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Outline for Week 2these notes represent 1/3 of the slides shown in class

Darwin’s Descent with modification– Geological evidence – Bio-geographical evidence

Logic of Darwin’s theory

Virus Evolution

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Darwin’s Ideas BEFORE his Voyage

• Fixity or Immutability of Species– each species was a single, unchanging

entity, separate from other species and constant in its fundamental, defining characteristics.

• Evolution or the Transformation of Species– was a speculative idea, espoused by

some (including his grandfather, Erasmus Darwin) but with little evidence and no mechanism(s).

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Key insights from Lyell for Darwin

• Gradualism - weak forces acting over long periods could produce as large geological changes as catastrophic forces produce in a short period of time.

• Uniformitarianism - geologic processes acting today are similar to those occurring throughout Earth’s geological history. They account for all of Earth's geological features: “the present is the key to the past.”

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Isabela

Darwin

Wolf

Pinta

Marchena Genovesa

Fernandia

SantiagoBartolomé

RåbidaPin zon

SeymourBaltra

Santa Cruz

Santa Fe

Tortuga

Española

San Cristobal

Floreana

EQUATOR

GalåpagosIslands

Galapagos Islands

Where are the Galapagos Islands?

What kind of islands are they?

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Bio-geographic Evidence from the Voyage

Oceanic Islands• Formed by Volcanoes.

• Far from Mainland.• Each island had a unique

group of species• Species on young islands

more similar to mainland species

• Descended from a colonizing ancestor with modification

• Unknown Mechanism

Continental Islands• Formed by breaking off

from the Mainland.• Near to Mainland.• Species were a subset of

those on the nearby mainland.

• Source of species, colonization from mainland.

• Recurrent migration

Two Kinds of Islands

Darwin’s Hypotheses

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Galapagos finchesDiffer in Beak Shape and

Diet

7Darwin’s Hypothesis

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Hawaiian honeycreepers ≈ Galapagos Finches

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Bio-geographic Evidence from the Voyage

South American Fossils• Extant species resembled

the fossils of S. A. more closely than fossils of Europe from the same strata.

• Extant Species Descended with modification from Fossil Species

• Unknown Mechanism

European Fossils• Extant species resembled

the fossils of Europe more closely than fossils of S. A. from the same strata.

Extant Species Descended with modification from Fossil Species Descent with modification

• Unknown Mechanism.

Darwin’s Hypotheses

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Darwin’s Ideas after the Voyage

• Fixity or Immutability of Species: Unlikely.

• Evolution or the Transformation of Species: Likely

– Much bio-geographic evidence but – No mechanism(s) to explain how or why

species change.

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The ONLY Figure in The Origin of Species.

It is THE most important Figure for you to understand inregard to this class.

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Darwin’s Theory of Descent with Modification

Questions: • How similar are two species? • How many traits do the two species share? Darwin’s Answer: When did the two species last

share a common ancestor?• If they share a recent common ancestor, then the

two species will be very similar and share many traits.

• If they share a distant common ancestor, then the two species will not be as similar and they will share few traits.

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Present:Living

Descendants

1, 2, and 3 Are living

Species today

Past:Fossil

Ancestors

1

2

3

SpeciesExtinctions:

branches that terminate before

the Present

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Present:Living

Descendants

Past:Fossil

Ancestors

Extinctions: branches that

terminate before the present

A NODE is the common ancestor of all species on the branches that radiate from the node.

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2

3

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Present:Living

Descendants

Past:Fossil

Ancestors

Extinctions: branches that

terminate before the present

Closely related species share a more recent common ancestor (MRCA) than more distantly related species

1

2

3

MRCA of 2 and 3

MRCA of 1 and 2

or1 and 3

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Darwin knew his theory would be controversial". . . We see this in the plainest manner by the fact that all the

most eminent paleontologists, namely Cuvier, Owen, Agassiz, Barrande, Falconer, E. Forbes, &c., and all our greatest geologists, as Lyell, Murchison, Sedgwick, & c., have unanimously, often vehemently, maintained the immutability of species. But I have reason to believe that one great authority, Sir Charles Lyell, from further reflexion entertains grave doubts on this subject. I feel how rash it is to differ from these great authorities, to whom, with others, we owe all our knowledge. Those who think the natural geological record in any degree perfect, and who do not attach much weight to the facts and arguments of other kinds even in this volume, will undoubtedly at once reject my theory. . . ." -Darwin, Charles On the Origin of Species, chapter 9.

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Darwin’s theory proposed to explain diversity by modification of descendants

through time, BUT he desperately needed a mechanism to explain how modification occurred and caused the

remarkable fit of organisms to their environments (i.e. adaptation).

Extraordinary claims require extraordinary evidence

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Interesting Additional Readings

Voyage of the Beagle

Darwin’s Autobiography

The Growth of Biological Thought

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The mechanism causing adaptation is Natural

Selection