Post by grrraaahhh on Nov 10, 2010 20:02:37 GMT -9
Excerpt from Max Hecht:
Rate of change and mode of evolution
Rate of change and mode of evolution. In 1974 Hecht, Eldredge and Gould debated the problem of punctuated equilibria as envisaged by Eldredge and Gould (1972). In this discussion I presented, as an example of phyletic evolution, the history of the polar bear which was later commented upon by Gould and Eldredge (1977). They and Stanley (J 980) said that the polar bear history was an example of punctuated equilibrium because of its rapid speciation. A reexamination of polar bear history can test whether it better fits the punctuated equilibrium or phyletic evolutionary model.
Kurten (J 964) demonstrated that the polar bear, a species of recent origin, was derived from ancestral populations of the brown bear, Ursus arctos. The polar bear invaded the ice pack and the adjoining tundras and as a result the two species are completely allopatric. The two species, U. maritimus and U. arctos, have not developed any genetic isolation mechanisms as indicated by the many hybrids and backcrosses found in zoos. The primary isolating mechanism is probably behavioural when given a free choice of mates. The morphological differences between the two species are clear and are correlated with the more carnivorous diet and the more aquatic habit of the polar bear. These two species are differentiated morphologically by skull shape, limb proportions and molar morphology.
The living polar bear populations are small because the total present world population is about 20,000 individuals. Based on studies of game populations, about 48 % are adult bears and slightly more than 60 % of these would be females. The females breed (at best) in alternate years after their fourth or fifth year and produce a single cub. The effective breeding populations of females must be under 3 000 individuals. Studies of the vagility of the bears revealed that some bears can be highly nomadic and individuals have traversed 3200 kilometers within a single year. Such vagility indicates possible gene flow of a degree to effectively make the entire species a single population. This observation is corroborated by studies on skull vanatJOn. Kurten (1964) indicates that early populations of the polar bear from the Late Wurm were characterized by dental variation more closely approximating the brown bear than the living polar bear populations. His chronosubspecies, U. 111. tyrannus, emphasizes another trend of size reduction from the early Wurm to the present. The phyletic trend within the polar bears show increasing carnivorous dentition and an elongated skull form with a concomitant reduction in total body size for the last 15,000 - 20,000 years. As late as 8,000 - 10,000 years ago, the polar bear populations still had a high frequency of arctos-like molars. This condition probably exists at very low frequencies in modern populations.
Chaline, J. 1983. Modalites, Rythmes, Mecanismes de L'Evolution Biologique: Gradualisme phyletique ou equilibres ponctues? Editions du Centre National de la Recherche Scientifique, Paris.
To be continued....
Rate of change and mode of evolution
Rate of change and mode of evolution. In 1974 Hecht, Eldredge and Gould debated the problem of punctuated equilibria as envisaged by Eldredge and Gould (1972). In this discussion I presented, as an example of phyletic evolution, the history of the polar bear which was later commented upon by Gould and Eldredge (1977). They and Stanley (J 980) said that the polar bear history was an example of punctuated equilibrium because of its rapid speciation. A reexamination of polar bear history can test whether it better fits the punctuated equilibrium or phyletic evolutionary model.
Kurten (J 964) demonstrated that the polar bear, a species of recent origin, was derived from ancestral populations of the brown bear, Ursus arctos. The polar bear invaded the ice pack and the adjoining tundras and as a result the two species are completely allopatric. The two species, U. maritimus and U. arctos, have not developed any genetic isolation mechanisms as indicated by the many hybrids and backcrosses found in zoos. The primary isolating mechanism is probably behavioural when given a free choice of mates. The morphological differences between the two species are clear and are correlated with the more carnivorous diet and the more aquatic habit of the polar bear. These two species are differentiated morphologically by skull shape, limb proportions and molar morphology.
The living polar bear populations are small because the total present world population is about 20,000 individuals. Based on studies of game populations, about 48 % are adult bears and slightly more than 60 % of these would be females. The females breed (at best) in alternate years after their fourth or fifth year and produce a single cub. The effective breeding populations of females must be under 3 000 individuals. Studies of the vagility of the bears revealed that some bears can be highly nomadic and individuals have traversed 3200 kilometers within a single year. Such vagility indicates possible gene flow of a degree to effectively make the entire species a single population. This observation is corroborated by studies on skull vanatJOn. Kurten (1964) indicates that early populations of the polar bear from the Late Wurm were characterized by dental variation more closely approximating the brown bear than the living polar bear populations. His chronosubspecies, U. 111. tyrannus, emphasizes another trend of size reduction from the early Wurm to the present. The phyletic trend within the polar bears show increasing carnivorous dentition and an elongated skull form with a concomitant reduction in total body size for the last 15,000 - 20,000 years. As late as 8,000 - 10,000 years ago, the polar bear populations still had a high frequency of arctos-like molars. This condition probably exists at very low frequencies in modern populations.
Chaline, J. 1983. Modalites, Rythmes, Mecanismes de L'Evolution Biologique: Gradualisme phyletique ou equilibres ponctues? Editions du Centre National de la Recherche Scientifique, Paris.
To be continued....
