Evolution, 4th Edition

538 CHAPTER 20

even if pollinators are scarce. The SI system almost never

re-evolves in SC lineages. Thus, both a genetic ratchet and

individual selection (sometimes) bias evolution toward self-

compatibility. However, a phylogenetic analysis showed that

the rate of diversification is greater in SI lineages than in

SC lineages, apparently because SC lineages have a higher

extinction rate (FIGURE 20.25). The great diversity of SI spe-

cies seems to be maintained by species selection.

Are there major trends in the history of life?

Do any trends or directions characterize the entire evolu-

tionary history of life? Although many have been postu-

lated, all have exceptions. Still, one might ask if there is any

feature that, on the whole, has evolved with enough consis-

tency of direction that one would be able to tell, from snap-

shots of life at different times in the past, which was taken

earlier and which later [54, 74]. Let’s consider two promis-

ing possibilities.

EFFiCiENCY AND ADAPTEDNESS There are innumer-

able examples of improvements in the form of features that

serve a specific function. The mammal-like reptiles, for

example, show trends in feeding and locomotion associ-

ated with higher metabolism and activity levels, which cul-

minated in the typical body plan of mammals. There might

well be a global trend toward greater efficiency [30]. But efficiency and effective-

ness must always be defined relative to the task set by the organism’s environ-

ment and way of life. We cannot meaningfully compare the level of adaptedness

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66 56 34

Nonplanktotrophic

Planktotrophic

Paleocene Eocene

Time (Mya)

FIGURE 20.24 A trend caused by species selection. Each bar

shows the stratigraphic distribution of a fossil species of volutid

snail. Although nonplanktotrophic species had shorter durations,

they arose by speciation at a higher rate, so the ratio of nonplank-

totrophic to planktotrophic species increased over time. (After

[39].)

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Solan

um

Ca

ps

ice

ae

Ioc

hro

min

ae

Lycium

Nolan

a

Ni

co

tia

na

Solanum melongena (eggplant)

FIGURE 20.25 A phylogeny of 356 species of Solanaceae

(tomato family). Blue tips indicate self-incompatible (SI)

species, and red tips self-compatible (SC) species that

can reproduce by self-fertilization. Primarily blue clades,

in which self-incompatibility is ancestral, often give rise

to SC species (in Solanum, for example), but primarily red

SC clades (such as Nicotiana) seldom give rise to blue SI

species. The predominance of self-incompatibility in this

family can be attributed, at least partly, to species selec-

tion. (From [33]; phylogeny courtesy of B. Igić.)

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