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Lecture 2

BIOL 1020U Lecture Notes - Lecture 2: Population Bottleneck, Stabilizing Selection, Intermediate Frequency

Course Code
BIOL 1020U
Annette Tavares, Mary O.

of 9
Darwin, Evolution and Natural Selection Lecture
Chapter 21 (Part 2) (pp. 21.8 - 21.16)
Importance of evolution?
Explains how organisms have arisen and diversified from earlier organisms.
Evolution is the change over time in allele frequencies in a population of
The theory is a foundation of modern biology, genetics, and ecology.
Darwin’s Theory: Descent with Modification by Natural Selection
In 1844, Darwin wrote an essay on the origin of species and natural selection
did not introduce his theory publicly. Why? It was controversial
June 1858 - Darwin received manuscript from Alfred Russell Wallace
Theory of natural selection similar to Darwin’s
Darwin quickly finished The Origin of Species and published it the next year
Darwin Challenged Traditional Views
On the Origin of Species by Means of Natural Selection (Nov. 24 1859) stressed
two major points:
1. Current species are descendants of ancestral species
Ancestral species differ from modern species
2. Natural Selection provided a mechanism for this evolutionary change
At start of 19th century, the general belief was:
The earth was young (6,000 years old)
Species remained unchanged since their creation
But there were a few doubts about this
Darwin really rocked the boat!
Darwin challenged these belief
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Prevailing Views Before Darwin
Aristotle (384-322 B.C)
Opposed evolution
species fixed and unchanging
scala naturae
Old Testament of the Bible
Species individually designed by Creator and perfect
Seemed to corroborate Aristotle’s views
Many scientists of 1700’s believed that Creator had designed each
species for a specific purpose
Carolus Linnaeus (1707-1778)
Binomial naming system
Classified species into hierarchy – increasingly complex, nested
Similarities between species reflected pattern of creation rather than
evolutionary relatedness
Other Scientists Influenced Darwin
Many of Darwin’s ideas came from studying the work of other scientists and other
Paleontology = study of fossils in sedimentary rock in layers or strata
Cuvier opposed idea of gradual evolutionary change but observed extinctions in
fossil record
Catastrophism = repopulation via immigration. When you lose fossils in
different layers because of a catastrophic event (earthquake, flood etc.)
Other scientists suggested change via cumulative effect of slow but continuous
Geologist James Hutton (1726-1797)
Perceived that changes in the Earth’s surface can result from slow,
continuous, gradual mechanisms still operating today
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Geologist Charles Lyell (1797 – 1875)
Took Hutton’s ideas and incorporated into his principle of
uniformitarianism = mechanisms of change are constant over time
Same geological processes operate today as in past and at same rate
Both Hutton and Lyell strongly influenced Darwin’s thinking
Darwin (and Wallace) also influenced by Thomas Malthus
Essay on the Principle of Population (1798)
Populations have potential to increase geometrically
Typically doesn’t happen in real populations
Limited resources and competition
Both Darwin and Wallace proposed that best adapted individuals in the
population would ‘win’ the competition (competitive advantage) next
generation would have more individuals well adapted to environment
Lamarck’s Hypothesis of Evolution
In 1809, Lamarck hypothesized that species evolve through:
Use and disuse and
Inheritance of acquired traits
Mechanisms unsupported by evidence (modern genetics)
Also thought evolution occurred due to an innate drive to become more complex
What did Darwin think about Lamarck’s ideas?
rejected evolution due to innate drive towards complexity
agreed with introduction of variation via inheritance of acquired
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