Evolution, 4th Edition

422 CHAPTER 16

in warmer places (see Figure 8.2). This is considered

an adaptation for conserving heat, because larger indi-

viduals have a lower ratio of surface area to volume. In

Chapter 2 we described several groups of birds, such as

hummingbirds and sunbirds, that have independently

evolved long slender bills for obtaining nectar from long

tubular flowers (see Figure 2.22). Convergent evolu-

tion is the foundation of the comparative method, which

consists of comparing sets of species to test hypotheses

about adaptation.

As we saw in Chapter 3, we can often infer the selec-

tion pressures responsible for a feature that has evolved

independently in many lineages by determining what

ecological or other factors are correlated with that trait.

We think long slender bills are adaptations for feeding

on nectar because that is what these several groups of

birds do. The comparative method is also used to test a

priori hypotheses (those that are developed before the

data are analyzed). For example, in species in which a

female mates with multiple males, the several males’

sperm compete to fertilize eggs. Our understanding of

sexual selection leads to a prediction: males that produce

more sperm should have a fitness advantage. In Chapter

3, we saw that in primates, the quantity of sperm pro-

duced is correlated with the size of the testes, and that

as our hypothesis predicts, males in polygamous species

have larger testes, relative to body mass, than males in

monogamous species (see Figure 3.20).

One important use of the comparative method is to

determine if two features tend to evolve together, which

might suggest that having one feature favors the evolution of the second, or that

both features are adaptations to the same environmental variable. Do warm-

blooded animals tend to have larger brains, perhaps because higher metabolic rates

allow growth of more nervous tissue? To answer questions such as that, we need

to be wary of a potential complication: the phylogenetic relations of the species. To

see why this is so, consider the correlation between the ability to fly and the mode

of reproduction in birds and mammals. Most birds fly and all lay eggs, while most

mammals do not fly and most give live birth. Does that suggest that those two traits

are evolutionarily linked because of some adaptive reason? Almost certainly not.

The correlation here results because the ancestor of all birds laid eggs and flew,

while the ancestor of all marsupials and placental mammals gave live birth and did

not fly. The many species of living birds and mammals have those same characters

simply because they inherited them from their ancestors. In statistical terms, we

would say that the various species are not independent data points because of their

shared evolutionary history.

We therefore need to test for adaptive coupling of characters during evolution,

while controlling for phylogenetic relations. One way to do that uses ancestral state

reconstruction. The first step is to estimate the states for two characters at each node

(or branch point) in the phylogeny. We then ask as we move along each branch of

the tree, is a change in the state of one trait correlated with a change in the second?

More sophisticated versions of this approach take into account uncertainty in the

phylogeny and in the ancestral states. When Paul Harvey and collaborators ana-

lyzed the primate data this way, they concluded that there had been many shifts

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_16.22.ai Date 01-03-2017

1975 1985 1995 2005 2015

Year of

outbreak

SL 2014

GN

DRC

Gabon

Congo

Gabon

Gabon

DRC

DRC

DRC

DRC

Gabon

Congo

2007–2008

2001–2005

2001–2003

1994–1996

1976–1977

1975

1988

1999

2004

Year

The Ebola virus strain in 2014 probably

had a common ancestor with the strain

in the smaller 2007–2008 outbreak in

Democratic Republic of Congo.

Q: Would this gure benet from a balloon caption? I’ve added one

as an example.

FIGURE 16.21 A gene tree for Ebola virus genotypes in dated

outbreaks in West Africa, with dates of common ancestors derived by

phylogenetic estimation procedures. The virus strain in the devas-

tating outbreak of 2014 probably had a common ancestor in 2004

with the strain in the smaller 2007–2008 outbreak in the Democratic

Republic of Congo (DRC). The other countries that suffered outbreaks

were Sierra Leone (SL), Guinea (GN), Gabon, and Congo. (After [14].)

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