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, closely-allied 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 claw-bearing 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, multi-criterial 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?