Download 11.4-11.6 Darwin

11.4 & 11.5 DARWIN’S OBSERVATIONS ON HIS
VOYAGE OF DISCOVERY
Charles Darwin (1809-1882)
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A naturist, who studied to become first a doctor, then a clergy member
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In 1831, left for a five year voyage on the HMS Beagle (fig. 3, p. 520)
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While on the Galapagos Islands, he noticed each island supported unique species of
plants and animals
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Wrote the book On the Origin of Species in which he said that organisms are a
result of a history of descent with modification from common ancestors
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Variations that appear in a population occur by chance
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Also explained the variety of organisms and their means of survival
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The mechanism behind evolutionary change is Natural Selection
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Natural selection acts on an individual within a population and over generations the
population would change as a result of acquiring adaptive traits
PROOF OF EVOLUTION
1. Fossil Records
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There are progressive fossils leading from an ancient primitive organism and
culminating in modern forms
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Macroevolution: species-to-species changes

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Fossil records provide evidence of this
Transitional organisms (“missing links”) have been found
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Tiktaalik: a fossil found in Arctic Canada that links land and water organisms (has
scales and gills like a fish and has the head of a crocodile)
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Archaeopteryx: transitional form between birds and reptiles
2. Comparative Anatomy
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Convergent Evolution
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Unrelated species might independently evolve superficially similar structures
given similar environmental demands
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Develop analogous structures
Analogous structures
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Structures similar in function but not in origin
e.g. wings of flies and birds
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Divergent Evolution
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Species evolve increasingly different traits due to different environmental
demands
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Develop homologous structures
Homologous structures
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Have same evolutionary origin despite differences in functions
Ex. dolphin flippers and human hands
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Vestigial structures/features
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Structures/features that serve no apparent purpose but are homologous to fully
functioning structures/features in related species
Ex. whales have small pelvic and leg bones, human appendix, wisdom teeth, tail
bone, body hair, erector pili (muscles causing goosebumps)
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Homologous and vestigial structures provide evidence that related organisms
adapted to different environments
Ex. forelimbs of birds and mammals have similar internal structure despite being
used for diverse functions
3. Embryology
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Stages of animals can provide evidence of common ancestry
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During early stages, embryos look similar (fig. 3, p. 524)
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Ex. human, pig, chicken and salamanders develop tails and gill slits because
ancestral vertebrates possessed genes for tails and gills
4. Genetics
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Biochemistry and molecular biology provide evidence of evolutionary relatedness of
all living organisms
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Ex. all living organisms have cells that contain DNA, RNA and use ATP
5. Modern Evolution in Action

Global Warming: In order to cope with changes in their environment, some species
have modified their behaviour
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Ex. Canadian squirrels have changed their breeding habits to correspond to an
earlier spring

Ex. Blackcaps (European bird) have changed their migration patterns due to
warmer weather in regions closer to breeding grounds
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Invasive Species: non-indigenous (exotic) species are entering new territory and
disrupting native species

Ex. Blue mussels are developing thicker shells as protection against an exotic
species predator (Asian shore crab)
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Microevolution: Changes in gene frequency within a population or species
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Ex. antibiotic resistance in bacteria
11.6 THEORY OF EVOLUTION BY NATURAL SELECTION
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Another naturalist, Alfred Wallace, independently came up with the same idea as
Darwin
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Darwin, after 20 years, was still not ready to publish but was encouraged to present
his research along with Wallace and a year later he released On the Origin of
Species
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At this time there was no way to date rocks and fossils and no understanding of
genetics and inheritance
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Darwin summarized natural selection, the mechanism for his theory of evolution as
follows:
“…can we doubt (remembering that many more individuals are born than can possibly
survive) that individuals having any advantage, however slight, over others, would
have the best chance of surviving and procreating their own kind? On the other
hand, we may feel sure that any variation in the least degree injurious would be
rigidly destroyed. This preservation of favourable variations and the rejection of
injurious variations I call Natural Selection”
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See Table 1, page 530
Unanswered Questions
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For natural selection to result in the evolution of entirely new species with new
adaptations and structures, a great length of time was required
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Not everyone believed that the Earth was many millions of years old
The fossil record in Darwin’s day did not show evidence of transitional species that
solidify the idea that species were changing over time
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Today, using radiometric dating, scientists have conclusive evidence that life on
Earth started at least 3.5 billion years ago

Fossil records are more extensive today and show evidence for intermediate
species, especially in the case of vertebrates
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Darwin’s theory did not offer an explanation for the source of new variations; in
other words, how could an entirely new structure be explained? It was widely
believed at the time that inherited traits from each parent were somehow blended in
offspring and that unusual or rare variations would become diluted over time
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In 1866, Gregor Mendel published his investigations of inheritance in garden peas
which were not widely accepted until 1900
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Today, discoveries in modern molecular genetics have revealed that mutation and
recombination are the source for new inheritable variations