From Darwin to the Present

EVOLUTION

Part I: Darwin and his Theory

“Sit down before a fact as a little child, be prepared to give up every preconceived notion. Follow humbly wherever and

to whatever abysses nature leads. Or you shall learn nothing.” Thomas H. Huxley

EVOLUTION BEFORE DARWIN

• Like all great theories, Darwin’s ideas did not spring from his brow like

Athena from Zeus’ brow.

• Many before him had observed nature and thought about the vast

diversity of life.

• Here are some observations and a few theories leading to Darwin,

whose masterstroke pulled them together in one great theory to explain

them all:

EVOLUTION BEFORE DARWIN:

OBSERVATIONS

• Taxonomy

• Fossil Record

• Homologies

• Embryology

• Vestigial Organs

• Biogeography

• Animal husbandry

EVOLUTION BEFORE DARWIN I:

TAXONOMY

• Since the early Greeks, if not before, naturalists recognized that some

kind of hierarchical relationship existed between groups of organisms.

• Aristotle (384-322 BCE)

• Organized animals into his “scala naturae” by degrees of complexity

(11 grades according to the complexity of structure and ability to

move)

• Developed a binomial nomenclature: Genos is broad category (e.g.,

mammals), Eidos is specific (e.g., tiger, horse, etc.)

EVOLUTION BEFORE DARWIN I:

TAXONOMY (CONT’D)

Aristotle• proposed several groups for animals:

Animals with blood•

Live• -bearing

Egg• -laying

Animals without blood•

Shelled•

Non• -shelled

Also separated marine mammals from fish, and grouped sharks and rays •separately as Selache (modern selachii).

EVOLUTION BEFORE DARWIN I:

TAXONOMY (CONT’D)

• Carl Linnaeus (1707-1778)

• Classified nature within a nested hierarchy, starting with two

kingdoms (animal and vegetable). Kingdoms were divided into

classes and they, in turn, into orders, and then genera, which were

divided into species.

• Linnaeus' groupings were based upon shared physical

characteristics.

• Linnaeus is credited with establishing the idea of a hierarchical

structure of classification which is based upon observable

characteristics and intended to reflect natural relationships.

THE LINNAEAN HIERARCHY

Kingdom

• Phylum

• Class

• Order

• Family

• Genus

• Species

EVOLUTION BEFORE DARWIN I:

TAXONOMY (CONT’D)

• But what was lacking was any idea of the cause for the

relationship.

• Most naturalists were satisfied with the theological

explanation that each species had been individually and

divinely created. Most held that “types” of organisms, divinely

created, were perfect and immutable.

EVOLUTION BEFORE DARWIN II:

FOSSIL RECORD

Fossils showed a progression of change in organisms over time.•

Fossils in older (deeper) strata were different, often in subtle ways, •

from those in newer (more superficial) strata, yet clearly they were

the same species of animal or plant. So species were not immutable.

Some fossils clearly resemble modern organisms.•

Some species (e.g. dinosaurs) had no modern counterpart. Hence some •

species died out.

The theological explanation of Noah• ’s flood was invoked to explain

this, but had difficulty with the layering.

EVOLUTION BEFORE DARWIN III:

HOMOLOGY

• Many animal structures show a

remarkable similarity even though

highly modified, indicating a strong

degree of relatedness:

• Hands, wings (birds and bats), fins

of lobe-finned fish), etc., all have

the same skeletal structure.

EVOLUTION BEFORE DARWIN IV:

EMBRYOLOGY

• Animals of very

different species show

remarkable similarities

in early embryological

stages. Only later do

they differentiate.

Again, this

demonstrates their

relatedness.

EMBRYOLOGY (CONT’D)

• Free-swimming larvae of

very different marine

crustaceans bear

remarkable similarity

EMBRYOLOGY (CONT’D)

All terrestrial •

vertebrates show at

some time in their

embryogenesis gill

arches. These later

develop into useful

structures.

Developmental anomalies:

EVOLUTION BEFORE DARWIN V:

VESTIGIAL ORGANS

• Many animals show vestigial (nonfunctional) organs:

• Human appendix, coccyx

• Teeth in baleen whales

• Eyes in some cave-dwellers

• Leg bones in whales, snakes

EVOLUTION BEFORE DARWIN VI:

BIOGEOGRAPHY

• Clearly related animals show discontinuous distributions. The groups,

while related, show modifications:

• Camels (Asia and Africa) and llamas (South America)

• Indian and Asian Elephants

• Finches of South America and the Galapagos

• Some groups on different continents are very similar (Europe and North

America) while others on other groups are only distantly related

(Australia)

EVOLUTION BEFORE DARWIN VII:

ANIMAL HUSBANDRY

• From earliest times farmers and herdsmen have known that

by selecting their breeding stock based on desirable traits,

and continuing this over many generations, the original stock

could be dramatically modified.

• E.g., domestication; breeds of dogs; etc.

EVOLUTION BEFORE DARWIN:

THEORIES

Several prominent theories sought prior to Darwin to •

explain the diversity of life :

Divine creation•• Scala Naturae (the Great Chain of Being)

o Began with Aristotle, but continued (with

religious modifications) up to Darwin

o Believed that there was some “intrinsic

force” in all living things that caused them

to change in the direction of (initially) man

or (later) “increasing complexity”.

EVOLUTION BEFORE DARWIN:

THEORIES• Erasmus Darwin (1731-1802)

• Charles Darwin’s grandfather

• Published “Zoonomia” (1794-1796), anticipating much of Charles’ thinking on evolution by over fifty

years:

• “Would it be too bold to imagine, that in the great length of time, since the earth began to exist,

perhaps millions of ages before the commencement of the history of mankind, would it be too

bold to imagine, that all warm-blooded animals have arisen from one living filament, which THE

GREAT FIRST CAUSE endued with animality, with the power of acquiring new parts, attended

with new propensities, directed by irritations, sensations, volitions, and associations; and thus

possessing the faculty of continuing to improve by its own inherent activity, and of delivering

down those improvements by generation to its posterity, world without end!”

• Also anticipated natural selection:

• “The strongest and most active animal should propagate the species, which should thence

become improved"

EVOLUTION BEFORE DARWIN:

THEORIES

• Lamarck (1744-1829) :

• Came closest to a real theory of evolution in his “Recherches sur

l’Organisation des Corps Vivants” (1802)

• Acknowledged that the descendants of organisms change over time

in response to environmental pressures (a marked reversal of his

earlier opinions, and possibly influenced by the French Revolution)

• Considered this modification due to the

“inheritance of acquired

characteristics” (Similar to Erasmus

Darwin’s idea).

• E.g., the giraffe’s neck

CHARLES DARWIN: A VERY BRIEF BIOGRAPHY

Born in Shrewsbury, • Shropshire, February 12, 1809. Father: Robert Darwin,

a wealthy society doctor; Mother: Susanna Wedgewood, of the well-to-do

Wedgewood family.

• Was sent to Edinburgh in 1825 to study medicine. Was

appalled by blood and found medicine “boring”.

Neglected his studies to spend time with others in a

naturalist society (the Plinian Society)

• His father, annoyed, sent him to Christ’s College,

Cambridge, for divinity studies. He graduated in 1831,

10th out of a class of 178.

• Signed on in 1831 as expedition naturalist on the HMS Beagle, on

a five-year voyage to chart the coastline of South America

BRIEF BIOGRAPHY (CONT’D)

Darwin’s observations on the Beagle gained him

immediate fame at home in England when he

returned in 1836

He soon became acquainted with Charles Lyell

(geologist: theory of uniformitarianism)* and Richard

Owen (paleontologist: fossils of extinct animals

related to those Darwin had found in South America).

From 1836 to 1858, he worked on his theory of evolution, carefully documenting it, as he knew it to be

controversial.

In 1838 Darwin read and “was impressed by” the theories of Malthus on population.

In January 1839, he married Emma Wedgewood and was elected a Fellow in the Royal Society. Henceforth

money was not a problem, though frail health was.

*provided Darwin the idea that the earth is millions of years old, and that small

changes accumulate over the millennia.

BRIEF BIOGRAPHY (CONT’D)

• July of 1844: puts down his musings about natural selection in a 230-page “essay” (as

insurance in case he were to die before publication, as he was rather ill).

• From 1842 to 1846, wrote two books, one on coral reef formation and one on

barnacles.

• In early 1856, Lyell read a paper by Alfred Wallace that seemed to reflect Darwin’s

ideas. Lyell showed the paper to Darwin, who was not overly worried, but began to put

his ideas into a short paper, later to be put into book form.

• In 1858 Wallace sent a more complete paper to Darwin for review. The paper had in

essence all of Darwin’s ideas on natural selection. Darwin was devastated.

BRIEF BIOGRAPHY (CONT’D)

• Darwin forwarded Wallace’s paper to Lyell for publication (even though Wallace had not requested that).

• His family in crisis with scarlet fever in the village and his own health precarious, Darwin asked Lyell and Hooker to adjudicate the matter of precedence.

• Lyell and Hooker had Darwin and Wallace both present their work to the Linnaean Society on July 1, 1858. The event was little-noticed outside the Society.

• Darwin finally published “On the Origin of Species” on 22 November 1859. It was an immediate best-seller.

A WELL-

RESEARCHED

BOOK SET

FORTH DARWIN’S

IDEAS AND THE

DATA TO BACK

THEM UP.

1859

THE REACTION WAS

IMMEDIATE, AND NOT

ALWAYS FAVORABLE

Brief Biography (cont’d)

BRIEF BIOGRAPHY (CONT’D)

• In spite of controversy and ill health, Darwin continued to publish.

• 1862: “Fertilization of Orchids”

• 1868: “The Variation of Animals and Plants under Domestication”

• 1871: “The Descent of Man, and Selection in Relation to Sex”

• 1872: “The Expressions of the Emotions in Man and Animals”

— 1875: "Insectivorous Plants" (sold faster and better than "Origin of Species". )

— 1876: "The Various Contrivances by which Orchids are Fertilized by Insects" and "The Effects of

Cross and Self Fertilization in the Vegetable Kingdom“

— 1877: "The Different Forms of Flowers on Plants of the Same Species“

— 1878: A biography of his grandfather titled, "Erasmus Darwin“

— 1879: "The Movement and Habits of Climbing Plants“

— 1881: "The Formation of Vegetable Mould, through the Actions of Worms, with Observations on

Their Habits"

— Darwin died 19 April 1892

DARWIN’S GREAT SYNTHESIS

• Darwin took the seven observations mentioned above

and, along with Malthus’ description of population

growth and Lyell’s theory of uniformitarianism,

combined them with his vast knowledge of the natural

world to make his theory of natural selection based on

four simple but powerful postulates:

POSTULATES OF NATURAL SELECTION

(IN MODERN LANGUAGE)

1. The individual organisms that make up a population vary in the traits they

possess, such as their size, shape, physiology, etc.

2. To the extent that a given trait has some genetic basis, some of the offspring

will inherit the variations in the trait.

3. In most generations, more offspring are produced than can survive. As a

result, only a subset of the offspring that are produced survive long enough

to reproduce.

4. The subset of offspring that survive and reproduce is not a random sample

of the population. Instead, individuals with certain traits are more likely to

survive and reproduce or to produce the greatest number of offspring. The

individuals with there traits are, in Darwin’s words, “naturally selected”

DARWIN’S FIVE MAJOR THEORIES1

The non1. -constancy of species (the basic theory of evolution)

The descent of all organisms from common ancestors (branching 2.

evolution)

The gradualness of evolution (no saltations, no discontinuities)3.

The multiplication of species (the origin of diversity)4.

Natural selection5.

1. Mayr, 2001

HOW NATURAL SELECTION LEADS TO THE ORIGIN OF SPECIES:

AN OVERVIEW

• Members of a species are never uniformly distributed across the entire

range of the species.

• Therefore different demes (breeding subgroups) of the species are

subject to slightly different environments and hence slightly different

selection pressures

• Anything that diminishes the flow of genes from deme to deme (physical

separation, behavioral differences, etc.) will allow natural selection to

begin to operate differently on the demes

• Given enough time and enough separation the differences between the

demes will become so great that they can no longer mate and produce

viable, fertile offspring.

DARWINIAN FITNESS

• Natural selection has often been described (as Darwin

did) as “survival of the fittest”

• Fitness, in Darwinian terms, is a measure of the ability to pass more

of your genes on into the next generation than your competitors.

• Thus natural selection operates only on those characters that affect

reproductive fitness.

This has some unpleasant consequences for those of us

beyond child-bearing age!

SOME SUCCESS STRATEGIES

Live short lives and have lots of offspring: e.g., insects, annual plants•

Live long lives and spend a lot of energy on parental care: e.g., us•

For males, be as promiscuous as you can get away with.•

For females, choose the most • “fit” mate

Disperse your gametes (or offspring) over the widest possible area: •

e.g., wind-pollination, burrs.

Others??

DOES EVOLUTION ANSWER THE QUESTIONS?

• Taxonomy:

• Organisms are related by descent from a common ancestor. The more remote the ancestor the more the difference (e.g., higher level taxa).

• Fossil Record:

• Change over time is evidence of evolution. Extinction can be explained by natural selection (among other reasons).

• Homologies:

• Descendants of common ancestors will share the same structure, although they may have been modified to different extents by natural selection.

HOMOLOGY VS. ANALOGY

Homologous structures arise due to common ancestry. Analogous structures

arise due to convergent evolution.

DOES EVOLUTION ANSWER THE QUESTIONS?

• Embryology:

• “ontogeny recapitulates phylogeny”. Well, not totally, but close enough.

• Vestigial Organs:

• Inherited from common ancestors, but diminished due to lack of positive selection pressure.

• Biogeography:

• If there are camels in Africa and Asia and again in South America, there must be camels in North America. Paleontology shows fossil camels North America in the Tertiary (ca. 30MYA). North America and Europe were once part of a larger supercontinent.

• Animal husbandry:

• A kind of natural selection.

RECOMMENDED READING

Accessible books:•

Knoll, A.H. • “Life on a Young Planet: The First Three Billion Years of Evolution on

Earth”, Princeton University Press, 2003

Gould, S.J., • “Wonderful Life: The Burgess Shale and the Nature of History”,

Norton, 1989

More Academic:•

Mayr• , E. “What Evolution Is”, Basic Books, 2001

And, of course,•

Darwin, C. • “On the Origin of Species” (a quite readable book. Amazon has a

December 2011 edition)