Evolution is the process of long-term change in biological systems.
Until 1859, most biologists believed in the fixity of species. Lamarck
and Geoffroy thought that species could adapt to their environments by
changes occurring within individual lifetimes. Darwin proposed instead
that natural selection resulted in "descent with modification."
Historical ideas before Darwin:
- Before 1859, most scientists and philosophers insisted that
species were fixed and unchanging; each was thought to be
an unchanging copy of a heavenly form (=type, idea, eidos).
Species were arranged in an unchanging, unbroken order of perfection,
the great chain of being (scala naturae).
- Paley's "Natural Theology" explained adaptations as the result of
God's great power and benevolence. Darwin was familiar with this viewpoint.
- J.B. Lamarck tried to explain environmental adaptations.
He claimed that the willful use of a part would strengthen and enlarge
it, while the disuse of a part would cause it to wither (Lamarckism,
or the theory of use and disuse). Lamarck also believed in a single-file
line of progress ("La marche de la Nature").
- E. Geoffroy Saint-Hilaire explained adaptation by direct effects
of the environment (Geoffroyism), a concept which Lamarck rejected.
- Both Lamarck's and Geoffroy's theories rely upon inheritance of
acquired characteristics, changes occurring during an organism's
lifetime. This possibility was later disproved by Weismann and others.
Charles Darwin (1809-1882): A voyage around the world aboard H.M.S.
Beagle convinced him of several facts that earlier theories could
not explain:
- Different continents had very different species, even in very
similar climates.
- Species sharing a land mass or island group were often related.
Darwin: deep organic bond
- Island species were usually related to those of the nearest continent.
Darwin: species on islands
- Similar environments did not always produce the same species or
related species (contrary to Lamarckism and Geoffroyism).
Darwin1.ppt
Darwin: species not immutable
- Reading of Malthus' Essay on Population
- Observations of animal breeders
- Work on barnacles
- Wallace's letter; Linnaean Society papers (1858)
On the Origin of Species (1859), in which
Darwin explained all the above facts by two new theories:
- branching descent with modification, a branching form of evolution very different from earlier theories.
- natural selection as a
mechanism for evolutionary change.
Evidences for branching evolution ("descent with modification"):
- Patterns of common descent are reflected in classifications, forming "groups within groups".
- Related species share many internal similarities (anatomical,
biochemical, or embryological homologies) despite different
adaptations. Homologies
- These homologies may include vestigial remnants of once-useful parts.
- Similar adaptations often occur under similar circumstances, even
in unrelated species (convergent adaptations).
Convergence
- Related species often inhabit certain land masses or island groups.
- Species on islands are usually related to those of the nearest continent.
- Many types of species are absent on oceanic islands.
Darwin: species on islands(2)
frogs on islands
- Fossils can often be arranged in evolutionary sequences.
- Some species vary from place to place, and the differences
are inherited.
Natural selection:
[see next topic outline for more details]
- All living species tend to over-reproduce.
- Most seeds, eggs, or hatchlings die without reproducing.
- All living species are extremely variable.
- Many of these variations are inherited.
- Inherited differences in survival and reproductive ability
(natural selection) bring about change in each generation.
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Study guide and vocabulary
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