Evolution is the phenomenonof modification with descent

(it is not natural selection)

Big questions in evolution

• Why do species change?

• How do new species arise?

• Why do species go extinct?

EquusHyracotherium (50MY)

Hawaiian Drosophila

A (very brief) history of evolutionary thought

• Aristotle and Plato

– Archetypes (i.e. no evolution, variation is meaningless)

• Buffon, Cuvier, Smith

– Importance of fossils. Earth may be old.

• Hutton, Lyell

– Current geological processes can explain earth, but only on a long time-scale

• Lamarck and Erasmus Darwin

– Transmutationism

– Inheritance of acquired characeteristics or ‘use and disuse’

– Universal common ancestor

Darwin and the Beagle: 1831-36

<<insert picture p432 Campbell>>

What Darwin said

Organisms produce too many offspring

Heritable differences exist in traits influencing the adaptation of an organism to its environment

Organisms that are better adapted have a higher chance of survival

Also Alfred Russell Wallace – letters to Darwin prompted publication of ‘On the Origin of Species’

Darwin’s finches

Geospiza

The Huxley – Wilberforce debate

I asserted - and I repeat - that a man has no reason to be ashamed of having an ape for his grandfather. If there were an ancestor whom I should feel shame in recalling it would rather be a man-a man of restless and versatile intellect-who, not content with an equivocal success in his own sphere of activity, plunges into scientific questions with which he has no real acquaintance, only to obscure them by an aimless rhetoric, and distract the attention of his hearers from the real point at issue by eloquent digressions and skilled appeals to religious prejudice.

Arguments against Darwin

• Biblical

– Earth is only about 4000 years old

• Tautological

– Which are the fitter organisms? Those which survive.

• Paley’s blind watchmaker

– How can complex structures arise by a ‘blind’ process?

• The problem of heredity

– How can natural selection work?

Gradualist steps in the origin of the molluscan eye

Darwin and heredity

• Darwin viewed heredity as a blending process

• BUT blending inheritance removes variation which is essential for natural selection

• Francis Galton’s law of heredity attempted to provide a mathematical basis for Darwin’s

views (later reinterpreted by Karl Pearson)

• Major controversy in the early 1900s arose between those who supported Darwin’s view

and those (led by Galton) who rejected them

What is the evidence for evolution?

• The fossil record

– Origin of diversity

– Continuity of form

– Simultaneous shifts in climate and fossil fauna

• Direct observation

– Industrial melanism

– Drug resistance

– Experimental

• Species complexes

– Hybrid zones

– Ring species

• Homology

– Similarity through common descent

– From anatomy to molecular biology

The fossil record – different forms of life

Ediacara Burgess Shale

Dickensonia~ 550 MY

Marella~ 500 MY

Burgess shale

Fossils – continuity of form

Homo erectus1.8 MY

Homo sapiens (early form)0.18 MY

Homo sapiens (modern)0.01 MY

How are fossils formed?• unaltered preservation

– like insects or plant parts trapped in amber, a hardened form of tree sap

• permineralization=petrification

– in which rock-like minerals seep in slowly and replace the original organic tissues with silica,

calcite or pyrite, forming a rock-like fossil - can preserve hard and soft parts - most bone and

wood fossils are permineralized

• replacement

– An organism's hard parts dissolve and are replaced by other minerals, like calcite, silica, pyrite,

or iron

• carbonization=coalification

– in which only the carbon remains in the specimen - other elements, like hydrogen, oxygen, and

nitrogen are removed

• recrystalization

– hard parts either revert to more stable minerals or small crystals turn into larger crystals

• authigenic preservation

– molds and casts of organisms that have been destroyed or dissolved

Evolution in action I – Industrial melanism

Melanic and wild-type Biston betularia

Frequency of melanic form in England(1959-1995)

Evolution in action II – Drug resistance in HIV

• RT inhibitors

– Block reverse transcription

• Nucleotide RT inhibitors

– Chain terminating nucleotide analogs

– Some escape mutations avoid incorporating altered bases

– Others can excise them if incorporated

• In absence of drug modifications are deleterious

– Compensatory mutations can occur which reduce/remove deleterious effect

Ring species

• Continuously interbreeding forms between two or more separate species

– e.g. Ensatina salamanders in California

Homology – inference of shared ancestry

• Structures similar by descent

– wings of birds and bats homologous to arms of humans and flippers of

dolphins

– subunit alpha- and beta-globin in haemoglobin similar through descent

Structural similarity between Acetylcholinesterase and Calmodulin (Tsigelny et al, Prot Sci, 2000, 9:180)

Is species change inevitable?

Coelocanth

Tuatara

Dicksonia tree fern

Further controversies in the evolution debate• Gradualist v punctuated?

– Fossil record shows abrupt changes of form separated by long periods of

stasis

– ‘Punctuated Equilibrium’ – Gould and Eldridge

– BUT fast in fossil record is millions of years

– Fossil record very patchy

– Gradualist evolution can occur fast (e.g. Homo sapiens)