CPSP118G Spring Semester: Earth, Life & Time Colloquium
The Evidence of Evolution
Thomas R. Holtz, Jr.
What is "Evolution?"
- Literally "unfolding" or "unravelling"
- Pre-1860s, term used for development of an embryo
- Generally used for "change through time":
- Sometimes for predetermined set of changes, such as stellar evolution or evolution of
- Also for the general process of change, as in "evolution of the automobile"
- More specifically, organic evolution, or the change of groups of living things
- Often summed up in terms of genetics: "changes of gene frequency through time" (literally
true, if a bit boring...)
- Darwin himself used the phrase "DESCENT WITH MODIFICATION" rather than "evolution"
- In other words, evolution in the broadest sense is no more than the observation that
"none of us looks exactly like our parents."
Fixity vs. Transmutation
Traditionally, people accepted the fixity of species just as they accepted that the world today is pretty much the same now as in the past.
Theological argument for fixity under the Biblical concept of the Plenum ("fullness"):
Many early naturalists accepted the Plenum, but evidence of extinction (man-made, as in
the dodo, and natural, as in fossils) showed that things could be removed from Creation. What
about adding to it?
- Ecclesiastes 1:9 and 3:14-15, if you want to look it up
- "Nothing new under the sun": nothing has been taken from Creation, nor removed from it
While some thinkers once thought that life as we see it now is the way it has always been, the discovery of the fossil
record showed that strange creatures once roamed the Earth that are no longer there. How to explain these observations?
Two main possibilities:
- The successive appearance and disappearance of different forms through time, without genetic connection (as
supported by Owen, Cuvier, and others)
- Transmutationism: direct lineal relationships between ancestor and descendant species. So living
species are descendants of earlier distinct species, which themselves were the descendants of even earlier ones. "Transmutationism"
became known as "evolution" after the work of Darwin and Wallace.
Transmutationism, a set of early evolutionary models, accepted by several prominent
scientists by the late 1700s. Among them were Jean Baptiste Pierre Antoine de Monet, Chevalier
de Lamarck (normally known as Jean Baptiste de Lamarck) and Erasmus Darwin (doctor,
scientist, surgeon, abolitionist, and INCREDIBLY rich).
Fossils demonstrated that the living component of the Earth
changed through time; shared homologies showed connections between groups; adaptations showed
organisms "fit" to their environment. Already accepted the central tenets of Evolutionary
But what caused the modifications?
- The Diversity of Living Things is the Product of Descent with Modification
- New species are the modified descendants of previously existing species
- Spontaneous generation of new lineages of organisms throughout time; thus,
many living things represent seperate origins at different points in Earth History
- Within each lineage, "driving forces" impel organisms towards improvement (i.e., simple
forms become complex) down predetermined pathways
- Inheritance is from use and disuses: characters acquired during the lifetime of
an individual are passed onto descendants
Problems with these ideas, however:
- Spontaneous generation doesn't work
- "Driving forces" never identified, and are more metaphysical than naturalistic
- Continuity of lineages through long periods of Earth history, rather than appearance, transformation,
- Also, fossils documented linkages between groups rather than separation
- Inheritance doesn't happen by use & disuse; transformations to adult are not passed onto
Darwin did not discover evolution, nor did its study stop with his work. At least some of the
evidence for evolution was long known before his time (although we've added a LOT, even to these
Initial Evidence for Evolution
- Homologies: the same anatomical structures ("body parts") are repeated in different
organisms. This allows us to recognize how they differ from each other, and how they resemble each
- Living things can be grouped using a nested hiearchy based on shared presence of
homologous stuctures of similar form
- System of classification codified by Carolus Linnaeus (18th Century Swedish botanist)
- Many of his principles, such as Latin names for organisms, and the use of genus and species still
- However, species are not fixed entities. They vary across their range, and they can often
hybridize with closely related forms
- Adaptations: any structure or behavior which allows an organism to interact with
its environment in certain specific ways
- Analagous structures: non-homologous structures found two or more organisms that
are adapted for the same function
- Vestigial structures: anatomical features which have some significant adaptive function
in some forms, but are reduced and non-functional (or nearly non-functional) in a related form
- Transitional fossils: extinct species intermediate in morphology between now-distinct groups
- Presence of similar phases and morphologies in embyros of organisms which are very
different and distinct as adults
- Biogeography: the non-random distribution of living things over the surface of the
Earth, both now and in the geologic past
- William "Strata" Smith's Stratigraphic Principle of Fossil Succession, indicating
a History to Life
- Descent with Modification
- Both the pattern of the history of life, and the processes that controls that pattern
- Next couple of lectures: how evolution works!
Last modified: 10 January 2008