The frog: a relic of evolution?
by David B. Carter
During conversations about creation and evolution it has become apparent to me that
most people, whichever one they believe in, regard the two ideas as merely philosophical
matters related only to the past. Nevertheless, in the sciences of geology and biology,
either model will profoundly affect the contemporary search for truth. The creationist
model is of great practical importance to me as a ‘metamorphosing’ zoologist
because it leads me to expect a rational, intelligent design in the living systems
I study.
A corollary of the theory of evolution is that there is a progressive order of ‘perfection’
or ‘superiority’ as the phylogenetic trees are followed through from
the primitive single cell. Progression from the aquatic to the terrestrial environment
is regarded as evolutionary advancement. This philosophy has resulted in a poor
understanding of the physiology and ecology of several animal groups of which the
Amphibia are probably the most maligned.
From the standard first year zoology textbook, the following libelous comments are
directed at the unfortunate creatures:
‘Although well adapted to the terrestrial environment in many ways, amphibians
have retained several features from their piscine ancestors that restrict most of
them to a rather moist habitat and prevent them from fully exploiting the terrestrial
environment. … First, most amphibians are unable to prevent a large loss
of body water (through the skin) when on land. … The structure of their kidney
tubules is not yet modified for the reabsorption of a large volume of water from
the urine during the process of urine formation.’7
Thus a result of the evolutionary approach is to regard the Amphibia as a poorly
adapted group of animals—merely a doomed relic of evolution. As Parker and
Haswell put it, ‘today they (amphibians) are phylogenetically senile.’3
Or Bentley:
‘Undoubtedly the greatest physiological disadvantage which the Amphibia have
to overcome in hot, dry habitats is their permeable skin and the most valuable evolutionary
novelty which they could invent to assist their survival there would be a more impermeable
integument. … Amphibia do make the best of the situation and utilise the
skin to collect water from damp surfaces.’1
In view of the enormous ‘handicaps’ of the amphibians a number of eminent
American scientists were intrigued by the presence of frogs in arid environments.
Their studies have shown that, although desert frogs are rather poorly adapted to
survive in a laboratory dessicator, they are ‘amazingly’ well suited
to the subterranean environments in which they live.4,5
During wet conditions, the skin absorbs water and the kidney processes dilute urine
which is stored in the bladder. The bladder is a huge water tank and consequently
a kidney which reabsorbs water in the tubules would defeat the purpose of filling
the tank. Desert frogs spend most of their lives burrowed in soil in a dormant state.
The permeable skin—far from being a detriment to terrestrial life—confers
on them the unique ability to absorb soil water even when the soil is apparently
quite dry. If the soil becomes too dry they accumulate their shed skins to form
a protective cocoon and survive by reabsorbing bladder water.
Of course the majority of frogs are semi-aquatic or live in moist habitats, to which
they are admirably designed. However, it was noticed that some tree frogs in Africa
and South America survive for months under rather extreme conditions. Studies have
shown that these frogs can make their skins quite impermeable by wiping a lipid
film over their bodies. They also excrete uric acid which can be excreted with very
little water loss. Thus their control of water loss is akin to the desert reptiles.6
I could elaborate at length about skin respiration, metabolic rates and energy storage,
the variation in reproductive modes, etc., but the main point from the wealth of
accumulating studies is that frogs are brilliantly, precisely and rationally
designed for the habitats they occupy. This is an obvious prediction
of a creation model of origins. While these findings are not irreconcilable with
the evolution model (frogs ‘make the best of a poor deal’) they were
unexpected and serve to illustrate one of my pet contentions: that the ‘advanced’
and ‘primitive’ hierarchy, which is an inevitable corollary of evolution,
is a major setback to the study of ecology and environmental physiology.
David B. Carter,
at the time of writing, was working for an Honours degree in Zoology at the University
of Adelaide.
References
- Bentley, P.J. (1966) Adaptations of amphibians to arid environments. Science.
152:619-623
- McClanahan, Shoemaker & Ruibal (1976) Structure and function of the cocoon of
a ceratophryd frog. Copeia 1976:176-185
- Parker, J & Haswell,W. (1962) Textbook of Zoology: Vertebrates. pp 386-387,
Macmillan, St Martin's press.
- Ruibal, Tevis & Roig (1969) The terrestrial ecology of the spadefoot toad.
Copeia 1959:571-584
- Seymour & Lee (1974) Physiological adaptations of anuran amphibians to aridity:
Australian prospects. Aust. Zool, 18(2):53-65
- Shoemaker & McClanahan (1975) Evaporative water loss, nitrogen excretion and
osmoregulation in phvllomedusine frogs. J.Como.Phvsiol. 100:331-345
- Villee, Walker & Barnes (1973) General-Zoology. W.B. Saunders Co, Philadelphia.
pp 701-705.
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