To show the involvement of developmental drive, it is necessary to compare existent and nonexistent morphologies.[1]
Centipede Segments
There exists at least 3000 species of centipede with trunk segment numbers that range from 15 to 191. There are two orders of centipedes in which developmental drive is evident.[2]
The order Lithobiomorpha consists of about 1100 species. All have 15 trunk segments at their adult stage. However, these centipedes hatch from the egg with fewer than 15 segments. The segments are added with age through a series of moults through which the hatchling grows towards adulthood. Lithobiomorph centipedes with an even number of trunk segments exist, but only as juveniles. However, given the role of heterochrony in evolution, there have been no shifts of relative timing so that reproductive maturity and a cessation of segment addition have occurred in an even-numbered trunk segment juvenile in at least one of the many lithobiomorph species. It is more plausible that there is a drive towards odd-numbered trunk segments than a constraint of even-numbered trunk segments.[3]
The order Geophilomorpha consists of about 1000 species with variable trunk segment numbers. The overall range of segment number for the order is from 27 to 191. Within this range almost all odd numbers are represented in individuals whereas all even numbers are completely absent. Despite the larger number of segments in geophilomorphs, all are formed during embryonic development. The tiny hatchling has its full adult complement of segments. There are no segments added in any series of moults in any geophilomorph species in postembryonic growth.[3]
All variant centipede ontogenies, from whatever starting point, are driven into odd-segment-number character states. Therefore, there is absolute drive in this direction, and equivalently absolute constraint regarding even numbers of segments.
- ^ Arthur, W. (July 2001). "Developmental drive: an important determinant of the direction of phenotypic evolution". Evolution & Development. 3 (4): 271–278. PMID 11478524.
- ^ Chipman, Ariel D; Arthur, Wallace; Akam, Michael (2004-07-27). "A Double Segment Periodicity Underlies Segment Generation in Centipede Development". Current Biology. 14 (14): 1250–1255. doi:10.1016/j.cub.2004.07.026.
- ^ a b Arthur, W. (October 2002). "The interaction between developmental bias and natural selection: from centipede segments to a general hypothesis". Heredity. 89 (4): 239–246. doi:10.1038/sj.hdy.6800139. ISSN 0018-067X. PMID 12242638.