Chapter 2 – Darwin and
Natural Selection
Charles Darwin (1809-1882)
– An avid hunter, with strong
interests in collecting, geology
and botany.
– At age of 16 began to study
medicine, later switched to study
theology at Cambridge
University
• He did not finish.
– In 1831, became the naturalist
onboard the ship H.M.S Beagle,
during a five-year voyage of
geographical exploration.
Fig 2.1 pg 21
Voyage of the Beagle
– Darwin read Lyell's Principles of Geology, and used it
to study geological features during the voyage.
– Discovered fossil bones in Argentina, such as the
6m-high giant sloth, Megatherium.
Fig 2.3 pg 24
Why are Giant sloths found in the
same continent as armadillo?
Tree sloth
Armadillo
Glyptodon
Galapagos Islands
– Small volcanic islands
off western coast of
South America.
– Darwin collected many
specimens of finch.
• He later noted that each
island had finches with a
unique beak shape.
Fig 2.4 pg 25
Darwin's Finches
– Each island has unique beak shape.
– Darwin noted that the variation in beak shape
provided advantages for the food source available
on each island.
– Beak shape modified, but how?
• Natural Selection
Fig 2.5 pg 26
Lamarckian Inheritance
• Lamarckism – adaptations driven by an inner vital
force.
• Principle of Use and Disuse Variation
– When a feature is used, it becomes elaborated, and
when it is not used it regresses.
– Proposed as means for variation among species, and
development of adaptations.
• Inheritance of Acquired Characters
– Adults pass on traits by somehow passing on somatic
cells.
Natural Selection
• Thomas Malthus (1766-1834)
– Wrote a short paper about population growth and
food sources.
• Populations grow geometrically (2, 4, 8,…)
• Food Sources grow arithmetically (1, 2, 3,…)
• Leads to food shortage
• Darwin realized food shortage acted as
selection force, driving variation among
species.
Artificial Selection
• Before publishing the Origin of Species,
Darwin amassed considerable evidence
for how adaptations occur.
– Studied chicken breeds, to show how over a
short period of time (generations),
adaptations can be selected for (artificially).
Fig 3.22 pg 59
Blending Inheritance
• Heredity was believed to be a blend of
both parents
– Red + White = Pink
– How can adaptive variation be passed on if
blended?
Pangenesis vs. Germ Plasm
Theory
• Darwin's solution to problem of blended
inheritance.
– small particles (pangenes) derived from
parent were incorporated in tissue of
offspring.
• Was later shown to be false
• Germ Plasm Theory: only gonads
contribute to offspring.
Fig 2.9 pg 34
Pangenesis Theory
Fig 2.9 pg 34
Discussion
• What is a better evidence supporting
natural selection?
1. a useful organ
2. a useless organ (vestigial structures)
Discussion
• What are the prerequisites for natural
selection to occur?
Darwin :
Reproductive ability
Environmental restrictions
Struggle for existence
Heritable variations
Homework Due next Tuesday
• Describe your study using Hypothesistesting style approach.
Phenomena現象: 真相or假相
Question問題: Proximate近因or ultimate遠因
Hypothesis: Falsifiable?可能被否定?
Testing: Denying or supporting實驗結果能否
定什麼?
打一針KCN,人就死了,是 是支持
KCN有毒的好証據?
邏輯分析
打一針X ,人就死了,是
是支持X有毒的好証據?
• 很多人打一針X,就死了,是
是支持X有毒的好証據?
打一針KCN,人就死了,是 是支持
KCN有毒的好証據?
未必.
打一針X ,人就死了,是
是支持X有毒的好証據?
• 很多人打一針X,就死了,是 是支持X有毒的好証據?
• 很多人打一針X,就死了,但是打生理食鹽水的都沒事,
是 是支持X有毒的好証據?