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population. Indeed, the time splits of the Bolivian population could be overestimated,
being lower than those presented in this work. Therefore, the smaller cerebral
capacity and the larger teeth number of the Bolivian Inia could not be considered as
the ancestral characters in Inia . In contrast, they seem to be derived characters, which
could represent a violation of Willingston' law. A strong gene drift and the prolonged
bottleneck effects could change specific morphological characters following a
genetic revolution (Mayr, 1963) or a ―flush-crash‖ process (Powell, 1978). If so,
these characters could simply be neutral and the natural selection action invoked by
Grabert (1984 a, b, c) on these morphological characters could be false. Similarly,
the arguments in favor of the original Bolivian form discussed by Pilleri & Gihr
(1977, 1980) and Pilleri et al. (1982) are now unsustainable from a genetics point of
view. Those morphological characters considered to be the oldest and most primitive
in the Bolivian population (BP), are really derived from those presented in the
Amazon population (AP): rostrum longer (BP) vs. rostrum shorter and more massive
(AP); premaxillares, one or one and a half times as long as wide with protuberances
in front of the nares only moderately elevated (BP) vs. premaxillares, twice as long
as wide with protuberances in front of the nares very prominent (AP); number of
teeth by ramus 33 (BP) vs. 26 (AP); profile of supraoccipital with an indentation
immediately behind the vertex (BP) vs. profile of supraoccipital with no indentation
behind the vertex (AP); no condylus tertius (BP) vs. condylus tertius very frequent
(AP); squamosum covers the half of the parietal (BP) vs. squamosum covering 2/3 to
4/5 of the parietal (AP); manus with fewer phalanges, variation in number of
phalanges and no phalanx in the pollex (BP) vs. manus with more phalanges in all
five fingers, more variation in number of phalanges and pollex often has one phalanx
(AP); IV cervical with neural arch with a distinct spine (BP) vs. only rudimentary
spine in the neural arch; VI cervical with upper and lower transverse processes broad
and wing-like (BP) vs. upper transverse processes small and conical, lower
transverse processes rod-shaped (AP); VII cervical with distinct rod-shaped lower
transverse processes (BP) vs. lower transverse processes rudimentary (AP); and
sternum with rostral incision wide (BP) vs. sternum with rostral incision narrow
(AP). Henceforth, these morphological characters are neutral and fixed in the
Bolivian population by genetic drift or they evolved by different natural selection
conditions in the newly colonized Beni-Mamoré River Basin regard to the original
Amazon river conditions.
5.
Within the Orinoco basin, there are at least two different mtDNA lineages that
derived from the Amazon population. Thus, Inia geoffrensis humboldtiana is a
polyfiletic taxa and is possibly a non-valid one. One, of these two mtDNA lineages,
was established during the Holocene (5,000-8,000 years ago), coinciding with the
opinion of Grabert (1984 a, b, c) and the possible formation of black-waters and
flooded forest. I quite agree with the opinion of Casinos & Ocaña (1979) that this is
not a different subspecies from the Amazon form, disagreeing with the Pilleri & Gihr
(1977, 1980)'s observations. However, the other mtDNA lineage could also be
generated in the Holocene, but could be older than the black-water and flooded forest
formation. If this last affirmation is certain, the Amazon and the Orinoco basins were
connected around 50,000 years ago. Additionally, one animal sampled in the Negro
River (at Nova Airao) was more related to the Orinoco haplotypes than to the
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