INTELLECTUAL BACKGROUND TO DARWIN'S DISCOVERY
Like any good intellectual pursuit , the problem of Origins can be traced back to Greek philosophers. World view of the time was one of an eternal world, or at least one where time was cyclical; a steady state world; concepts of time were very different from ours today: issue was on origins; evolution (change) was less of a concern (according to E. Mayr, The Growth of Biological Thought, Harvard University Press).
Plato's ideas dominated this line of thinking for next approx.
2000 years; his dogmas had the effect of being antievolutionary
(but not by intention): Essentialism, belief in a constant eidos,
a form or fixed idea. This idea was distinct from the phenomenon of appearance:
there is an ideal professor and I am but an imperfect personification of
that ideal; there is an ideal Brown undergraduate and, alas, you are all
imperfect examples varying around that Platonic ideal. Animate cosmos,
the universe is a living harmonious whole. A creative power, a demiurge
(not a god-like creator). "Soul". All of these ways of thinking
probably stifled evolutionary thought; the emphasis was on origins. That
an evolutionary thinking did not emerge probably stemmed from the fact
that evolution would disturb the harmonious whole.
Aristotle, a great naturalist and the founder of Natural History. Nature passes from inanimate objects through plants to animals on to man in a "great chain of beings"; perpetuated the Platonic view of the fixity of species and held that the natural order was eternal and unchanging
These views, held in a world where time was cyclical, all stifled (or
at least diverted attention away from) mechanistic/evolutionary thinking.
Jump through the fall of Rome to Christianity: the Word was that of God written in the Bible; again focus was on origins (Creation in Genesis), but now time was unidirectional: the earth and organisms were created once, Christ died once and the Judgment day will come but once!
Despite linearity of time, the fixity of species was still held in the
context of Creation. Platonic essentialism was still influential: God created
the heavens, the earth and the organisms and since God was a perfect god,
the natural world thus must fit a pattern: the Scala Naturae (a
legacy of the Aristotelian great chain of beings). All things that God
conceived of , did exist and held their fixed place in nature. God would
not have conceived of a species and then not created it. This is the essence
of the concept of plenitude: as many organisms that could exist,
did exist; this was all for the good. Extinction was not possible because
a perfect God would not have permitted it. As we'll see, fossils might
present a problem for plenitude.
Natural Theology (Natural Philosophy) developed as one path to
god; Nature was studied for the greater glory of God since one path to
that God was through the contemplation of his works (e. g. nature). Ironic
that natural theology set the stage for evolutionary thinking rather than
The scientific reawakening that took place in the 16th and 17th centuries
further set the stage for evolutionary thinking (Descartes, Newton). Cosmology
discovered the infinity of space and lead to the development of
the infinity of time. They still held onto creationism and Christianity,
but the advances in the physical sciences came to challenge the all-powerful
God as having a hand in everything. God created the world, but physical
laws kept it running. Still a major barrier to evolutionary thought
was the inability to observe evolution. One could watch a rock drop and
explain the process; evolution had to be inferred and hence a mechanism
for evolution did not yet develop.
Advances in Natural Theology and Geology began to challenge the fixity
of species. Linnaeus was a voracious cataloguer "for the greater
glory of God" (Systema Naturae, 1735). Collected widely throughout
Europe and had huge shipments of material from all over the world sent
to him by students and colleagues. Became clear that some of his own varieties
and those sent to him from other areas were not the same. Variation
was being forced upon the Naturalists
The age of discovery throughout the 15, 16, 17th centuries also began
to notice different faunas in different parts of the world. Buffon
wrote Histoire Naturelle: 35 volumes! (1749-1788) notes on many of the
species that Linneaus had catalogued. Developed the discipline of Biogeography.
If the Fauna from Noah's ark is what populated the earth after the flood,
different faunas were hard to reconcile: "what the heck are these
things?" Fossils were also found that were not now alive (not represented
in Noah's ark): big trouble for the principle of plenitude.
Georges Cuvier (1796-1832), founder of paleontology and comparative
anatomy, worked in the strata of the Paris basin. Discovered many fossils
and described the exact stratigraphy. Found that the lower the strata,
the more different the fossils were from the organisms today. Maybe
there were a few floods?? and man was involved in the last one??. This
rationalization fit into then widely held view of Catastrophism,
periodic catastrophes that shaped the earth. His data ultimately provided
the best evidence for evolution, but he remained opposed to the concept
to his death.
Uniformitarionism, proposed by James Hutton (1785), was
contrary to Catastrophism in that it proposed that the forces that formed
and shaped the earth were the forces that are acting today. Erosion and
sedimentary deposition have been going on for millennia: earth had "no
vestige of a beginning, no prospect of an end"
Uniformitarionism was made conceivable by William Smith, a British
canal surveyor and engineer, walked the English countryside following geological
strata. Found that one could follow strata for miles, and more importantly,
each stratified bed had a different flora and fauna. Claimed that stratigraphic
record was a record of sequence of deposition and hence time (1815). Put
a temporal scale on the fossil discoveries.
Sir Charles Lyell, in Principles of Geology (1830), formalized
Hutton's and Smith's work in strong support of Uniformitarionism. Darwin
read it on the Beagle. Had a major influence on Darwin in thinking about
gradual biological change.
Many of these ideas and new bits of data were converging at the end
of the 18th and beginning of the 19th centuries. The accumulating evidence
initiated belief that organisms could change over time. Now we need a MECHANISM.
Lamarck set the Scala Naturae in motion. Extinction was a problem,
but "solved" by arguing that lower forms changed in to higher
forms (thus up the great chain of being) Philosophie Zoologique (1809).
As to mechanism, acknowledged that the environment was ever changing
and created new "needs" ("besoin") in the organism.
The changing environment stimulated "activities" in the
organism to meet these needs. Important(1): the environment did not directly
induce the new characters, it stimulated the organism to enhance these
characters. Important (2): no volition invoked; stimulus was a "need"
not a "desire". Came to be called the Inheritance of Acquired
Characteristics. Nevertheless, initiated thinking about the Descent from
a common ancestor.
Darwin claims he got nothing useful from Lamarck's work. The milieu
in which Darwin began his scientific career, extinction was accepted, fossils
were viewed as real evidence of previous life. Evolution of species (in
the sense of serious questions about the fixity of species) was being considered.
The patterns were in place; Darwin came up with the process.
Some Darwiniana: A devoted naturalist from an early age. Loved
to ride and hunt and walk outdoors. Heavily influenced by his grandfather
(Erasmus Darwin) a successful doctor and author of Zoonomia
in which notions of evolution were described (heretical for that time).
As a teenager, Darwin was active in the Plinian Society, a discussion
group seeking physical, rather than supernatural, explanations for scientific
phenomena. Many freethinking democrats involved. Darwin presented
his first scientific paper to this group and was intoxicated by the notion
of discovering previously unknown facts of nature. Probably fostered a
critical view of the world, rather than accepting the religious dogma of
Darwin was part of a very comfortable family. His father, too, was a
successful doctor and his mother was a Wedgwood (the wealthy family of
potters). Darwin was a brilliant naturalist, but a poor student. He hated
medical school and withdrew (couldn't stand the sight of blood). Went on
to Cambridge with intention of ultimately becoming a clergyman (a common
profession for respectable, affluent men). Squeaked through Cambridge overspending
his allowance, but developed important relationships with his botany professor,
Henslow, with whom he frequently "botanized"
Henslow recommended to the powers that be, that Darwin should be selected
as the unpaid naturalist aboard the Beagle, a navy ship involved
in a variety of missions (e.g., charting the South American coast). Darwin's
father, Robert, felt that going on such a mission was a further postponement
of 'getting on with his life' and said "if you can find any man of
common sense who advises you to go I will give my consent"
Darwin found this consent in his uncle Josiah Wedgwood (mom's brother)
and a meeting with Josiah, Robert Darwin and son Charles settled the matter.
Charles had previously declined the offer to be the Beagle's naturalist,
so he quickly dashed off to Cambridge and then London to meet with the
ships captain, Robert FitzRoy. The interview with FitzRoy went extremely
well and FitzRoy had insisted that he would not take as a naturalist anyone
with whom he could not get along well personally (good idea for a 2-4?
year trip on a small ship). Voyage of the Beagle 1831-1836.
Darwin's road to discovery is a model for many (most) organismal biologists:
sail around the world for five years, read books, collect specimens, observed
different lands and people, come home and digest it all and write up a
theory that is a cornerstone of human intellectual history.
All this occurred with some important help. John Gould, the ornithologist
who was analyzing the mocking birds Darwin had brought back from the Galapagos,
pointed out (March, 1837) that the birds were distinct between three different
islands. Darwin later wrote in the Origin how arbitrary the distinction
was between different varieties and species. The fixity of species held
by the creationists and the essentialists, and along with it Darwin's concept
of what a species is, was forever changed. Reading (September, 1838) Malthus's
Essay on the Principle of Population (1798) which argued that unlimited
growth in human populations would lead to famine. This led him to realize
that the favorable variations would tend to be preserved,
i.e., a "struggle for existence" would lead, over time, to the
origin of species by Natural Selection.
Simultaneous discovery of the essential concepts of natural selection by Alfred Russel Wallace (letter to Darwin in June 1858; see figure 1.3, pg. 9). Darwin had been amassing a huge body of information to support his theory. He knew that his ideas were going to cause a stink, so he had to overwhelm the opposition with his "big book". Wallace's letter initiated the publication of an "abstract" (about 500 pages): 1859
Variation among individuals
Brought inter-species variability down to the intra-species level
The variation is heritable
Had no clear understanding of genetics, but pigeon breeding convinced him of all that he needed to know: variation could be enhanced by selecting out of a variable population
More individuals are produced than can survive each generation
the more favored forms in this variation will be preserved
The fact of evolution was probably accepted reluctantly, but Darwin's theory of evolution by natural selection was challenged continuously. Genetics was the problem. Blending inheritance (like mixing paints) would homogenize all individuals and there would be no variation on which to select. Even with the rediscovery of Mendel's laws, the differences between species were seen as Mendelian mutants, so species arose by discrete mutations and the gradual change inherent in natural selection was inconsistent with the view of mendelism at the time (figure 1.7, pg. 14)
In the 1920's and following, R. A. Fisher, J. B. S. Haldane
and Sewall Wright (figure 1.8, pg. 15) showed that the mechanism
of natural selection could operate on mendelian factors, and more importantly
that many continuously varying traits had genetic bases that could
be "mendelized' (shown to be the combined effects of several loci).
Thus a rigorous mathematical and genetical body of theory was developed
that provided overwhelming support for Darwin's mechanism. This came to
be known as the Modern Synthesis. The first half of this course
will deal with this Neo-Darwinian approach to Evolution.
2. THE EVIDENCE FOR EVOLUTION: AN OVERVIEW
Diversity and Adaptation
Not (by themselves) good evidence: genetic basis; creationism; Intellegent
Change over Time
Fossil horses: Gradual changes in toe number, body size,
skull morphology. Clear linear (actually sequential hierarchical) sequence.
Correspondence of Jaw/tooth morphology to habitat changes.
Extinct/Extant mammals: the extinct forms on any continent
are closely related to the extant forms on that continent. Australia:
extinct Tertiary mammals are mostly marsupials, as are the extant ones.
South America: extinct Quaternary mammals are prominently armadillos and
sloths, as in the extant fauna
Industrial melanism The peppered moth Biston betularia:
Preindustrial times the melanic forms were about 1% in museum collection;
post industrial times the melanics had reached 90% in some areas. Dramatic
frequency change of an allele at a single Mendelian locus in 100
years. Kettlewell placed the different colored moths on tree trunks of
different background color and showed that bird predation was higher
on the more visible form. Recent evidence indicates that the moths don't
always sit on the tree trunks and there are viability differences between
the two genotypes, but the dramatic frequency change is documented nonetheless:
Resistance to pesticides and antibiotics: resistant strains of
insects and bacteria have emerged in short periods of time after the application
of the insecticide/antibiotic. Either the variant was there and it was
swept to high frequency or recurrent mutations arose that happened to be
Microevolution and macroevolution. The fact of evolution can
be documented at either of these two dramatically different time scales.
Both are change over time, both are evolution
Descent with Modification
Homology: Attributes of two organisms are homologous when they are derived from an equivalent characteristic of the common ancestor: vertebrate forelimbs, DNA sequences (figure 3.6, pg. 54).
Embryos look alike while adults look different. This was viewed
by Darwin as "by far the strongest single class of facts in favor"
Vestigial organs: pelvic girdle, leg bones in snakes; non-functional
toes in horses (figure 3.9, pg. 58).
Artificial selection Dogs all descended from Wolf-like ancestor, selectively bred to have different traits. Pigeons (Darwin's example), also selectively bred; clear evidence for heritable variation
Natural selection: Differential survivorship and/or reproductive success.
Convergence Australian marsupials; Succulent plants; Alpine plants