676 



ECOLOGY AND EVOLUTION 



substances, while closely related parasites 

 are dependent upon their hosts for certain 

 vital essentials (p. 695). 



Although the phylogeny of the viruses 

 is almost wholly unknown, it is possible 

 that these protein molecules, which resem- 

 ble genes in their protein constitution and 

 their autocatalytic reproduction, may be the 

 extreme in regressive evolution of the cell, 

 still dependent through parasitism upon the 

 cellular constitution of other organisms 

 (Burnet, 1945; Darlington, 1944). 



In some instances, convergent degenera- 

 tion of homologous structures in parasites, 

 coupled with convergent evolution of adap- 

 tive structures, makes the study of the 

 phylogenetic relationships difficult (Van 

 Cleave, 1941). 



Regressive evolution may take place at 

 any level of biological integration (p. 693). 

 The simplicity of Amoeba is probably the 

 result of regressive evolution from the more 

 complex Flagellata. Most of the cases of 

 phylogenetic regression cited in the pre- 

 ceding pages are multicellular organisms. 



The absence of a free-hving tadpole 

 stage in the Surinam toad (Pipa pipa), and 

 of the free-living larva of the tsetse fly 

 (Glossina) and the sheep tick {Melophagus 

 ovintis) must be interpreted as regressive 

 evolution of a certain stage in the life cycle 

 of the metamorphic organism. In these vivi- 

 parous flies, the larvae are nourished within 

 the body of the female and pupate imme- 

 diately after emergence from the mother. 



Stages in the life cycle of cyclomorphic 

 populations may also be lost during evolu- 

 tion. Many rusts (Uredinales) exhibit a 

 rather definite tendency toward a simplifica- 

 tion of their life history as compared with 

 that of the ancestral types with greater 

 polymorphism, more highly developed 

 sexuality, and heterecious adjustments (p. 

 614). The life cycle of the Anthozoa and of 

 hydra, with polyps only, and possibly of 

 the Scyphozoa with reduced polyps or with 

 medusae only, may be presumed to be an 

 evolution from ancestral coelenterates with 

 metagenetic life cycles. 



Integrated polymorphic populations such 

 as those found in the social Hymenoptera 

 have on numerous occasions lost their 

 worker caste in association with the evolu- 

 tion of social parasitism. Examples include 

 the wasp, Vesptila austriaca, which lives in 

 the nest of Vespula rufa in Europe; Ves- 



pula adulterina, which lives in the nest of 

 Vespula arenaria in the United States (Tay- 

 lor, 1939); various species of the bee genus 

 Psithtjrus, which parasitize species of the 

 bumblebee genus, Bombus, and doubtless 

 have evolved from Bombus; and ants of 

 fourteen genera and seventeen species, in- 

 cluding Wheeleriella santschii, a parasite of 

 Monomorium salomonis in North Africa, 

 and Anergates atratulus, a parasite of Tet- 

 ramorium coespitum in Europe. Many 

 gradations are known that indicate the phy- 

 logenetic stages leading toward extreme 

 social parasitism. 



Degeneration of the nesting and paren- 

 tal instincts has occurred convergently 

 several times among birds (Friedmann, 

 1929; Miller, 1946). Examples are known 

 among cowbirds (Icteridae), cuckoos (Cu- 

 culidae), weavers (Ploceidae), honey- 

 guides (Indicatoridae), and ducks (Anati- 

 dae ) . These are all extreme examples in 

 which the parasites lay their eggs in the 

 nests of other unrelated species, and the 

 young are raised by the foster parents. 

 Closely related species show various de- 

 grees of loss of the nesting and parental in- 

 stincts that may be considered to represent 

 phylogenetic series. First came the loss 

 of nest construction (Molothriis badius) . 

 Second, the brooding and feeding of the 

 young was lost (M. rufo-axillaris) . Finally, 

 territoriality and pair formation disap- 

 peared (M. ater). 



The gradual evolution of infertility, hy- 

 brid inviability, or noncrossability, often 

 exhibited between species, is to be consid- 

 ered a regression of the reproductive func- 

 tion between populations, with a conse- 

 quent permanent reproductive isolation be- 

 tween the species (pp. 622, 623). This 

 phenomenon is so common that many 

 authors define the species as physiologically 

 incapable of interbreeding with other spe- 

 cies, either because of psychological, 

 physiological, or genetic incapacities (Dob- 

 zhansky, 1941, p. 373; Mayr, 1942, p. 120; 

 see also p. 626). 



The genetic basis for the evolution of 

 infertility and inviability has been dis- 

 cussed particularly by Muller (1942). In- 

 teresting cases are known among plants in 

 which fertility has regressed through a lack 

 of chromosomal balance, but may be re- 

 gained through allopolyploidy (Darlington, 

 1940; see also p. 625). Polyploid species 



