Evolution: A start
Life’s variety and changes
Wallace’s Law
• Alfred Russell Wallace was the co-discoverer of
natural selection. He was a biogeographer who
made a living collecting specimens in South
America and Southeast Asia.
• In 1855 he published a paper on a pattern he
had observed in living things and the fossil
record:
– Every species has come into existence coincident
both in space and time with a pre-existing, closelyallied species.
Kangaroos again
Darwin
• How do we explain the often-striking
resemblances between relatives?
• By descent & heredity.
• But we can extend this idea to these kangaroos–
if they are descended from common ancestors,
we have an explanation of two things:
– Why they all live in and around Australia.
– Why they share all the traits that make them
kangaroos.
• This is also the obvious explanation of Wallace’s
law…
Feet and thumbs and design
• Young discusses the tree kangaroos’ feet:
• These kangaroos would climb much better
if they could actually grip branches with an
opposed toe. But kangaroo hind feet are
already heavily specialized for running on
the ground– so they lack opposing toes.
• We can make a similar case regarding
panda thumbs.
The Panda’s Thumb
Details
• The ‘thumb pad’ that pandas use to hold
bamboo and strip it is made from the radial
sesamoid bone of the wrist.
• In most mammals (and in particular in the
carnivora that the panda belongs to) this is a tiny
bone in the wrist.
• In the panda, it has been modified and enlarged
to the point where it can be used to pin bamboo
branches against the palm of the ‘hand’.
Comparison with a bear
• The difference in size
is obvious.
• So is the close
resemblance in the
rest of the paw’s
bones.
• In particular, both
have five clawbearing digits, the first
has 3 bones and the
rest have 4.
Makeshift
• These limits and imperfections in living things are direct
evidence for evolution, as opposed to design.
– Evolution holds that living things today are modified descendants
of earlier living things.
– The starting point that each form of life received in its ancestors
places limits on the kinds of changes/ adaptations it is able to
acquire.
– The forepaws of carnivores would be very difficult to transform
into the flexible, opposable digits of primates.
– The hindfeet of kangaroos are similarly limited.
– So the panda bear has a useful, but suboptimal, extension of its
wrist bone instead of a real thumb, and the tree kangaroo has a
useful but suboptimal modified hind foot that helps it to climb, but
not as much as a real gripping digit on the foot would.
The history of life
• This is a theme we’ve already explored, but the
comments and illustrations here add to it.
• New evidence about our near-relatives the great apes,
made Darwin’s avoidance of human evolution futile: a
close relationship between humans and these animals
was an inevitable conclusion for evolutionary thinkers.
• The role of religion here is also not news to us:
geologists and biologists were not skeptics eager to
undermine religion– quite the opposite, in fact. And the
religious community did not generally regard their work
as hostile to important religious doctrines and principles,
either. A ‘two books’ view of the matter was widely
accepted.
Expanding knowledge of the world
• This is also a theme we’ve touched on.
• But again, it’s beautifully illustrated here: The
gradual shift to more careful, more detailed,
more communicable and replicatable
observations is a fundamental change in how
naturalists worked.
• The mixture of myth, observation and symbolism
of early bestiaries contrasts with the detailed
skeletal comparison of a human and a bird.
• A link here to Foucault’s ‘clinical gaze’
John Ray, Carolus Linnaeus
Catalogue of Cambridge Plants
• 1660: 626 local plants described; Francis
Willughby joins Ray and supports his work.
• 626 is too few: Many new plants discovered,
both on closer search in Europe and in wider
investigation elsewhere.
• Microscopic life as well.
• A system of classification is needed to organize
all this (can’t just look at the book page by page
until you find a picture/description that fits what
you see).
Towards a system
• Classification shouldn’t be based on how we use or think
of these species (as food, as poisonous, as attractive,
noble, or vile and lowly…)
• It should be based on their intrinsic features and their
resemblances and differences from other species.
• Logical division (Aristotle) is an efficient way to organize
information and searches.
• We have a genus (kind) and species of the genus
(distinct members of the kind).
• Aristotle also ranked animals by ‘perfection’ on a ‘scale
of perfection’ or ladder of nature…
Ornithology
• An attempt to give a first-hand, observation based
account of various kinds of birds across Europe.
• 1676: Ornithology published.
• Logical division: land and water birds, curved and
straight beaks,… down to the level of species.
• Species listed & described one by one.
• This is much harder to do for plants, where different
individuals of the same species growing in different
environments can look very different.
• Perpetuation by seed the key to sorting this out.
Natural groupings
• Some birds clearly resemble each other
closely.
• Birds of prey, for instance: taloned feet,
hooked bills, sharp eyes, …
• They belong together naturally.
• Classification should reflect this.
Hierarchical Taxonomy
• This is a kind of taxonomy by logical division (except that
we allow more than two species per genus).
• But the chosen divisions are not arbitrary, because they
reflect many shared traits, not the presence or absence
of a single, arbitrarily chosen trait.
• So, for instance, all mammals have hair, suckle their
young, have 7 cervical vertebrae, teeth of different
shapes, etc. It’s natural to group them together in our
classification of animals because these and other traits
unite them all and distinguish them from other animals.
• Similarly for turtles, lizards, alligators and crocodiles; for
even-toed ungulates, for the various members of the
horse family, for passerine (perching) birds, for flowering
plants, for jellyfish, crustaceans, etc.
Linnaeus’ Ranks
• So we wind up with a number of ‘layers’ in our
taxonomy.
• We are animals (kingdom animalia),chordates
(phylum chordata), mammals (class mammalia),
primates (order), of the great ape family
(homminidae), human (genus Homo), and finally
humans of our particular species– the only living
species of genus Homo, sapiens.
• For short (as with all other species) we go by our
genus and species names, with the genus
capitalized: We are Homo sapiens.
Linnaeus
• Proposed genus, species binomials for each
species he described, coining the oldest
accepted scientific terms for many species of
animals and plants.
• Proposed the further, higher ranks of kingdom,
class and order.
• We’ve added families (grouping genera and
grouped together into orders), phyla (grouping
classes grouped into kingdoms), and
superkingdoms (grouping the kingdoms into the
three main divisions of life, archaebacteria,
bacteria and eukaryotes).
The Tree of Life
• The result of this work is a tree of life, with
each species occupying one twig of one
branch of some larger branch, etc.
• This structure of groups within groups,
reflected not in an arbitrary choice of how
to group these things but in strong, multicriterial similarities linking the groups
together, is a challenge to biology: How
should it be explained?
Teleology
• Since Newton physics has done without final
causes.
• That is, nothing in physics is aimed at any
particular outcome– what happens is the result
of the forces & motions of physical objects.
• These are the efficient causes that bring about
change over time: gravity works the same in
causing objects to fall and planets to orbit; if the
circumstances are such that these forces lead a
comet to strike the earth, that is what it will do–
but if they are such that it will miss, it will miss.
Teleology in Biology
• But it’s different in biology– even today, we think
of organisms as aiming at ends– as seeking
nourishment, mates, safety etc.
• And we think of their parts as serving purposes
too– hearts don’t just pump blood, that’s what
they’re for, and lungs don’t just oxygenate our
blood, that’s what they’re for.
• At the same time, if we want to explain how the
heart pumps blood, it’s a matter of structure and
forces, that is, the explanation is based in
efficient causes just like physics.
Ray on the eye
• So we find John Ray discussing the eye.
• On one hand, the new science of optics provides a much
more detailed efficient causal understanding of how the
eye works.
• On the other hand, the intricacy and refinement of the
various parts and their functions is all the more striking
as we understand all these details.
• So, while we understand that it works in a way that
physics can explain, how the intricate arrangement of
parts needed to make sight possible came to be remains
a mystery.
• For Ray, it’s a case of the natural world pointing beyond
nature to a designer.
Our place in the chain
• From nature’s chain whatever link you strike,
tenth or ten thousandth, breaks the chain alike.
Alexander Pope.
• The chain of being is an alternative to
hierarchical (groups within groups) classification.
• It orders everything from mineral to man and
beyond in a scale of perfection, each separated
from the next by a very slight difference.
• As the ‘highest’ of the animals, humans occupy
the top of the chain of living things; only angels
(of varying degrees) and God rank above us.
Filling in the chain
• But the doctrine of the chain suggests there should be
animals that are only a little bit below us… what are
they?
• The chimpanzee made a very good candidate, as Tyson
showed with his detailed anatomical comparison.
• This comparison is none the worse for having been
made in the service of a view of taxonomy that is now
known to be wrong.
• We can say the same for many other observations made
during this time, both in biology and in geology. Careful
observations and measurements rarely go to waste,
even if our initial understanding of them is misguided.
Noah’s ark, in passing
• Thinking about the flood was stimulated by
Kircher’s unseaman-like work on the ark.
• Burnet followed up with his view of the
present world as the ‘wreck’ of paradise.
• John Woodward competed with Burnet,
trying to use the specific gravity of different
materials (and fossils) to explain the layers
he thought had been laid down during the
flood.
The System of Nature
• Linnaeus, as we’ve seen, rejected the chain of
being in favour of a hierarchical taxonomy
grouping species, genera, orders and classes
together.
• He chose (somewhat arbitrarily) to classify
plants by their sexual parts (numbers/
arrangments of stamens/pistils).
• But still succeeded in organizing both his
hierarchical taxonomy and a binomial
nomenclature of living things. These have been
central to biology ever since.
Buffon
• Buffon didn’t follow Linnaeus’ taxonomy.
• But he did contribute to developing/ setting out new
evidence of species around the world.
• An early ecological principle: The earth makes the
plants; the earth and the plants make the animals.
• But oddly, plants and animals differ around the world
even when they live in very similar climates/soils/ etc.
• Comparisons of different species that are clearly very
nearly related led Buffon to suppose that (for instance)
buffalo and European cattle were really just different
‘branches’ of the same species.
• Buffon regarded such changes as due to ‘degeneration’,
but couldn’t offer much of a story about this process.
Biogeography
• Further work on the distribution of organisms led
to the notion of regional floras and faunas.
• The idea that life had spread out from one place
grew less and less tenable as these different
faunas and floras (and the barriers to easy
migration) became better known.
• Here another sort of final cause was proposed:
the ‘harmony of nature’. Plants and animals in a
region ‘fit together’ into an optimal sort of
interaction, in which each (even the smallest and
lowest) served a role in maintaining the system.
Elephant bones
• There are three species of elephant today– African
(Loxodonta africana), Asiatic (Elephas maximus), and
the African forest elephant (Loxodonta cyclotis).
• Fossil hunters soon appreciated that the mastodon (with
its tusks etc.) and mammoth were also forms of
elephant.
• Cuvier showed not only that the two known species were
distinct– so distinct they belonged in different genera–
but also that the fossil species differed from the African
and Asiatic elephants so much and so consistently that
they had to be different species. This was widely
accepted as the final proof of extinction.
Elephants: L. africanus, E.
maximus, Mammuthus primigenius,
Mammutus americanum
Elephant teeth
Cuvier’s Rules
• Cuvier proposed two main rules for
understanding/ interpreting organisms.
• The ‘correlation of parts’ held that every animal’s
parts were constrained by its way of life, so that
we could infer one part’s form from an others’.
(Carnivore teeth and claws; herbivore teeth and
hooves…)
• The ‘subordination of character’ declared that
the parts most useful to classification were those
least altered by the specific needs of an animal’s
way of life.
Cuviers’ Conclusions
• For Cuvier the correlation of parts ruled out evolution,
since each part had to be ‘just so’ for it to serve the
animals needs properly. This concept of ideal adaptation
played an important role in thinking about biology for
some time.
• The subordination of character, on the other hand, made
it clear just how much different animals resemble each
other.
• But Cuvier’s work in comparative anatomy set a new
standard of detail and accuracy.
• His success at reconstructing fossil animals showed that
there was something to his notion of the correlation of
parts– animals very similar in one part often turned out to
be very similar in many other respects.
Paleontology
• Cuvier also did a lot of work in
paleontology, from very recent fossils (still
obviously distinct from todays) in the
gravels around Paris to much older and
very different fossils (paleotherium) in
tertiary formations.
Early evolutionary ideas
• Erasumus Darwin:
– a drive to adapt, rooted in lust, hunger, etc. (i.e. in an
organism’s needs), leads species to change.
• Lamarck.
– Traits developed in active struggle are passed on to
offspring.
• Evolution as an alternative to extinction:
– History needs new forms of life, but does it require
extinction, or just gradual change in the old forms?