Charles Darwin (1809-1882)
The voyage on the Beagle (1831-36):
Reads Lyell’s Principles: uniformitarianism and deep time
Darwin’s finches in the Galapagos with their gradation of beaks, and each
island being like a little world, with a high number of endemic species.
Darwin’s The Origin of Species (1859) many editions with enlargements
and modifications. The two basic ideas are that of the tree of life and
of natural selection.
Tree of life: living organisms are organized as the limbs of a great
tree, with more general groups branching into lesss general ones. Moreover:
- species change over time (transmutation)
- some species go extinct (extinction)
- some species keep diverging, splitting eventually into multiple descendent
species (common descent).
NOTE:extinction and common descent are not necessary for transmutation. For
example, Lamark rejected extinction and common descent (species don't split).
Natural Selection is the principal mechanism for descent with modifications
(evolution). Its existence can be established by considering the following:
First Fact: The Struggle for Existence
- intraspecific: two canids struggling for scarce food;
several mistletoes parasitizing the same tree and trying to attract birds
to disseminate their seeds, etc.
- interspecific: the same mistletoes competing with other
fruit bearing plants; a plant’s seed germinating in a crowds field, etc.
- environmental: a plant at the edge of the desert struggling
against drought, etc.
NOTES:
In equatorial areas, the struggles is mostly intra and inter-specific, while
in hostile environments (e.g., Arctic regions, or high mountains) the competition
is mostly environmental.
Intrageneric struggle is very intense because species of the same genus are
close in structure and often in habits. This is the basis for the Principle
of Divergence, by which the more diversified the descendants from one species
become, the more successful they’ll be.
The struggle for existence “inevitably follows” from the high rate at which organisms tend to increase if unchecked. Examples:
- an annual plant producing only 2 seeds a year will have 2n
descendants in n years. In general, organisms tend to increase
in geometric ratios.
- domestic animals run wild in new environments often have population
explosions.
“The face of Nature may be compared to a yielding surface, with ten thousand sharp wedges packed close together and driven inwards by incessant blows, sometimes one wedge being struck, and then another with greater force.”
Second Fact: Individual Natural Variation and Its Transmissibility to Progeny
This is evidenced in the selection of breeders concerning plants and animals (dogs, cats, pigeons).
NOTE: as to the mechanism of variation and transmission, D. is silent in The Origin. However, in The Variation of Animals and Plants under Domestication (1868) he adopts pangenesis (gemmulae and quasi-Lamarckism)
Importance of cross pollination (the mixing of traits from the parents).
Struggle for existence plus individual natural variation and transmissibility to progeny produce Natural Selection, the mechanism of evolution. Natural selection consists in the preservation of favorable variations and the elimination of injurious ones, while neutral variations are left fluctuating.
The result of natural selection is Evolution (descent with modifications). Species, then are just “strongly marked and permanent varieties.”
All animals descend from four or five projenitors, and so do plants; moreover, given the similarities between plants and animals (e.g., in their reproductive processes) it’s not absurd to believe that all life may descend from a common origin.
NOTE: Saying that an rganism originated because of Natural Selection does NOT
amount to saying that it is the product of chance