Required Reading:

Waser, N. M. & M. V. Price. 1981. Pollinator choice and stabilizing selection for flower color in Delphinium nelsonii. Evolution 35(2):376-390.

Waser, N. M. & M. V. Price. 1985. The effect of nectar guides on pollinator preference: experimental studies with a montane herb. Oecologia 67:121-126.

v General equation for evolution by natural selection

Ø Change in frequency of A allele = (new frequency – old frequency)

v Note structure of this equation

Ø Average fitness of A allele is

§ So, expression inside square brackets is mean fitness of A allele minus population mean fitness

§ Which means that change in frequency of A allele (p) is a proportional to the difference between fitness of A allele and the average fitness of alleles in the population

Ø If A allele is absent (p = 0), then

§ No change in allele frequency

§ No evolution

Ø Similarly, change in frequency of a allele

v Try using these equations to explore the effects of different genetic mechanisms on response to selection

Ø e.g. What happens if homozygous recessive is lethal?

Ø In this case,

§ w_AA = w_Aa = 1

§ And, w_aa = 0

Ø Hence,

Ø Allelic frequency after selection is

Ø The change in allele frequency is

§ Which is a negative number

Ø Thus, frequency of a allele will decline

Ø Examine right side of equation more closely

§ Frequency of a allele declines most steeply when q is close to 1

· Because numerator is large relative to denominator

· Means that frequency declines by almost half every generation

§ Frequency of a allele declines very slowly when q is close to 0

· Because as q declines, numerator shrinks much faster than denominator shrinks, and delta q becomes a very small number

· Allele frequency change becomes almost zero per generation

Ø An empirical example from a population cage experiment involving fruit flies with a lethal allele that affects eye size and shape in heterozygotes

§ Frequency declines steeply during first few generations, but never reaches zero during the experiment

(from Hedrick, P. W., 1985, Genetics of Populations)

Table 4.4 (Hedrick, 1985) illustrates these points with numerical examples

q0	qt	t
0.5	0.25	2
	0.1	8
	0.01	98
0.1	0.05	10
	0.01	90
	0.001	990
0.01	0.005	100
	0.001	900
	0.0001	9900

Third column (t) indicates the number of generations needed to reduce frequency of recessive lethal allele from initial frequency (q₀) to the frequency in column q_t , where q is the frequency of a recessive lethal

§ When recessive lethal is common (q = 0.5)

· It is reduce to half that frequency in just 2 generations

§ However, when recessive lethal is rare (q = 0.01)

· It takes 100 generations to reduce that frequency by half

Ø Cystic fibrosis

§ Common recessive lethal in humans is cystic fibrosis or "CF"

· Homozygous individuals have salty sweat and severe digestion problems. Congestion of bronchi and lungs leads to many secondary infections

§ Life expectancy has grown from 2 years to over 30 since 1940

· But, both males and females are sterile, so the condition is still a recessive lethal

· Extra information can be found online at http://www.ncbi.nlm.nih.gov/htbin-post/Omim/dispmim?219700

§ About 1 in 20 whites heterozygous for cftr gene

· Why so many heterozygotes?