Transcript DARwinism - smithlhhs

Predictions From Darwinism
1.
2.
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Natural Selection is the only significant
mechanism of evolution
Evolutionary change is slow and
gradual
All evolutionary change is adoptive
(does not happen if it doesn’t increase
chance of reproductive success)
Macroevolution is the same as micro
(human change same as bacterial)
Criticisms
Relative absence in fossil record of any
transition types
1.
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Punctuated Equilibrium (Steven Jay Gould)
proposed as an alternative
Possible limits to genetic variation
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Only so much could be contributed to
random, small change in genes
Blended inheritance
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Favorable variation “swamped out” by
backcrossing with “ordinaries”
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Mendelian genetics helps in this regard
Non-adaptive traits
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Example: peacock’s tail - more energy to
make, attractive to predators, etc
Darwin’s response was sexual selection
Evolution of Populations
A key in Darwin’s ideas was variation
 Although he may not have understood
the mechanism(s) then, we have a much
better grasp now
 One concept he did observe was
geographic variation
 The example here would be the finches

But what exactly causes variation?
 One method is mutation
 The genetic code (order of nucleotides)
determines which amino acid gets
expressed
 The linkage of specific amino acids
makes a protein
 If the order is changed – new amino acid
– new protein
 But not all mutations result in new
proteins
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A point mutation could do absolutely
nothing (silent), or it could have
disastrous consequences
Gametes
Another aspect of variation is the
rearrangements that occur during
meiosis
 One example is independent assortment
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There are even further possible
combinations when crossing over
(production of recombinant
chromosomes) is included
Causes of Microevolution
The Hardy-Weinberg principle states that
there are five requirements for stability
Any deviation from the requirements below
indicates the population is evolving
1. Very large population size
2. Isolation from other populations
3. No net mutations
4. Random mating
5. No natural selection
Monitoring
So now the question becomes, how do
we watch for changes
 Recall the Hardy-Weinberg principle
 There is also an equation which can
predict the relative frequencies of alleles
in a gene pool

Example
Imagine a wildflower population of 500
plants
 In this, there are pink alleles A,
completely dominant over white alleles a
 20 are white, so they would be aa
 320 are AA
 160 are Aa

Since they are dipliod, the pink allele
accounts for 800 of the 1000, and white
200 of the 1000
 We can use math to determine relative
frequencies in the population
 Any deviation from that means the
population is evolving

Example
Batten disease is a rare recessive
neurodegenerative disease, affecting 3 out
of every 100 000 people in North America.
Based on this knowledge, what
percentage of people are carriers and
could pass it onto their offspring?
Answer
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We define the dominant, normal allele
as B, and the recessive as b
Since occurrence is 3 out of 100 000,
b2=0.00003
So, frequency of recessive allele is
b=√0.00003 = 0.005
The frequency of the dominant allele is
B = 1-b = 1-0.005 = 0.995

The frequency of carriers would be
2Bb=2 x 0.995 x 0.005 = 0.0095
 So, approximately 1% of the population
are carriers for this disease
Example (try on your own)
It is believed that approximately 4% of
Canadians of South American decent are
carriers for the recessive condition sickle
cell anemia. If 98% of the alleles in this
population are dominant, what should the
prevalence of sickle cell anemia be?