GEOL 104 Dinosaurs: A Natural History
Fall Semester 2006
Evolution II: On the Origin of Species by Means of Natural Selection
Jean-Baptiste Lamarck (French: late 1700s-early 1800s) produced predominant theory of
evolution prior to Darwin. In fact, "Lamarckian" evolution (as used in most textbooks) is just
the theory of inheritence from acquired characters, which wasn't original to him. In general,
Lamarck and other early evolutionists believed in:
Flaws with Lamarckian Evolution:
- Multiple independant origins of life throughout time
- Predetermined changes in form from ancestor to descendant
- Theory of inheritence from acquired characters ("use and disuse") (Actually not
original to Lamarck, but often associated with him)
- During their lifetimes, individual organisms acquire structures or skills
useful in dealing with environment
- These acquired structures are passed on to their descendants
- Over time, the accumulation of acquired structures changes one type of organism to
- Fossil and modern record did not support multiple independant origins: instead, seemingly
different groups were found to have been more similar to each other earlier in Earth history
- No evidence for predetermination in changes in history of life
- VERY IMPORTANTLY: characters acquired during the lifetime of an
organism are not passed on to descendants!
- Homology as features passed on from common ancestors
- Adaptations as the net result of characters acquired in the lifetime of organisms
- Vestigial organs as the net result of non-use of organs during lifetime of organisms
Darwinian evolution differs from Larmarckian ideas in three main aspects:
- Theory of Common Descent: all living things stem from common ancestors which diverged
during Earth history
- Theory of Individual Variation: all living things have some differences from all
other living things. Variation is the reality; ideal types are human constructs. (This contradicts
early "idealist" models of taxa, such as held by Linnaeus).
- Theory of Natural Selection: the differential and survival of variants in a population resulting
in a net change in the phenotype of the descendants. Natural Selection recognizes that the
reality of individual variation, coupled with basics of population dynamics, limited resources,
and the immensity of geologic time, yielded a mechanism to produce many diverse lineages from
a single ancestor.
[NOTE: Remember that "theory" doesn't mean "guess" or "hypothesis", but rather some model of
broad explanatory power.]
Although we call the above set of theories "Darwinian Evolution", it was actually independantly
discovered to by two different naturalists during the mid-1800s:
Their basic observations which led to the discovery of Natural Selection:
- Two individuals were Alfred Russel Wallace and Charles Darwin:
- Both British
- Both trained in England, but traveled to far countries (Darwin to South America and
Galápagos, Wallace to Amazon
River basin and Malaysia)
- Independently made same basic observations and conclusions
- Mutual friends decided to present papers of both (in 1858) on their behalf, so both
could get credit
- The following year (1859) Darwin published On the Origin of Species by Means of
Natural Selection: an instant "best-seller".
I. Variability: In Nature, there is some variation in all populations
Thus, IF some variation gives the individual a slight advantage (bigger, stronger,
smaller, smarter, less tasty, whatever) at surviving; and IF that variation is inherited;
THEN there is a somewhat better than average chance that organisms with that variation
will survive to bear the next generation. Over the long expanse of geologic time, the
accumulation of these variations will change the population from one form to another:
the origin of species.
II. Heritabiliy:Some (but not all) variation is inherited (genetic, in modern terms)
III. Superfecundity: More are born to every population than can POSSIBLY survive
This process is analogous to artificial selection (i.e., domestication), and thus
called natural selection.
NOTE: Natural Selection is NOT "survival of the fittest", as commonly thought. Or more accurately,
"fitness" is NOT a measure of individual strength, speed, intelligence, or so forth, but
rather is an index of reproductive success.
NOTE ALSO: Darwin did not use the word "evolution" very often; instead, preferred the
phrase "descent with modification".
Darwin pointed out a subset of Natural Selection: Sexual Selection, where the
variation is "being more sexy" (and thus have better than average chance of breeding,
and thus passing on "sexiness", compared to other members of the population): peacock
tails, bird song, etc.
Many similarities to Lamarckism: homology due to common ancestry, for example.
Primary difference: adaptations due to differential survival of variations in a
population, not to accumulation of characters acquired in the lifetime of the organisms.
Evolution by natural selection explained a lot:
- Homologies are shared structures inherited from common ancestors
- Adaptations are produced by the net effect of differential survival of particular
variations in the populations, favoring a particular interaction with the environment
- Life is organized by nested hierarchy because it IS a tree of descendants branching
out from common ancestors
- The Principle of Fossil Succession works because Life had a single, unique,
- Vestigial organs exist because variants with reduced versions of these organs have
a better than average survival rate (perhaps because physiological resources aren't being
used up for less-used structures) than those with fully developed organs
- However, vestigial organs represent existence of ancestor with more fully developed
version of that organ: vestigial legs on whales & snakes indicate legged whale- and
- Common pattern of embryology between closely related forms because they share very
recent common ancestors; more distantly related organisms with different forms of
development because longer times between divergence
- Fossil record of groups intermediate between distinct modern groups represent common
ancestors (or relatives of the actual common ancestors) of these groups
- Because life has a history of divergence from common ancestors, regions with shared
history of particular creatures will have closely related species
- New species are the modified descendants of older species
For those interested in some of the original literature on the subject, click here for
the joint Darwin & Wallace paper of 1858 (actually, it was a series of short papers and
letters published together) and
here for the text of the first edition of The Origin of
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Last modified: 14 July 2006