o 



o 

 > 



cules change at a regular rate and thus pro- 

 vide a timepiece for dating the moment at 

 which hneages leading to different organ- 

 isms first separated. The rough regularity 

 of the rate is due to the steady accumula- 

 tion of errors; the process of copying DNA 

 can lead to mutations, just as the copying 

 of manuscripts by hand can lead to 

 changes in a text. To estimate how far 

 apart the pairs of species had drifted ge- 

 netically, we looked for differences in the 

 sequence of the four DNA nucleotides on 

 a part of the circular DNA molecule found 

 in the mitochondrion (the energy-produc- 

 ing engine of the cell). We also looked for 

 differences in proteins that are determined 

 by DNA in the nucleus of the cell. 

 When we combined our data from the 



A Pacific species of snapping shrimp, 

 Alpheus sulcatus inhabits coastal 

 waters from western Mexico to Peru. 

 Snapping shrimps wield their enlarged 

 claws in territorial battles; when 

 rapidly closed, the claw makes a sound 

 reminiscent of com popping. 



behavioral and molecular studies, we 

 found a clear pattern. The shrimp pairs 

 that were least aggressive to each other 

 had the most similar mitochondrial DNA 

 and nuclear proteins, while pairs that 

 fought vigorously showed much greater 

 molecular divergence. What could ac- 

 count for some sister species being more 

 closely related to their transisthmian coun- 

 terparts than others? 



Ecological differences among the vari- 

 ous shrimps — especially the kind of habi- 

 tat they prefer — suggest that not all pau-s 

 parted company at the same time. The 

 most closely related, and thus the most re- 

 cently separated, species were those that 

 inhabit shallow, turbid waters — exactly 

 the kind of conditions that would have 

 characterized near-shore habitats immedi- 

 ately preceding the final emergence of the 

 isthmus and the closing off of any connec- 

 tion between the Pacific and Caribbean. 

 The more divergent pairs, however, both 

 in behavior and in their mitochrondrial 

 DNA and proteins, were those typically 

 found in slightly deeper water or in the 

 clearer waters of offshore islands, perhaps 

 indicating that they had already moved 

 away from the turbid edge of the emerging 

 isthmus before the land barrier was com- 



plete. How long before? Molecular diver- 

 gence rates and paleontological evidence 

 suggest that the four most closely related 

 pairs of snapping shrimps were separated 

 about the time of final closure of the isth- 

 mus, some three milhon years ago; five 

 were isolated four to six million years ago; 

 and two were separated at least seven mil- 

 hon years ago. 



Judging from the combination of be- 

 havioral tolerance and infertility demon- 

 strated by the most similar males and fe- 

 males from opposite sides of the isthmus, 

 three milhon years appears to be just ade- 

 quate for the creation of new species of 

 snapping shrimps. A study of closely re- 

 lated, but geographically isolated, fruit 

 flies resulted in a similar estimate of the 

 amount of time required to create new 

 species. In both cases, the separated or- 

 ganisms remained almost identical in out- 

 ward appearance. In contrast, during the 

 same length of time, the transition from 

 Australopithecus to our own extremely 

 different species, Homo sapiens, took 

 place. The isthmus, and the sibhng species 

 on either side of it, give us one measure of 

 the background rates of routine evolution- 

 ary change against which great evolution- 

 ary developments can be compared. 



68 Naitjral History 6/94 



