Evolution, 4th Edition

440 CHAPTER 17

organisms, with only one cell type, may have evolved because large size protected

them from being swallowed by unicellular predators. The advantage of more com-

plex multicellularity was almost surely the division of labor between different cell

types with different functions [34, 65]. Multicellularity enabled the evolution of

large size and elaborate organ systems. In the origin of animals and plants, and

perhaps in the other multicellular lineages as well, the first step seems to have

been the evolution of cell adhesion, followed by the evolution of new signaling

molecules and transcription factors, as well as intercellular bridges that facilitate

the movement of nutrients and signaling molecules [49]. Simple multicellularity,

based on adhesion of cells formed by cell division, has evolved in laboratory cul-

tures of yeast [75].

The Cambrian Explosion and the

origins of Animal Diversity

Animals are most closely related to the unicellular choanoflagellates (Choano-

zoa), which have cell adhesion proteins and form colonies by cell division. They

resemble certain cells in sponges (FIGURE 17.6). Choanoflagellates and animals

Futuyma Kirkpatrick Evolution, 4e

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Core jakobids

Diplomonads

RetrotamonadsParabasalids

Oxymonads

Pelobionts

Lobose amoebas

Dictyostelid slime molds

Plasmodial slime molds

Protostelid slime molds

Glaucophyte algae

Chlorophyte algae

Charaphyte algae

Prasinophyte algaeRed algae

Radiolarians

Chlorarachinophytes

Cercomonads

Foraminiferans

Euglyphid amoebas

Land plants

All members are multicellular

Clade contains unicellular and colonial/multicellular species

Unicellular with rare multicellular forms

All members are unicellular

Choanoagellates

Animals

Microspondia

FungiChoanozoaRoot

Dinoagellates

Cillates Bicosoecids

Opalinids Labyrinthulids

Cryptophytes

Haptophytes

Acrasid slime molds

Vahlkampid amoebas

Euglenoids

Trypanosomes

Leishmanias

Oomycetes

Diatoms

Brown algae

Apicomplexans

Pl

an

ts

Rhizaria

Alveo

late

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He

te

ro

ko

nt

s

Dis

cic

ris

ta

te

s

Exc

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tes

Opis

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FIGURE 17.5 Multicellularity has evolved many times from unicellular ancestors. Five

taxa (yellow circles) are entirely multicellular (red algae, land plants, dictyostelid slime

molds, plasmodial slime molds, and animals). Nine taxa (half-yellow circles) include

some multicellular or colonial species, and two (open circle) include a few multicellular

species. Some currently understood relationships differ from those shown in this phy-

logeny, which was published in 2003. (After [34], with phylogeny from [6].)

17_EVOL4E_CH17.indd 440 3/22/17 1:36 PM