680 



ECOLOGY AND EVOLUTION 



little evolution), and (4) taxonomic 

 (groups once highly varied and now re- 

 duced to a few species). A single relict spe- 

 cies may fit several of these categories, and 

 all these types are probably indications of 

 evolutionary retardation as the result of 

 competition with more successful and more 

 recent groups (p. 661) or of rapid changes 

 in the habitat occupied. 



The most extreme examples of relicts are 

 those monotypic famiUes and orders that 

 the systematists recognize as single repre- 

 sentatives of large branches springing from 

 near the base of the phylogenetic tree. 

 Examples include the ginkgo tree {Ginkgo 

 hiloha), the only remaining species of the 

 order Ginkgoales, which, through the pos- 

 session of motile speiTn cells, represents the 

 primitive transition leading to the higher 

 seed plants in which a passive sperm nu- 

 cleus is transported in the pollen tube; the 

 recently discovered lobe-fiimed fish {Lati- 

 meria chalumnae) found off South Africa, 

 which belongs to a group that had been 

 thought to be extinct since Cretaceous 

 times; the famous Sphenodon of New Zea- 

 land, a generalized reptile of the order 

 RhynchocephaUa, long known from fossils 

 found in other parts of the world; and the 

 primitive Australian termite, Mastotermes 

 darwiniensis, the sole living species of the 

 family Mastotermitidae, which displays 

 many transitional characters connecting 

 modern termites to their roachhke ances- 

 tors. 



Besides these monotypic branches, nu- 

 merous groups of primitive types may have 

 a number of living species, but show little 

 indication of evolutionary change over long 

 periods of time. Illustrative examples are 

 provided by the horseshoe crabs, branching 

 from the base of the chelicerate arthropods, 

 and including the Atlantic and Indo-Pacific 

 genus Limidus, which has existed from 

 Triassic times to the present; the brachio- 

 pod genus Lingula, which exhibits httle 

 evolution since the Ordovician; the lung- 

 fishes, with the genera Neoceratodus, Lepi- 

 dosiren, and Protopterus, little changed 

 from the Triassic to the present; and the 

 opossums, only slightly modified from their 

 Cretaceous forebears. 



Simpson (1944) shows that land carni- 

 vores have, on the average, evolved about 

 ten times faster than pelecypods. He also 

 concludes (on p. 143) that slowly evolving 



(bradytelic) groups have the longest geo- 

 logical history; lines evolving at a standard 

 rate (horotelic) are less fikely to survive 

 over long periods; and the rapidly evolv- 

 ing (tachytelic) groups become extinct 

 more quickly. Relicts may be found in 

 groups exhibiting all these evolutionary 

 rates. 



Stebbins (1945), after discussing a num- 

 ber of bradytehc higher plants, concludes 

 that slow evolution is brought about by 

 population structure, environment, and type 

 of adaptation, rather than through inherent 

 genetic properties of the species (see Or- 

 thogenesis p. 638). 



The known factors that might tend to 

 produce stability or retardation of evolution 

 over long intervals may be listed as follows: 

 (1) long life cycle; (2) prevention of he- 

 reditary reassortment; (3) lack of mutation; 

 (4) small, relatively homozygous popula- 

 tions; (5) equihbrium as a consequence of 

 Mendelian mechanisms; (6) lack of partial 

 or complete reproductive isolation; (7) ab- 

 sence of certain types of selection pressure, 

 particularly competition; (8) the limita- 

 tions of successful mechanisms; (9) an ex- 

 cellent, balanced adaptation to a relatively 

 stable and long-existing habitat, with severe 

 selective elimination of new variations (see 

 Cain, 1944, pp. 376-382). 



Long time intervals between generations 

 might be presumed to retard evolution (pp. 

 600, 654, 662; see also Worthington, 

 1937), but Simpson (1944, p. 137) indi- 

 cates no paleontological proof that number 

 of generations influences rate of evolution 

 in a given period. The rapid emergence of 

 strains among the asexual bacteria may be 

 in part the result of a rapid sequence of 

 generations. 



Reassortment being impossible in asexual 

 or completely parthenogenetic forms (p. 

 628), variabiUty is possible only through 

 mutation. With the absence of sexual re- 

 production, therefore, we may expect to 

 find the pace of evolution slowed or stop- 

 ped, unless there is a compensatory in- 

 crease in mutation pressure (see p. 641), 

 population size, or reduction of time be- 

 tween generations. A possible example of 

 such stability is found in the flagellate pro- 

 tozoan (Macrotrichomonas pulchra) occur- 

 ring in the intestines of widely separated 

 species of the termite genus Glyptotermes 

 (Kirby, 1942), that probably originated in 



