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BIO 370
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Lecture 4
Charles Darwin and the development of
Evolution by Natural Selection
I. Charles Darwin (1809-1882).
Introduction –Read and digested Lyell’s Principals of Geology, and became involved with Lyell’s empiricism; the
necessity to EXPLAIN FROM OBSERVATION, rather than TO it (as favored by the “rationalistic approach of
“logical deduction.”
a. This may in part explain the mass of detail that is the strength of Darwin’s book, as it was Lyell’s.
b. Both men restricted themselves to empirical data, data that could be observed.
A. CD Biographical sketch.
1. Born the same day as Lincoln, to prominent (but not aristocratic) family in Kent, England.
2. Education - Darwin was not a splendid student.
a. Went to Edinburgh to study medicine, washed out, partly because he couldn't stomach the brutal conditions.
b. Then to Christ College, Cambridge, with intention of studying for the clergy. Natural theology was the
accepted mode.
c. (1831) J.S. Henslow, (botanist sets him up as civilian naturalist on the surveying ship H.M.S. Beagle,
commanded by Capt. Robert FitzRoy.
2. He took only 2 books with him; Paley's Natural Theology, which argued for a literal interpretation of natural
history in light of the bible, and Lyell's Principles of Geology which argued for interpretation from visible
evidence.
a. He left believing implicitly in the former, and returned a convert to the latter.
II. Influences of the voyage on Darwin’s thinking.
A. Human1. Darwin got to see the range of "adaptiveness' in humans; Feugean natives educated in England.
B. Geology.
1. Noted that fossils he collected resembled (but were not identical to) living organisms in the same region.
Not consistent with Catastrophism.
D. Biology
1. Galapagos - Tremendous variations among what were clearly similar species.
a. Tortoises on different islands have different shells; so much so the locals could tell at a glance which island
a tortoise came from.
b. Groups of species (like Darwin's finches) which are obviously related, but are ecologically very different.
2. South American flora and fauna from different regions were distinct from the flora and fauna of Europe.
3. He also noted that temperate species of South America were taxonomically closer to species living in tropical
regions of South America than to temperate species of Europe.
4. This extended to fossil record. In Patagonia he collected giant fossil ground sloths, elephants,llamas and
armadillos- all clearly extinct, but very much akin to he living animals of the area.
III. The Origin of Species
1. Discusses population growth with Robert Thomas Malthus.
a. Malthus was among first to recognize that population growth is a multiplicative (or exponential)
b. Darwin recognized that any species has a reproductive capacity high enough to rapidly overpopulate its
environment, BUT THEY DON'T.
2. This combined with his observations of nature gave him the evolutionary mechanism he was looking for - he
eventually called it natural selection. Sits on it. Starts writing in 1856, when….
3. In 1858, he received a letter from Alfred Russell Wallace asking his advice about a paper he (ARW) was
planning to write which outlined a theory of evolution essentially identical to CD's.
5. He nearly gave up, thinking himself scooped. Lyell intervened, persuaded CD to publish his theory
6. CD then put aside his big book on natural selection, and in less than a year, wrote a 513 pp manuscript, called it
"The Abstract". That was published, became an instant bestseller (1st printing sold out in one day). Title: The
Origin of Species by Means of Natural Selection, or the preservation of favoured races in the struggle for life.
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IV. The building blocks to Darwin's theory.
A. Variation under domestication.
1. Individuals of domesticated species are always variable - individuals differ.
2. This variation is inherited - offspring bear a non-random resemblance to parents.
3. Artificial selection - proves inheritance; also analogy for evolution in nature.
a. Small changes each generation accumulate and can lead to great changes.
b. Breeders have caused changes so great different breeds of animals resemble different species.
c. Some so great the ancestors of modern varieties are uncertain.
4. If artificial selection can operate in such a short period of time, then natural selection should be capable of
considerable modifications over hundreds of thousands of years.
B.
1.
Variation under nature.
He finds an astonishing array of variation in barnacles and had great difficulty in classifying them into species
and subspecies.
C. Heritability
1. Heritability- the proportion of the variation observed in a population that is due to variation in
the effects of genes.
a. That is, the estimate of heritability attempts to eliminate the influence of environment on
phenotypic expression.
2. Heritability is determined via regression analysis.
a. Regression analysis provides an r2. This tells you how much of the variation in Y is a
result of X. E.g. That is, of the variation in beak depth of offspring, how much can be
explained by the beak depth of the parentals.
D. The struggle for existence. (And excess offspring)
1. Darwin recalls Malthus.
2. He observed nature - not uncontrolled growth. How come? Most individuals born don't survive.
3. Population size reduced by ecological factors.
4. On average, the most fit individuals pass their genes onto more offspring than less fit individuals.
E. Is Survival and Reproduction Nonrandom?
1. Great question by Darwin! He considered the null hypothesis. We the observations he was making solely due to
chance alone?
a. Natural selection requires more than surviving, it means surviving to reproduce. And it means reproducing
more than other phenotypes.
2. Example of Geospiza fortis – the work of Peter and Rosemary Grant (and students, e.g. Peter Boag)
F. This leads to-: Natural selection.
1. Natural selection is the differential success in reproduction, and its product is adaptation of organisms to their
environment.
"This preservation of favorable variations and the rejection of injurious
variations, I call Natural Selection."
2. Even slight advantages of some variations will accumulate in a population over many generations of being
perpetuated by natural selection.
3. Natural selection thus occurs from the interaction between the environment and the inherent variability in a
population.
4. Variations in a population arise by chance, but natural selection is not a chance phenomenon since environmental
factors set definite criteria for reproductive success.
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G. To summarize natural selection
1. Thus, variation + inheritance + struggle for existence = evolution by natural selection. If these three
preconditions are met, evolution necessarily results.
2. Results in adaptation.
3. Unique features of CD's theory:
a. Entirely materialistic and mechanistic - depends entirely on forces or factors that can be observed.
b. No limit to the amount of change possible.
c. Non-progressive - proper icon for history of life isn't ladder (scala naturae), but tree.
4. Important implication: Natural selection favors only successful individuals, not coexistence of species. Contrast
with popular views on the harmony of nature. Improvement of one species can even Ö extinction of another.
V. The Nature of Natural Selection
A. Natural Selection acts on the individual but its consequences are seen at the population level.
B. Natural Selection acts on the phenotype, but the result is a change in genotype (change in allele frequencies).
1. This is validation for the concept that for natural selection to occur, traits must be hereditary.
C. Natural Selection cannot predict the future (and neither can organisms)
1. The phenotype of a generation is a product of the environment that preceded it. If the environment stays the
same, so will the phenotype of the population.
D. Natural selection can produce new traits, even though it operates on existing traits.
1. Natural selection can only select from the range of variation that already exists in the population.
2. Paradoxically, natural selection can produce new traits.
a. Mutation creates more variants for N.S. to act on.
3. Occasionally, novel characteristics can be generated through preadaptation. (e.g Panda’s thumb)
b. This occurs when a pre-existing trait is used in a novel way and is elaborated upon by selection.
E. Natural Selection does not predict perfection
1. Natural selection can only favor those that are fittest from among the range of genetic variants available. But, no
population contains the full range of genetic variation that might exist, so the range of genetic variants might be
limited.
2. Organisms come to match their environments by being ‘the fittest available’ or ‘the fittest yet’; they are not ‘the
best imaginable’.
F. Fitness is not Circular
1. E.g. Of course cacti are more likely to survive and reproduce in the desert, they have the traits that permit them
to do so.
a. The weakness in our thinking is we forget the fundamentals of how natural selection works.
G. Natural Selection Acts on Individuals, not groups.
1. Groups of individuals are not selected for. Neither are species.
a. Self-preservation will be selected for unless genetically closely related.
H. Miscellaneous.
Fitness.
1. The fittest individuals in a population are by definition, those that leave the greatest number of descendants.
2. Fitness is a relative term, not an absolute term.
Historical Factors
1. Organisms live where they do for reasons that are often, at least in part, accidents of history.
2. Moreover, they have lived in environments that were quite different from the present: some of the features
that they acquired in other environments now hang like millstones around their necks.
3. These characters that they have acquired limit and constrain where they can now live and what they do.
4. These characters can also constrain how these organisms respond to additional changes in their
environment.
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VI. Problems with the theory of Natural Selection-What Darwin didn’t know.
A. Source of variation.
1. Darwin did not know the biological basis for heredity or its variations, and his arguments were weakest in these
areas.
B. Laws of inheritance- the demise of Neo-Lamarckism
1. If natural selection was to work, traits had to be hereditary.
2 Recognized this as the greatest weakness of theory. He relied on blending inheritance in 1st ed, was shot down blending inheritance reduces variability.
3. CD needed something that would maintain variability -"pangenesis"
a. pangenesis - all parts of the body produced hereditary units, which traveled via the circulatory system to the
reproductive organs for transfer to the next generation. Shot down in face of experimental evidence.
3. August Weismann (1834-1914) chopped the tails off of 22 generations of mice, and the mice still bore tailed
offspring (could be argued that it test Lamarck's hypothesis of disuse).
a. Weisman offered the germplasm theory - that the heritable genetic material was sequestered in germplasm,
and that the body (somatic tissue) had nothing to do with inheritance. Here comes the origins of genotype
and phenotype.
C. Gregor Mendel.
1. Mendel's experiments - used garden pea, successfully deduced laws of heredity by choosing simple discrete
characters (e.g. flower color), keeping detailed records of his experimental results, and mathematical analysis
(unusual at the time). Major result: inheritance is particulate.
a. Mendel demonstrates that traits are discretely heritable. Disproves blending theory.
2. Mendel lucked out. Peas had no surprises in their reproductive system. (R.A. Fisher demonstrated that Mendel had
a 1:14000 chance of getting the results he reported.
a. Mendel also worked with Hawkweed (Hieraceum) which was a total failure. Hawkweed is apomictic
(asexual).
D. Orthogenesis –straight-line evolution.
1. Espoused by paleontologists who discovered progressive trends in the fossil record.
2. The variation that arises is directed toward fixed goals, and that a species evolves in a predetermined direction
without the aid of selection.
3. Such trends need not be adaptive, and could drive a species toward extinction. (contrast with teleology)
4. E.g. Irish elk (Megaceros giganteus) reputably became extinct because it could not stop evolving larger and
larger antlers which became too unwieldy for the beast.
5. No mechanism was proposed for orthogenesis and it was discarded during the Evolutionary Synthesis.
F. Mutationism – based on the principles of Mendelian Genetics.
1. Darwin argued that slight differences among individual that we see results in continuous variation in a
population.
2. The Mendelians followed Mendel in emphasizing discrete variations, inherited as single factors. Continuous
variation had no genetic basis.
3. DeVries asked where variation came from.
a. Worked with evening primrose (Oenothera lamarckiana), an apt omen.
b. Coined the word mutation for variants of evening primroses that appeared spontaneously and differed
substantially from parents. He considered these to be new species.
c. Ergo, mutation was sufficient to account for the origin of species, natural selection was superfluous.
4. Mendelians therefore believed that evolution proceeded in jumps by macromutations.
5. The snapper was the experimental organism – Mendel was lucky, DeVries was not.
a. Oenothera (in hindsight) has heterologus translocations in its chromosomes – portions of chromosome are
exchanged, linking the chromosomes together. In Oenothera, all 7 chromosomes of the haploid complement
are linked together, and in meiosis, they travel as a unit.
b. Two morals here. 1) Beware of extrapolation a theory from a single organism, especially if you don’t know
its biology. 2) In hindsight, do population studies – which DeVries did not.
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VII. The New Synthesis
A. The people
1. During the early 1900’s, the work of Mendelian genetics laid the groundwork for the understanding of how
evolution could operate.
2. Systematists and paleontologists were at odds with the geneticists
3. The evolutionary synthesis or modern synthesis was the reconciliation of Mendelian genetics with the
mechanisms of natural selection.
a. R.A. Fisher, Sewell Wright and J.B.S. Haldane all contributed.
b. Theodosius Dobzhansky is given credit because as a practical person, a naturalist and a systematists, he
possessed many of the same attributes that made Darwin a great synthesizer.
c. He masterfully united the theory of population genetics with the systematists’ and naturalists’ view of the
discreteness of species in his 1937 book, Genetics and the Origin of Species.
d. The classic works of theoretical population genetics demonstrated that natural selection could work with the
kinds of variation observable in natural populations and the laws of Mendelian inheritance.
e. No other processes were needed, nor are the inheritance of acquired characteristics, directed variation, or
macromutations that produced ‘hopeful monsters’ necessary.
4. In 1940, these scientists were able to fully integrate genetics and Darwinian theory to argue persuasively that
mutation, recombination, natural selection, and other processes operating within species account for the major,
long-term features of evolution (macroevolution).
5. Twenty tenets were set down, involving contributions from laboratory genetics, population genetics, natural
history, systematists and paleontologists.