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.