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Transcript
The Species Question
More trees and natural selection,
too.
Vestiges
• Robert Chambers book was influential and
popular, but scientifically sloppy and
inaccurate.
• It embodied the progressive spirit of the
age, viewing evolution as an upward climb
to greater and greater perfection.
• But it did bring the issue of species (and
especially where they come from) to public
attention.
Lyell
• Volume II of Principles of Geology took on the history of
life in detail.
• Among other things, Lyell concluded that species have a
‘real existence’– hybrids between species, though not
unusual, are rarely fully fertile, and cross-breeding is rare
under natural conditions. So the line between species
seems to be substantial.
• Lyell concluded that species started out more or less as
they are now.
• But, while he had a clear account of how species came
to be extinct, he had no credible story to tell about how
new ones arise.
Further observations
• The existence of distinct ‘floras’ and ‘faunas’ in different
places, for Lyell, showed that collections of species are
linked to particular regions.
• This suggests an adaptational claim: That the species of
a region are interdependent. (The beginnings of
ecological thinking here…)
• Some species clearly can do well outside their actual
range; their restriction to a particular region is explained
by geographic barriers to migration/spread.
• The lack of evidence for the development of new species
is not surprising– it would be unusual and local. We may
never witness it!
• This is ugly–part of our understanding of life is beyond
observation (& out of our theories’ reach as well).
Accounts of ‘Creation’
• Primary cause: God.
• Secondary (or intermediate) cause: a
regular natural process of some kind.
• If God simply does something directly,
then it will be beyond science to account
for it.
• But if God employs secondary, regular,
natural causes, then we have some hope
of understanding them.
Scientific method
• Vera Causa: A true cause. For Herschel, a true
cause is one that we can confirm to be in
operation, and that plays a systematic
theoretical role (rather than being a mere
observed regularity).
• Herschel clearly favoured secondary causes as
the source of new species: The evidence, he
thought, shows that God does not work directly
in the world, but via intermediate causes.
• But the open question remained: what causes
could be responsible for producing new
species?
Whewell
• Thought we needed a very new sort of cause to
account for new species.
• Emphasized adaptation in defending this view:
without a natural cause for adaptation, divine
design was the only game in town.
• Emphasized consilience in his philosophy of
science– the role that explanatory ideas play in
unifying the phenomena, bringing many distinct
aspects of the world under a single principle.
Darwin
• Born February 12, 1809.
• An indirect route to becoming a naturalist: Edinburgh,
then Cambridge (with an eye to a life as a clergyman).
• But an interest in natural history was clear from the
outset (considered a good hobby for a clergyman at the
time).
• Encouraged by Henslow, met Sedgwick and Whewell.
• Planned a naturalist trip to follow his graduation, but it
fell through (got some training in geology with Sedgwick
in preparation for the trip).
• Invited to sail on the Beagle (Dad refused, but relented
after Darwin’s grandfather intervened).
The trip
• Left in December 1831. Circumnavigated the
world, from England to South America through
the Straits of Magellan & up the coast, then to
the Galapagos, through the south sea islands, to
Australia and across the Indian Ocean to South
Africa, briefly back to South America and home
to England in late 1836.
• First scientific fame for his work on geology
during the trip.
Afterwards
• The collections of animals and plants provided much
work for Darwin and others (including Owen, Gould).
• Noticed many important facts: The resemblance of life
on the Galapagos to South American species, even
though the species (say, of land birds) were clearly
different, and even different from island to island.
• Absence of amphibians, most mammals on the
Galapagos.
• The idea of migration followed by evolution.
• The presence of fossils showing resemblance to animals
typical of the regions today (giant armadillo, sloth in SA).\
• All fit with an evolutionary hypothesis.
But what makes evolution happen?
• Evolution, as Darwin realized, would give us a
branching pattern, like that of taxonomy,
development and (as we’ll see) the fossil record.
• But to say that evolution explains the origin of
new species is just a start. Darwin shared
Whewell’s concern about adaptation. And
without some cause for evolution that could
explain adaptation, the theory remained
unsatisfactory.
The dismal science
• Darwin is aware, as many are, that natural
selection has an influence on wild populations,
destroying the weak and sickly while preserving
the strong and healthy.
• Finally, after reading Malthus, Darwin finds a
‘theory by which to work’.
• Malthus argued that our capacity for
reproduction is so great that it will outstrip any
resources we can draw on to keep ourselves
alive. There is no escaping the struggle for
existence.
Reproduction, variation and
heredity
• Consider a variation that enables an individual to
do better than others in the population at
surviving and reproducing.
• If the variation is heritable, then any organism
that has it will not only tend to out-reproduce its
fellows, its offspring are also likely to have that
same advantage.
• Over time the variation will spread through the
population (i.e. be had by a greater and greater
proportion of the members of the population).
Vera Causa
• Experience with breeding makes it very clear that this
can happen.
• The Malthusian argument shows that a similar (but
undirected!) process of natural selection will occur in wild
populations.
• So it’s clear that Darwin has a vera causa here, by
Herschel’s standards.
• But even his friend Hooker didn’t accept the theory on
first reading. Worse, Vestiges’ harsh reception showed
clearly that evolutionary ideas were not widely
acceptable.
• Darwin was cautious– he chose to work on his theory,
refining and extending it, before publishing.
More tree stuff
• Richard Owen: homologies and
archetypes.
• A homology is ‘the same organ in different
animals under every variety of form and
function’.
• An archetype is the basic structure shared
by all the members of a group (in
particular of very broad groups such as
Cuvier’s embranchements).
Same organ?
• Owen was pointing out here the detailed patterns shared
between flippers, hands, paws, etc. among mammals.
• Similar patterns of detailed shared structure (that is not
essential to function) unite swim bladders in fish and
lungs in land-dwelling vertebrates, eyes in all
vertebrates, hearts in all vertebrates, etc.
• But what does he mean by ‘same organ’ here? For
Owen (after some consideration) it was an abstract
notion– a kind of correspondence between different
animals with no more basic explanation in biology.
Archetypes
• A shared pattern uniting a
large group of animals.
• Again, not essential to
function.
• But a key part of
classification.
• For Owen (after
consideration) another
abstract item: An idea in
the mind of God that
guides/shapes the form
each individual species of
vertebrate has.
Application
• Applying homologies and archetypes to
classification helped Owen re-think how to
classify both fossils and present life.
• It gave a more detailed, precise approach
to arriving at the taxonomic tree.
The fossil record
• Not only a progression from older (more
different) life to more recent (familiar) life.
• Within each group, there is a shift from animals
having the most general features of the group to
a wider range of more specialized types.
• That is, the fossil record also demonstrates a
tree structure, as Owen, Carpenter and esp.
Heinrich Bronn showed.
Biogeography
• Wallace (1855) and others reinforced Darwin’s
observation that plants and animals from a region tend to
show family resemblance– both in the case of fossils and
living forms.
• So biogeography also has a kind of tree structure from
the point of view of taxonomy: many closely related
types (the marsupials of australia, sloths & armadillos in
South America, Darwin’s finches in the Galapagos,
honey creepers in Hawaii & many more) appear together
in particular regions.
• Wallace’s law: Every species has come into existence
coincident both in space and time with a pre-existing,
closely allied species.
Finches & Honeycreepers
The upshot
• Darwin had undertaken many projects to back up his
idea– pigeon breeding, correspondence with breeders,
biologists, collectors, a detailed examination of the
biology of barnacles…
• Wallace came up with the same idea, natural selection,
and sent his paper to Darwin.
• In the end two papers were read, one by Darwin (built
out of the 1844 essay and a letter to Asa Gray) and
Wallaces. Natural selection was in the open at last.
• Darwin gave up working on a very large and scholarly
book, and prepared instead a short abstract of his views
and evidence: On the Origin of Species by means of
natural selection.