o 



o 

 > 



On the 

 Importance 

 of Nothing 



Doing 



An exhaustive study of tiny 



bryozoans supports the idea of 



punctuated equilibrium 



by Jeremy Jackson and 

 Alan Cheetham 



From Charles Darwin's day until about 

 twenty years ago, biologists imagined that 

 the evolution of new species was a slow 

 and gradual process. The record of the 

 rocks, however, has always told a different 

 story. While some hneages can be seen de- 

 veloping in a series of transitional species 

 over the ages, most fossil species appear 

 abruptly — without intermediate forms — 

 and survive apparently unchanged for mil- 

 hons of years. Darwin attempted to dis- 

 miss this problem by invoking the 

 fragmentary and incomplete nature of the 

 fossil record. Trying to interpret it, he said, 

 was like trying to read a book that was 

 missing many of its pages and even whole 

 chapters. 



This explanation was widely accepted 

 until the 1970s, when paleontologists 

 Niles Eldredge and Stephen Jay Gould 

 noted that, contrary to Darwin's expecta- 

 tions, a century of fossil discoveries had 

 confirmed a pattern of stasis and abrupt 

 change. Why not, they urged, accept the 

 story told in the fossil record at face value? 

 Long periods without change in organisms 

 and relatively abrupt appearances of new 

 species (punctuated origins) must be in- 

 corporated into any valid evolutionary 

 theory. The Gould-Eldredge notion of 

 "punctuated equilibrium" set new hmits 

 on how speciation usually occurs. 



Amid the resultant controversy, paleon- 

 tologists set out to see if they could dis- 

 prove punctuated equilibrium. Some 

 thought that by measuring the anatomical 

 details of an extensive sampling of fossil 

 organisms, they could confirm that specia- 

 tion is gradual. When they attempted such 

 studies, however, they found that most 

 species (by about ten to one) show punctu- 



56 Natural History 6/94 



ated origins and then remain so stable that 

 specimens differing by millions of years in 

 age are visually and statistically indistin- 

 guishable. 



Still, skeptics believed that long periods 

 of species stability (stasis) were only occa- 

 sional occurrences and doubted that simi- 

 lar-appearing organisms, separated by 

 millions of years in the fossil record, could 

 really be considered the same species. 

 Within fossil skeletons, they suggested, 

 dwelled subtly different species. (Exam- 

 ples of such "cryptic species" exist among 

 some groups of living animals. Even ex- 

 perts cannot distinguish among a dozen 

 kinds of black flies or certain salamanders 

 without the aid of molecular genetic tests. 

 One discovery has documented two spe- 

 cies of apparently identical African ele- 

 phant-nosed fish that are distinguishable 

 only by the electrical pulses they produce.) 

 The question of whether skeletons alone 

 can be used as species markers is thus fun- 

 damental to accepting the fossil record as 

 evidence of evolution. Only if a creature's 

 skeleton is highly correlated with its ge- 

 netics can paleontologists study its evolu- 

 tion with some confidence. 



We addressed this question using both 

 living and fossil cheilostome bryozoans. 

 These common but little-known animals 

 live in colonies in the sea and are superfi- 

 cially similar to corals. While the individ- 

 ual creatures are microscopic, one sees 

 them in the aggregate attached to sea bot- 

 toms, tide pool rocks, and even aquatic 

 plants. These colonies can resemble a 

 mossy covering on an undersea rock or a 

 clump of miniature trees about three to 

 four inches high. The hard-shelled body is 

 topped by a soft, circular feeding organ, 

 the lophophore, composed of ciliated tent- 

 acles surrounding the mouth. The moving 

 ciha create a current of water that directs 

 plankton — microscopic algae, bacteria, 

 and flagellates — into the mouth. 



Bryozoans are not only among the most 

 abundant, wefl preserved, and diverse ma- 

 rine fossils, they also provide a fine case 

 study of evolutionary patterns because 

 their evolution is punctuated in the ex- 

 treme. Cheilostomes — a major group 

 characterized by a lidlike structure that 

 covers the aperture through which the 

 lophophore protrudes — first appeared 

 about 140 million years ago, and some 



#■ 





Magn f cat on approx mately X 90 









