Transcript Lecture 3: Natural Selection - University of British Columbia

Complexity: questions
How can mutation and selection explain
complex traits such as:
eyes?
blood clotting cascade?
Intelligent design claim: natural selection
and mutation cannot explain such traits
Complexity: a metaphor
• How likely is it that a chimp would type
“Methinks it is like a weasel”?
Complexity: metaphor II
• How likely is it that a chimp would type
“Methinks it is like a weasel”?
• Probability: 28 sites.
• 27 possibilities (26 letters, plus space).
• 27 * 27 * . . . . 27 = 2728 = 1.12 x 1040
Complexity metaphor III; A faster
way
• Odds of one correct letter:
• How long to get it right? A simple
simulation:
Complexity metaphor IV:
simulation
Gen.
Gen.
Gen.
Gen.
Gen.
Gen.
Gen.
01
02
10
20
30
40
43
WDLTMNLT
WDLTMNLT
MDLDMNLS
MELDINLS
METHINGS
METHINKS
METHINKS
DTJBKWIRZREZLMQCO P
DTJBSWIRZREZIMQLO P
ITJISWHRZREZ MECS P
IT ISWPRKE Z WECSEL
IT ISWLIKE B WECSEL
IT IS LIKE I WEASEL
IT IS LIKE A WEASEL
Complexity: What good is half an
eye?
Fig. 3.15
Complexity – eyes: How can lens
proteins evolve?
Origins of lens proteins
Complexity: Blood clotting
(“What good is half a scab?”)
“Irreducible
complexity?”
Complexity: blood clotting
serine proteases
serine
protease
Complexity: blood clotting
Evolutionary hypothesis?
• 600 million years ago, an invertebrate
ancestor . . .
• Low pressure circulatory system
• White blood cells present, can form plugs
• Injury releases cAMP, leading to smooth
muscle contraction
• and proteases that cut circulatory proteins
into fragments (which can clot)
Complexity: blood clotting
Evolutionary hypothesis
• Duplicate serine protease (trypsin: digestive
enzyme)
• Express duplicate gene; product goes to
circulatory system
• Proteases are inactive until cleaved
• Cellular proteases at wound activate protease
• More fragments, better clot.
• 1st clotting – specific gene.
Next step: mutation
• Recombine blood protease with an EGF
domain (epidermal growth factor)
• Would bind to cell surfaces when active.
Complexity: blood clotting
Prediction from evolutionary hypothesis
• Clotting proteins had earlier functions
• Therefore, expect similar proteins found in
organisms without clotting system
Complexity and Natural selection
• If variation, heritability, and selection occur
(=differential survival and reproduction),
populations will evolve due to natural
selection
• Natural selection retains useful variation
• Natural selection can lead to complex
traits that could not evolve by mutation
alone
Additional readings and questions
Miller, K. 2003. Finding Darwin's God. (Contains some excellent examples of
the evolution of complex traits, including the clotting system. A very good
account of intelligent design and its flaws, by a Christian evolutionist)
Xu, X and R. F. Doolittle. 1990. Presence of a vertebrate-like fibrinogen in an
echinoderm. Proceedings of the National Academy of Sciences 87: 20972101.
Question: In Darwin's Black Box (1996), Behe argues that irreducibly complex
systems, such as blood clotting, the immune system, and the eukaryotic
flagellum, could not have evolved by natural selection.
“By irreducibly complex I mean a single system which is composed of several
well-matched, interacting parts that contribute to the basic function, wherein
the removal of any one of the parts causes the system to effectively cease
functioning. An irreducibly complex system cannot be produced directly by
slight, successive modifications of a precursor system, because any
precursor to an irreducibly complex system that is missing a part is by
definition nonfunctional . . . Since natural selection can only choose systems
that are already working, then if a biological system cannot be produced
gradually it would have to arise as an integrated unit, in one fell swoop, for
natural selection to have anything to act on.”
Critique Behe's thinking using the evolution of blood clotting systems. What are
the key pieces of evidence that would be necessary to test whether blood
clotting is "irreducibly complex"?