Telling the Evolutionary Time: Molecular Clocks and the Fossil Record

Figure 1.2

Rates of amino acid replacement for ADH,

AMD, DDC, GPDH, SOD, and XDH in dipterans.

The time unit (abscissa) is 10 million yea

rs (myr).

White circles indicate comparisons made between

Drosophila

species, grey circles between the drosophilid genera, and black circles betwe

en tephritids and

the drosophilids. The rates on the right are for replacements

×

10

−^10

per site per year. Da is the rate for comparisons between species of the

Drosophila

genus,

Di

for comparisons between drosophilid genera, and Te between

tephritids and the drosophilids. The average values in

Figure 1.2

are from

Table 1.2

for

Di

(row 4) and Te (row 5). For Da the values used are 32, 36, 19, 2, 3

2, and 32, respectively for ADH, AMD, DDC, GPDH, SOD, and

XDH.

Averages (with their standard errors) are calculated to minimize

the impact of the phylogenetic structure of the sequence data shown in

Figure 1.1

. Thus

,

for example, for the

Drosophila

melanogaster

species-group the average amino acid distance from XDH (0.085

5±0.0243) is the arithmetic mean of the pair-wise

distances

D. ananassae

to

D. melanogoster

(0.0971) and

D. ananassae

to

D. erecta

(0.0723). The rates are obtained by linear regression, with the intercept

constrained to be the origin. The rate 5 on the right of the SOD graph

corresponds to comparisons between species within the

melanogaster

and

obscura

groups. Note the broken ordinate on the ADH graph to accommodate

the regression line for the comparison between tephritids and

the drosophilids.

MOLECULAR CLOCKS: WHENCE AND WHITHER? 13