The Modern Synthesis

1
The Modern Synthesis Goals Understand how
evolution operates at the level of
genes. Sub-goal Understand the issues (2)
surrounding blending inheritance. Sub-goal
Model genotype and gene frequencies in a
population.
2
What would happen if the blending model of
inheritance were correct? Selection wouldnt work
(very well).
3

• Then why does it sometimes seems as if blending
  inheritance does occur?
• Co-dominance.
• Polygenic traits.

4
What happens in evolution is that some genes
become more common, displacing their alternative
alleles. How does that happen? Except in gametes,
genes occur in diploid genotypes. So Lets
begin with genotype frequencies
5
Evolutionary dynamics of genes and
genotypes. Genotypes first. Assume a population
of 10,000. genotype of individs. freq. of
genotype aa 2000
2000/10,0000.2 Aa 4000
4000/10,0000.4 AA 4000 4000/10,0000.4
6
Evolutionary dynamics of genes and
genotypes. Knowing the frequencies of these
genotypes, can we compute gene (allele)
frequencies? Yes. For example, Each AA
individual has two copies of A. Each Aa
individual has one copy of A. Just add up copies
of A across the population.
7

genes qfreq. of a pfreq. of A only 2
alleles, so pq1
genotypes aa0.2 Aa0.4
AA0.4
p (2 X freq of AA) (2 X 0.5 X freq of Aa)
2 x 1.0 p (2 X
0.4) (2 X 0.5 X 0.4)
2 x 1.0 p (0.8) (0.4) 0.6
2.0
8

genes qfreq. of a pfreq. of A only 2
alleles, so pq1
genotypes aa0.2 Aa0.4
AA0.4
q (2 X freq of aa) (2 X 0.5 X freq of Aa)
2 x 1.0 q (2 X
0.2) (2 X 0.5 X 0.4)
2.0 q (0.4) (0.4) 0.4
2.0
9

genotypes aa0.2 Aa0.4
AA0.4
genes qfreq. of a pfreq. of A only 2
alleles, so (pq)1
q 0.4, p 0.6, (q p) (0.4 0.6) 1
10
What happens when these individuals
reproduce? Gametes are haploid. Whats the chance
that a gamete carries A? Whats the chance that
a gamete carries a? The same as the gene
frequencies in the populationfor both sexes of
parents
11
S
s
SS
Ss
S
s
Ss
ss
12
60 40
S
s
SS
Ss
S
40 60
s
Ss
ss
13
Thus q 0.4, p 0.6, (q p) (0.4 0.6)
1
Freq of AA p2 0.6 x 0.6 0.36 Freq of Aa
2pq 2(0.6 x 0.4) 0.48 Freq of aa q2 0.4
x 0.4 0.16 Check 0.36 0.48 0.16 1.00
14
What are gene frequencies in the offspring
population? Freq of AA 0.36 Freq of Aa
0.48 Freq of aa 0.16 p (2 X freq of AA) (2
X 0.5 X freq of Aa) 2
x 1.0 p (2 X 0.36) (2 X 0.5 X 0.48)
2.0 p (0.72) (0.48) 1.2
0.6
2.0 2.0
15
What are gene frequencies in the offspring
population? Freq of AA 0.36 Freq of Aa
0.48 Freq of aa 0.16 q (2 X freq of aa) (2
X 0.5 X freq of Aa) 2
x 1.0 q (2 X 0.16) (2 X 0.5 X 0.48)
2.0 q (0.32) (0.48) 0.8
0.4
2.0 2.0
16
p (0.72) (0.48) 1.2 0.6
2.0 2.0 q
(0.32) (0.48) 0.8 0.4
2.0 2.0 Hence, no
evolution. Is that good or bad for the
theory? There was no selection, so there was no
evolution.
17
What would selection look like? Freq of AA p2
0.6 x 0.6 0.36 Freq of Aa 2pq 2(0.6 x
0.4) 0.48 Freq of aa S(q2) S(0.4 x 0.4)
S(0.16)