The Development of Evolutionary Theory

1
The Development of Evolutionary Theory

• A Brief History of Evolutionary Thought
• The Scientific Revolution
• The Path to Natural Selection
• John Ray
• Carolus Linnaeus
• Comte de Buffon
• Erasmus Darwin
• Jean-Baptiste Lamarck
• Georges Cuiver
• Charles Lyell
• Thomas Malthus
• Charles Darwin
• Alfred Russel Wallace

• Natural Selection in Action

• Constraints on Nineteenth-Century Evolutionary
  Theory
• Opposition to Evolution

2
A Brief History of Evolutionary Thought
The individual most responsible for the
elucidation of the evolutionary process was
Charles Darwin. However, the theory of Natural
Selection was independently developed by another
Englishman Alfred Russel Wallace. Medieval
concept of statsis all aspects of nature
including all forms of life and their
relationships to one another, were seen as fixed
and unchanging. Christianity was taken quite
literally. God had created all life forms
exactly as they existed in the present.
Alterations were seen as impossible because
they would have run contrary to Gods plan. This
belief is know as fixity of species. The
notion that species, once created, can never
change.
3
Accompanying the notion of fixity of species was
the belief that all Gods creations were arranged
in a hierarchy that progressed from the simplest
organisms to the most complex Great Chain of
Being. First proposed by Aristotle Another
common notion the Earth was full and that
nothing new (such as species) could be added.
The plan of the entire universe was seen as the
Grand Design that is Gods design. This was
called the argument for design, anatomical
structures were viewed as planned to meet the
purpose for which they were required.
Archbishop James Ussher (1581-1656) The date
the Grand Designer started his work was at noon
on Sunday, October 23, 4004 B.C. The
prevailing notion of the Earths brief existence,
together with fixity of species, provided a
formidable obstacle to the development of
evolutionary theory WHY?
4
The Scientific Revolution
What, then, upset the medieval belief in a rigid
universe of planets, stars, plants, and
animals? The discovery of the New Word and the
circumnavigation of the globe in the 15th century
overturned some very fundamental ideas about the
planet. Earth was round awareness of biological
diversity Copernicus heliocentric model
challenged Aristotles geocentric
model Heliocentric model further supported by
Galileo Other notables, Keppler, Decartes, and
Newton establishing the laws of physics, motion
and gravity. In essence, the scientific
achievements increasingly came to direct as well
as reflect the changing views of Europeans.
5
The Path to Natural Selection
John Ray (1627-1705)- defined the concept of
species. First to recognized that groups of
plants and animals could be distinguished from
other groups by their ability to reproduce with
one another and produce offspring. He called
this category species Ray also recognized that
species frequently shared similarities with other
species. He labeled this group genus.
However, Ray was an adherent of fixity of
species. His 1691 publication, The Wisdom of God
Manifested in the Works of Creation, was intended
to demonstrate Gods plane in nature. He
stressed the deliberate outcome of the Grand
Design.
Ray is often referred to as the father of natural
history in Britain.
6
Carolus Linnaeus (1707-1778)- One of the leading
naturalists of the 18th century. He is best
known for developing a classification of plants
and animals The System of Nature (Systema
Naturae) in 1735. Linnaeus standardized Rays
more sporadic use of two names (genus and
species) for organisms, establishing the binomial
nomenclature. He added two more categories
class and order. His four-level system for
classification became the basis for taxonomy, the
system used today. Another innovation by
Linnaeus was the inclusion of humans into the
classification system into the genus Homo, and
species sapiens. While he was a believer in the
fixity of species, this view did change in his
later years based on the mounting evidence to the
contrary.
Carolus Linnaeus. Portrait by Per Krafft (The
University of Uppsala Art Collections).
7
Comte de Buffon (Georges-Louis Leclerc
1707-1788)- He believed neither in the
perfection of nature nor in the idea that nature
had purpose as declared by the argument from
design. He did recognize the dynamic
relationship between the external environment and
living forms. Wrote Natural History (1749) where
he repeatedly stressed the importance of change
in the universe, and he underlined the changing
nature of species. Migrating groups would be
influenced by and adapt to the environment. He
rejected the idea that one species could give
rise to another.
Comte de Buffon, portrait by Garnerey del., P. M.
Alix sculpt. Paris, chez Drouhin.
8
Erasmus Darwin (1731-1802)- Grandfather of
Charles Darwin. Freethinking high living
physician was well known in literary circles.
Chief among his writings was his Zoonomia,
which evolutionary concepts were expressed in
verse. Erasmus Darwin had expressed similar
views on natural selection. Charles was fond of
his grandfathers writings however, it is not
known how much he was influences by them.
Erasmus Darwin, grandfather of Charles Darwin
9
Darwin's interests extended well beyond the
practice of medicine. He was dissatisfied with
Linnaeus's theory of the immutability of species
and instead proposed the gradual evolution of
animals and plants. He published his ideas in
Zoonomia or the Laws of Organic Life in 1794.
This work has been called "the first consistent
all-embracing hypothesis of evolution." The
publications of his French contemporaries, Cuvier
and Lamarck, strengthened Darwin's position and
provided the foundation for later studies in the
field, most notably for that of Charles Darwin,
grandson of Erasmus. It should be noted,
however, that Charles Darwin rejected his
grandfather's strict mechanistic,
semi-experimental approach and even claimed that
Zoonomia had no effect on his famous work, On the
Origin of Species (1859).
10
Jean-Baptiste Lamarck (1744-1829)- the 1st
European scientist who attempted to explain the
evolutionary process. He postulated a dynamic
interaction between organic forms and the
environment, such that organic forms could become
altered in the face of changing environmental
circumstances. Therefore, as the environment
changed, so too did the animals activity
pattern. This resulted in the increase and
decrease use of certain body parts. As a result
of use or disuse, body parts became altered.
Physical alterations occurred as a function of
perceived bodily needs. Inheritance of
acquired characteristics or use-disuse theory
11
Georges Cuvier (1769-1832)- Most vehement
opponent of Lamarck. Specialized in vertebrate
paleontology and it was he who introduced the
concept of extinction to explain the
disappearance of animals represented by fossils.
He never grasped the concept of nature and was
a proponent of fixity of species. Instead of
assuming the similarities between certain fossil
forms and living species indicated by
evolutionary relationships, Cuvier proposed a
variation of the theory know as catastrophism.
held that the earths geological features were
the results of sudden, worldwide cataclysmic
events. To be consistent with the fossil
evidence, Cuvier proposed that destroyed regions
were repopulated by new organisms of a more
modern appearance and that there forms were the
results of more recent creation events. This
account avoided the notion of evolution while
still explaining change through time.
12
Charles Lyell (1797-1875)- He was a barrister by
training and a geologist by avocation. He is
considered to be the founder of modern geology.
He was for many years friend and mentor to
Charles Darwin. Principles of Geology
(1830-33) Uniformintarianism- he argued that the
geological process observed in the present as the
same as those that occurred in the past. This
theory flew in the face of Cuviers
catastrophism. Lyells theory implied immense
geological time scale. He changed the framework
within which scientists viewed the geological
past. This the concept of deep timeremains
one of Lyells most significant contribution to
the discovery of evolutionary processes.
13
Thomas Malthus (1766-1834)- an English clergyman
and economist. Wrote An Essay on the Principle
of Population, which inspired both Darwin and
Wallace in their separate discoveries. Malthus
pointed out that if not kept in check by limited
food supplies, human population growth could
double in size every 25 years. That is,
population size increases exponentially while
food supplies remain relatively stable.
14
Charles Darwin (1809-1882)- Grew up in a wealth
family. Attended medical school at the
University of Edinburgh. Being dissatisfied
after two years he left for Christs College,
Cambridge. Natural Science After graduation
in 1831 at the age of 22 he left on a scientific
voyage to circle the globe on the HMS Beagle.
Believed in fixity of species. However, he
began to have doubts. Galapagos Islands noted
that the flora and fauna of South America showed
striking similarities to those of the Galapagos,
as well as intriguing differences.
Darwin as a young man
15
He recognized that the various Galapagos finches
had all descended from a common, mainland
ancestor and had become modified in response to
the varying island habitats and to altered
dietary preferences. However, it was only in
retrospect that he considered the factors that
lead to the modification of 1 species into 13.
By the late 1830s, Darwin recognized that
biological variation within species was
critically important. He acknowledge the
importance of sexual reproduction in increasing
variation.
16
1838- Darwin read Malthus essay where he found
the answer to his question of how new species
came to be (populations increase at a faster rate
that do resources). From this he inferred
that in nonhuman animals, increase in population
size is continuously checked by limited food
supplies. He accepted Lyells observation that
in nature there is a constant struggle for
existence. It at once struck me that under
these circumstances favourable variations would
tend to be preserved, and unfavourable one to be
destroyed. The result of this would be the
formation of a new species (F Darwin, 1950, pp.
53-54).
17
Alfred Rusell Wallace (1823-1913)- Born into a
modest working class family. Had little
education. 1848 he became interested in
collecting plants and animals when he joined an
expedition to the Amazon. 1854 he sailed for
Southeast Asia and the Malay Peninsula to
continue his study and to collect bird and incest
specimens. In 1855, Wallace published a paper
suggesting that species were descended from other
species and that the appearance of new species
was influenced by environmental factors. 1858,
Wallace sent Darwin a paper titled On the
Tendency of Varieties to Depart Indefinitely from
the Original Type. Urged by Lyell and others,
in December 1859 Darwin published his greatest
work On the Origin of Species by Means of Natural
Selection, or the Preservation of Favored Races
in the Struggle for Life.
18
Darwins Explanatory Model of Natural
Selection Fact 1 Every population has such high
fertility that its size would increase
exponentially if not constrained. Fact 2 The
size of populations, except for temporary annual
fluctuation, remains stable over time (observed
steady-state stability). Fact 3 The resources
available to every species are limited Inference
1 There is intense competition (struggle for
existence) among members of a species. Fact 4
No two individuals of a population are exactly
the same (population thinking). Inference 2
Individuals of a population differ from each
other in the probability of survival (I.e.,
natural selection). Fact 5 Many of the
differences among the individuals of a population
are, at least in part, heritable. Inference 3
Natural selection, continued over many
generations results in evolution.
19
Constraints on Nineteenth-Century Evolutionary
Theory
Darwin argued eloquently for the notion of
evolution in general and the role of natural
selection in particular, but he did not entirely
comprehend the exact mechanisms for evolutionary
change. Natural selection acts on variation
within a species. However, no one understood the
source of this variation. Also, Darwin did no
completely understand the mechanism by which
parents transmitted traits to offspring.
Scholars of the time were confused about the
laws of heredity, and the popular consensus was
that inheritance was blending by nature. As it
turned out, the rules of heredity had already
been worked out by an obscure Augustinian monk,
Gregor Mendel. However, this was not recognized
until the beginning of the 20th century.
20
Opposition to Evolution
The topic of evolution was a hotly debated topic
with On the Origin of Species turning the
controversy into an inferno. The very idea
that species could give rise to other species was
particularly offensive to many Christians because
is was in direct conflict with events depicted in
Genesis. People were horrified that humans
could be biologically related to other animal and
especially that they might share a common
ancestor with the great apes. The mechanisms of
evolution are complex and do not lend themselves
to simple explanations. Must have a strong
background in biology and genetics. While most
religions offer explanations for natural
phenomena, and some even feature the
transformations of individuals from one form to
another, none really proposes biological change
over time. 1996 Pope John Paul II issued a
statement to the Pontifical Academy of Sciences
acknowledging that fresh knowledge leads to
recognition of the theory of evolution as more
than just a hypothesis.
21

• Darwins Contributions
• Lots of data
• Changed focus from types to population and the
  variation in population
• Mechanism for evolutionary change Natural
  Selection.
• Data survival of the fittest
• struggle/competition
• Observations
• Organisms reproduce more that their own numbers
• Numbers remain fairly constant over time.
• There is variation within and between species
• Concluded competition and struggle for existence
  both within and between species.
• Observed but could not explain
• In this struggle for existence those organisms
  with favorable variation will live longer and
  thus reproduce more. Natural Selection