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Lecture 5

EVOLUTION

I. Introduction

Thus far we have seen how the geological time scale was constructed and during that same period, dinosaurs were discovered.

A major part of this course will be the evolutionary relationships of dinosaurs and how the evolution of key innovations in groups of organisms modifies the Earth system.
But what is evolution, how do we know about it, how does evolution function, and how do we know about in the past?

II. SCALA NATURAE

The western belief in linear order - in 18th and 19th century Europe and North America made worth looking for order as a product of God a meaningful pursuit.

One very important aspect of that order was called the SCALA NATURAE (Scale of Nature) - a concept attributed to Aristotle, ranking things from the inorganic to humans and angels.
In this view the universe was absolute and full - a concept called the Principle of Plenitude
Concept related to Platonic concept of absolute ideals.
Supported feudal social stratification as well as putting everything in its place - we still have strong vestiges of that concept.

III. Species problem

Through the 1600 - 1800's the idea of "The path to God is by contemplating his works" was a romantic and popular one which at least justified (if not motivated) much early natural history work.

Led to great attempts at classifications.
One fundamental problem was - what are the units of life to be classified?

IV. Carolus Linnaeus (Carl von Linné)

(1706-1778) was a Swedish naturalist who took it upon himself to classify the entire natural world.
He became known as "God's Registrar"

Linnaeus recognized that interbreeding showed that there was a natural break between organisms that would freely interbreed and those that would not. This was a fact known well by breeders of dogs and other domestic animals.

Linnaeus saw that there was a "unity of type". He showed that the basic unit of natural classification was not the individual but the "species", and that they were recognizable by the fact that the individuals within a species would freely interbreed.
In other words a species is held together by sex.

In a monumental work on the sexual system in plants Linnaeus showed that the "biological definition of a species" holds true just as well for animals.

Nonetheless, in most cases species are recognized by the fact that members of a species tend to look much more like each other than they do to members of other species.
Thus, most species are identified by morphology. Testable by interbreeding.

V. In 1758 Linnaeus published his grand opus: SYSTEMA NATURAE

In this work he outlined not only the known species of animals and plants, but also what has become to be known as the "Linnean Hierarchy" of Taxonomic levels.

He noticed that while the fundamental unit was the species, that species could be grouped by similarity of structure into larger groups: genera, families, orders, classes, phyla, and kingdoms (and others in between). We call any entity within this hierarchy a taxon (plural - taxa). Thus a species is a taxon as is a family. This is an inclusive hierarchy.
In other words, kingdoms consist of and contain classes, classes consist of and contain orders, orders consist of and contain families, families consist of and contain genera, genera consist of and contain species.

In this "Linnean Hierarchy" speciesare handled specially - a fact you need to know and be familiar with.

Here is the Linnean Hierarchy for us: THE LINNEAN HIERARCHY FOR HUMANS

        Phylum Chordata
                Class Mammalia
                        Order Primates
                                Family Hominidae
                                        Genus Homo
                                                Species Homo sapiens

VI. That this classification method worked became clear very quickly and it was universally adopted. BUT WHY DID IT WORK ?

Linnaeus catalogued species as if invariable, but the occasional hybrids between species suggested to him they were not quite so fixed.

In fact, although Linnaeus began by believing that species were fixed entities created by God as is, later in life he began to believe otherwise:
that they were "children of time", capable of transformation in form though time ó in other words species could evolve.

VII. Jean Baptiste Lamarck

(1744-1829) came up with the first reasonable theory of organic evolution. He recognized that:

1) organisms change in their environment
2) that they discontinuously change and reorganize
But he could not could not deal with concept of extinction.

Lamarck believed that species could change through time by passing on traits acquired during an individual's life to their offspring - the so-called "acquired characteristics".

In this way all variation now and in the past were united by a "Great Chain of Being". It was the Scala Naturae in motion.

VIII. Our friend Georges Cuvier

(1769-1832) believed very strongly that extinction was real!

His studies made him think all traits of organisms are designed to be a perfectly functional whole.

He believed that there were several different basic plans (bauplans) of organisms - separate creations.
He also believed that the history of life consisted of separate periods of creation and periods of and extinctions - catastrophes - much like the French Revolution that he survived.

With this belief system Cuvier ran right into Lamarck. Cuvier was a successful political animal and managed to have Lamarck ostracized.

There were other evolutionary concepts around, but like Lamarck's ideas, none took hold because they lacked a clear and observable mechanism

IX. Darwin - Wallace and Natural Selection

Charles Darwin (photo) became the ships naturalist on the H.M.S. Beagle. He saw many unfamiliar patterns of geography and organismal variation that needed to be explained.

In particular he noted the parallel between species, geography and adaptation and fittedness to environments.
To Darwin, this became the central problem - how did organisms within a species become adapted to their roles in the world?
He spelled this out long after his voyages with the Beagle in his Origin of Species (On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life, 1859).

One key was Thomas Malthus (1766-1834), an English political economist concerned about the consequences of poverty such as he saw in London. A critical work was his "Essay on the Principle of Population" (1798).

Malthus showed that populations if allowed to grow unchecked increase at a geometric rate.
The world could not possibly hold so many, so natural checks are in place. Poverty, disease, war, and famine were all outcomes of increasing population that in fact kept populations of all organisms from increasing. Many more organisms are born than can possibly survive.

Darwin took this concept and combined it with a critical observation.

Variation of morphology and behavior was a fact of life.

The old idea was one of "Ideals" or "Types" of Platonic origin. We still hold to that informally. In the "typological" concept of species, variation is some error around a true ideal form.

But Darwin recognized that variation was at the core of what a species is. There is a statistically average condition, but each variant is as real as any other and there is no ideal condition. This concept of type is unfortunately still very much part of racial thinking and has no place in modern biology.
Darwin was espousing "populationist thinking".

Darwin realized that this natural variation led to differential survival based on that variation. For a given environmental context some variants would fair better than others in the "struggle for existence".

Origin of Species

Darwin was a domestic pigeon breeder (as a part of his research) and knew that humans had long taken advantage of natural variation to select for breeds; this is artificial selection. Hence Darwin called his explanation "Natural Selection".

Darwin noted clearly the fact that the most closely related (similar) species were often found closest together in geography.

There are 5 parts to Darwin's Natural Selection:
1. Heredity of most features
2. Heritable Variation in the population
3. Variation leads to differential rates in survival and reproductive success among the variations.
4. Differential survival and reproduction leads to a shift in the frequency of characters, leading to a shifting of mode within the species.
5. If this process goes on long enough, parent and daughter species can no longer interbreed.

Remember: Populations and species evolve, individuals do not.

Two wonderful examples, studied by Darwin and still studied today, are the so-called "Darwin's Finches", members of the family Geospizidae in the Galapagos Islands, and the islands' giant tortises.

Book by Wallace Alfred Russell Wallace (1823-1913) came up with vitually the same concept of natural selection more or less independently through his studies on the Malay archipelago. Darwin panicked because he was not ready with his book yet!

Both gave papers at the same time and the same subject at the Linnean Society meeting of 1858 (thanks to Charles Lyell and Joseph Hooker).

By the way, the phrase "survival of the fittest" comes originally not from Darwin but rather from Herbert Spencer who adopted Darwin's ideas to a general progessionist philosophy (many say inappropriately), although Darwin adopted the term in his later editions of the Origin of Species on advice from Wallace.

X. BUT HOW DO WE KNOW EVOLUTION OCCURS?

1. PRESENTLY TESTABLE

2. STRAINS OF BACTERIA adapting to changes

3. IN PAST - PREDICTIONS OF INTERMEDIATES - BIRDS - ARCHAEOPTERYX

Berlin Archaeopteryx

We know about evolution from 5 classes of order in organisms:

ORDER IN CHARACTERS (comparative anatomy)
ORDER IN GENETICS
ORDER IN DEVELOPMENT (embryology)
ORDER IN GEOLOGICAL RECORD
ORDER IN GEOGRAPHIC DISTRIBUTIONS

XI. From our present perspective then we can recognize at least three different kinds of evolution

1. microevolution
2. evolution of taxa
3. evolution of key innovations

Even within this we can recognize two modes:

ANAGENESIS VS. CLADOGENESIS (see also discussion in Transitional Vertebrate Fossils FAQ)

Then rates of evolution are a different matter again:

GRADUAL VS. PUNCTUATED

DARWIN'S GAPS

As an aside, it is worth noting that Darwin initially regarded the fossil record as a bit of an embarrassment. In 1859, there were no fossil intermediate forms known between major groups from the fossil record. To argue his point Darwin needed to insist that the fossil record was very incomplete because of gaps in sedimentation. Of course this begs the question of rates of evolution. It now seems that some of the apparent gaps are due to very rapid change indeed.

XII. Now what favors cladogenesis - the splitting of lineages and the proliferation of species?

IN OTHER WORDS, WHAT FAVORS SPECIATION?

1. small numbers
2. isolation
3. extreme conditions (intense selection)

XIV. EVOLUTION CREATES DIVERSITY - BUT EXTINCTION IS OPPOSITE

The geological record of diversity shows the overall pattern of life.

SO THIS IS WHERE WE GET WHAT WE SEE IN THE GEOLOGICAL RECORD - CREATION OF DIVERSITY

From this comes:

THE PRINCIPLE OF BIOTIC SUCCESSION of William Smith

HOW, IN DETAIL, DO WE WORK OUT THE EVOLUTIONARY RELATIONSHIPS OF FOSSIL AND RECENT SPECIES?

Go to lecture 5

References

http://www.astro.virginia.edu/~eww6n/bios/Aristotle.html (Eric's Treasuretrove of Science: Aristotle of Stagira (384-322 BC) - home page at http://www.astro.virginia.edu/~eww6n/)

http://www.astro.virginia.edu/~eww6n/bios/Plato.html(Eric's Treasuretrove of Science: Plato (ca. 427-ca. 347 BC) - home page at http://www.astro.virginia.edu/~eww6n/)

http://userwww.sfsu.edu/~rsauzier/Darwin.html (Glass Bead Game (Das Glasperlenspiel): Darwin, Charles (1809-82)) - home page at: http://userwww.sfsu.edu/~rsauzier/Biography.html#Biology)

http://www.clements.umich.edu/Photogal/bib/Darwin.html (Portraits from the Photographic Collections of the William L. Clements Library: Half length photograph of Charles Darwin) - home page at: http://www.clements.umich.edu/Photogal/people.html)

http://www.literature.org/Works/Charles-Darwin/ (An Online Literature Library: Charles Darwin - http://www.literature.org/)

http://www.dickinson.edu/~nicholsa/Romnat/cuvier.htm (Contains and English translation of Cuvier's "Discourse on the Revolutionary Upheavals on the Surface of the Earth" by Ian Johnston.
http://www.dickinson.edu/~nicholsa/Romnat

http://www.ucmp.berkeley.edu/history/malthus.html (Enter Evolution: Thomas Malthus (1766-1834) - http://www.ucmp.berkeley.edu/history/evolution.html)

http://www.hyperhistory.com/online_n2/people_n2/persons2_n2/plato_ideas.html (History of the World: Greece (Plato's theory of ideas) - http://www.hyperhistory.com/)

http://www.cc.columbia.edu/acis/bartleby/bartlett/423.html (PROJECT BARTLEBY ARCHIVE:Charles Darwin. 1809-1882 - http://www.columbia.edu/acis/bartleby/)

http://www.talkorigins.org/faqs/archaeopteryx/info.html (The Talk Origin Archive: All About Archaeopteryx by Cris Nedinhttp://www.talkorigins.org/)

http://www.mq.edu.au/hpp/Ockham/z3602.html (Macquarie University: PHIL360 - Later Medieval Philosophy; Tape 2: Scotus's proof of the existence of an infinite being).

http://www.ucmp.berkeley.edu/history/evolution.html (Enter Evolution)

http://landow.stg.brown.edu/victorian/economics/econov.html Victorian Economics (on, in part, Thomas Malthus)
http://landow.stg.brown.edu/victorian/victov.html (Victorian Web)

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