FROM WHERE DO NEW SPECIES COME?
Historically have been two primary competing views about life:
- Species do not change, but are fixed.
- Life changes over time.
Both ideas can be found in ancient Greek writing, and might have been even older.
Much new consideration of topic during Age of Enlightenment and afterwards:
- New knowledge of diversity of life, particularly of marine forms and foreign animals
& plants
- Beginning to realize the existence of a geologic past and fossil (extinct) organisms
Prior to knowledge of geological ancient past and its extinct creatures, could only relate
modern animals to each other; and thus assumed any transitions had to be between known
modern forms.
With rise of knowledge of both fossil creatures and geologic time, new models of
evolution were proposed. In these models the earlier forms were the ancestors of
modern taxa.
Jean-Baptiste Lamarck (French: late 1700s-early 1800s): predominant theory of
evolution prior to Darwin:
- 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
another
Flaw with Lamarckian Evolution: characters acquired during the lifetime of an
organism are not passed on to descendants!
(The test: mice had their tails cut, bred with other de-tailed mice. However, mouse
tail length did not decrease over time, but instead remained the same).
Still, during the early 1800s was the best model available. Explained:
- 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
During early-mid 1800s, two natural historians independently developed an alternative,
and superior model: Natural Selection.
- Two individuals were Alfred Russell 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”.
Their basic observations:
I. In Nature, there is some variation in all populations
II. Some (but not all) variation is inherited (genetic, in modern terms)
III. More are born to every population than can POSSIBLY survive
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.
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.
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,
non-repeating history
- 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
snake-ancestors
- 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
Species.
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