Evidence for Evolution Graduate Seminar

Introduction and overview

The three main claims of Darwinian evolution

• Living species are related by common ancestry

• Change through time occurs at the population not the organism level

• The main cause of adaptive evolution is natural selection (and related mechanisms)

The three main claims of Darwinian evolution

• Living species are related by common ancestry

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Common ancestor

The importance of common ancestry

• If two different species descended from a single ancestor then change (=evolution) is implied

All differences evolved along the lineages

What did people believe before Darwin?

• Separate ancestry (many versions)

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A special case: Jean-Baptiste Lamarck

• French Naturalist (1744-1829)

• The first scientific theory of evolution

• Struggled to reconcile evolution and the Scala naturae (great scale of being)

A special case: Lamarck’s theory

• Life progresses upward due to an internal drive towards perfection

A special case: Lamarck’s theory

• Life progresses upward due to an internal drive towards perfection

• Why are “primitive” organisms still around?

A special case: Lamarck’s theory

• Life progresses upward due to an internal drive towards perfection

• Why are “primitive” organisms still around?– Spontaneous generation of new

life constantly

Lamarck’s view

Because all species follow the same trajectory of origin: they will form a ladder of advancement

Lamarck’s view is basically separate ancestry

past

Lyell, C. Principles of Geology, Vol. II, Chap. 1

Common Ancestor

Species 2Species 1 Species 3

Common Ancestor

First articulation of common ancestry

Darwin envisaged evolution as a tree

The affinities of all the beings of the same class have sometimes be represented by a great tree. I believe this simile largely speaks the truth………The green and budding twigs may represent existing species; and those produced during former years may represent the long succession of extinct species…..….the great Tree of Life….covers the earth with ever-branching and beautiful ramifications

Charles Darwin, On the Origin of Species; pages 131-132

The only figure in The Origin of Species

Two claims

• That major groups of organisms share descent from common ancestry– vs. separate ancestry

• That all living organisms share descent from common ancestry– vs. several origins

Evidence for common ancestry (against separate ancestry)

• Fossil record

• Homology

• Vestigial Structures

• Classification

• Hierarchical distribution of traits

• Agreement between gene trees

• Evolution during domestication

Evidence for common ancestry• Fossil record

– Transitional fossils (sometimes in temporal sequence): consistent with the existence of real common ancestors

Living sister group

Major clade

Traits

Transitionalfossils

• Distantly related organisms share structural similarities

• Function varies

• Explicable by common ancestry

grasping

leaping

flying

swimming

running

Deep Homology

“fish” Amphibia Reptilia

Pentadactyl limb

human whale bat

Vestigial structures

• Structures that are non-functional (but functional in related species). For example:– Human appendix, tail bones,

– Gill slits in mammal embryos

– Hip bones of whales and snakes

– Eyes in cave fish

Trees explain patterns in trait distribution

Fur; milk

Amnion

Four legs; lungs

Vertebral column

Trees explain patterns in trait distribution

Fur; milk

Amnion

Four legs; lungs

Vertebral column

Applies a forteriori to molecular data

Molecular phylogeny of Hawaiian and other Campanulaceae

(Givnish et al.)

Hawaii

Biogeography: closely related species live near each other

Correlation among gene trees(Penny et al. 1982. Testing the theory of evolution by comparing phylogenetic trees constructed from five different protein sequences. Nature 297: 197-200.)

• When we estimate the phylogeny from different genes, we get trees that are much more similar than could happen by chance during separate ancestry

• Amenable to statistical analysis

We see diverse forms that are descended from single ancestor

Brassica oleracea

What about the claim of a single ancestor of all living organisms?• Shared biochemistry (e.g., same 4 nitrogenous bases, same

20 L-amino acids, ATP)– There are many possible nitrogenous bases and amino acids; Many

sugars could have form the NA backbone

– No chemical reason for L- vs. D-amino acids

• Shared structures (ribosomes, lipid bilayer membranes)

• Shared metabolic pathways (e.g., glycolysis)

• Share information processing (genetic code)– The code is a “frozen accident”

The three main claims of Darwinian evolution

• Living species are related by common ancestry

• Change through time occurs at the population not the organism level– No organism level mechanism is currently

plausible

• The main cause of adaptive evolution is natural selection (and related mechanisms)

The main cause of adaptive evolution is natural selection

• Claim 1: Natural selection happens– vs. Natural selection does not/cannot happen

• Claim 2: Natural selection is sufficient to explain even the most complex traits of living organisms– vs. natural selection is not sufficient

The main cause of adaptive evolution is natural selection

• Claim 1: Natural selection happens

• Evidence: – Artificial selection and rapid natural selection– Theoretically must occur if only minimal

assumptions are met• Genetic variation

• Limited resources

The main cause of adaptive evolution is natural selection

• Claim 2: Natural selection is sufficient to explain even the most complex traits of living organisms

• “Evidence”– On short time scales it is very effective– Time is long– No other natural mechanisms are known