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).
- 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.
- Island species were usually related to those of the nearest continent.
- Similar environments did not always produce the same species or
related species (contrary to Lamarckism and Geoffroyism).
On the Origin of Species (1859):
In this book, Darwin explained all the above facts by "descent with
modification," a branching form of evolution very different from
earlier theories. This book also suggested natural selection as a
mechanism for evolutionary change:
- 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.
Darwin quote
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.
- These homologies may include vestigial remnants of once-useful parts.
- Similar adaptations often occur under similar circumstances, even
in unrelated species (convergent adaptations).
- Related species often inhabit certain land masses or island groups.
- Fossils can often be arranged in evolutionary sequences.
- Some species vary from place to place, and the differences
are inherited.
Evidence for natural selection:
- All living species are highly adapted to their way of life.
- Many adaptations cannot be explain by environmental influence alone.
Examples:
- Unrelated but ecologically equivalent species live on different continents.
- Some embryonic structures (e.g., a flap in the human heart that
seals closed at birth) develop before they become useful
- Some behavior (like bird migration or nest building) occurs
in advance of its usefulness.
- Some adaptations are less than perfect, contrary to an earlier theory
that used perfect adaptation to prove divine creation.
- Natural selection has repeatedly been documented (e.g., among
peppered moths in England), and has resulted in changes over time
in natural populations.
- Artificial selection by animal and plant breeders has produced many new
adaptations, some of them similar to adaptations occurring naturally.
Mimicry and camouflage:
Many species gain protection against predators by resembling
their background (camouflage) or by falsely resembling other
species (mimicry).
In Batesian mimicry, a palatable
species resembles a distasteful or harmful one.
Mullerian mimicry
is resemblance among distasteful or harmful species.
- Mimicry works only when certain models are present,
a fact explained easily by natural selection, but not by Lamarckism
or similar theories, nor by theories of special creation.
- Mimicry may vary geographically, with the same mimic species
resembling different models in different places. Natural selection
can explain this; Lamarckism cannot.
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