Evolution, 4th Edition

506 CHAPTER 19

The shapes of phylogenies

We can also learn about patterns of diversification through time by studying the

phylogenies of living species. Using a molecular clock, the ages of nodes on the

phylogeny can be estimated. Changes in the rates of splitting on the tree (cor-

responding to speciation events) can then be followed through time, forming a

lineage-through-time (LT T) plot [49, 52]. This approach is complicated by the fact

that extinction events cannot be seen on the phylogeny, and for that reason the

approach is somewhat controversial.

For example, FIGURE 19.17A is a time-calibrated phylogeny of wood-warblers in

the genus Setophaga. In this case, most of the branch points are close to the base of

the tree. This is reflected in the shape of the LTT plot, which increases steeply at

first but then increases at a lower rate (i.e., it tends to level off; FIGURE 19.17B). T his

decline in the diversification rate (D) contrasts with the exponentially increasing plot

we would expect if the rate of diversification had been constant (see Figure 19.2). A

declining rate of diversification could be caused by a declining rate of speciation (S),

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_19.17.ai Date 12-12-2016

Au: Should the callout “Setophaga” (6 main green branches up from the red ones) be altered? Or ok as is?

(A)

Cumulative frequency of lineages Base Time Tips

0.2

0.4

0.6

0.8

1

0

(B)

Setophaga

radiation

Constant average

diversication rate

6 5 4 23 1 0

Sequence divergence (%)

S. fuscus

S. pensylvanica

S. petechia

S. striata

S. magnolia

S. cerulea

S. caerulescens

S. ruticilla

S. tigrina

S. angelae

S. pharetra

S. plumbea

Catharopeza

Mniotilta

Seiurus

Basileuterus

S. pityophila

S. occidentalis

S. nigrescens

Setophaga graciae

S. townsendi

S. virens

S. adelaidae

S. discolor

S. coronata

S. dominica

S. pinus

S. palmarum

S. castanea

FIGURE 19.17 A phylogeny showing a decline in

the diversification rate over time. (A) Phylogeny of

a clade of American wood-warblers, Setophaga,

and the closely related genus Catharopeza, based

on mtDNA sequences. The green branches repre-

sent outgroup genera in the wood-warbler family.

The branch lengths are proportional to degree of

sequence divergence, which is assumed to indicate

time since speciation. (B) A lineage-through-time

(LTT) plot of the Setophaga clade, on an arithmetic

axis. Note that the increase slows down and starts to

level off. A theoretical curve in which the number of

lineages grows exponentially (with a constant diver-

sification rate) is shown for comparison. The LTT plot

accurately portrays the history of diversity if extinction

has been constant (or zero). (After [39].)

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