individuals of a population.
Other observations:
iv. Individuals within a population have varying
v. Much of the variation is heritable (genetic).
The inferences from these observations are that:
2. Individuals with traits that allow them to outcompete
other individuals will survive and
3. These favorable traits are maintained in the future
generations, leading to gradual changes in the population.
 This is Darwin’s Theory of Natural Selection!
Natural selection occurs because of differential success in
reproduction. It is driven by interactions of variable
individuals with their environments. Those individuals with
the most favorable traits will succeed to produce more
The product of natural selection is adaptation (evolution) of
organisms to their environments.
- Populations are members of a species that interbreed.
- A population is the smallest unit that can evolve.
- Natural selection occurs because of interactions between
organisms and their environments.
- Natural selection works to increase or decrease the
occurrence of heritable traits. Examples??
- Individuals are selected, but populations evolve. Examples:
- See Fig. 22.10
– Evolution of insecticide resistance in insect populations.
– Evolution of drug resistance in HIV.
5. Evidence of evolution
a. Homology refers to a similarity in characteristics due to
common ancestry.
Figure 22.14 (p. 449) – Homologous structures: anatomical
signs of descent with modification.
- Examples of anatomical homology include skeletal
components of forelimbs, and vestigial organs in some
organisms.  Anatomical structures are not uniquely
engineered. There are similarities between species showing
possible links in natural selection and evolution.
 Embryological evidence: all vertebrates have pharyngeal
pouches – gills. See also, Fig. 22.15, p. 449.
 Molecular evidence: DNA/RNA is a universal genetic code.
 Other molecular evidence: homologies basically follow
the tree of life. More closely related organisms have more
homologous molecules.
See also, Figure 22.16 (p. 450)
b. Biogeography is the geographic
distribution of species.
- Species found in the same area on earth are typically
closely related.
Figure 22.17 (p. 450) – Different geographic regions, give
rise to different mammalian “brands.” Although these
two organisms look alike, that is they evolved to fill the
same niche, they are not closely related.
c. Fossil Record
- We can see the development of present day species
from ancestral species.