Classifying the Diversity of Life
A major lesson that students learn is that biological classifications
and evolutionary trees are hypotheses. They are only as good as the
data used to construct them. They also learn that, as stated by the
well-known Harvard biologist, E.O. Wilson (1992), “Systematics is mostly
science but also a bit of art.”
Over 50 freshwater species are found in North America, most of which are found in subterranean habitats (e.g., caves; Holsinger, 1976). Gammarus species are most common in relatively cool springs, streams, ponds, and lakes. Gammarus minus, the species used in the present exercise, is largely restricted to springs, springbrooks, and cave streams throughout much of the Appalachians and west to Illinois, Missouri, and Arkansas. Cave populations are morphologically distinct with degenerate eyes, long antennae, and bluish coloration (Culver, Kane & Fong, 1995).
G. minus only occurs in relatively alkaline hardwater (usually
pH > 6.0; Glazier, Horne and Lehman, 1992). The absence of amphipods
from acidic softwater is thought to be related to a deficiency of calcium
for carapace formation and to difficulties of maintaining energy and ionic
balance (Glazier, 1998). Often extremely abundant in hardwater springs
(~ 600-8,000 m-2 in central Pennsylvania), amphipods are readily
collected and can be easily studied and maintained in the laboratory.
In addition, they are sensitive indicators of water chemistry and pollution,
and thus are widely used in environmental toxicology studies (Maltby, 1994;
Animals compete not only for resources (e.g., food and shelter), but also for mates. Those individuals who do the best in this “competitive game”, produce the most offspring and thus their genotypes are favored and proliferate relative to less successful genotypes. This is natural selection. A special form of natural selection is sexual selection, which results from competition for mates. Sexual selection can be simply defined as “differential mating success” (Halliday, 1980). Those inherited characteristics that increase mating success (e.g., larger body size, stronger weapons, and more elaborate ornamentation in males) will be favored. This is because those animals that mate the most will have the most offspring and thus their genetic characteristics will be preferentially passed onto future generations.
Apparently because of intense competition for mates, precopulatory mate
guarding has evolved in many crustaceans, including amphipods (Conlan,
1991; Jormalainen, 1998). The male grasps a female before she is
ready to lay her eggs, and carries her beneath him (= precopula or amplexus)
for a few days until she molts. Immediately after molting the male
fertilizes the female’s eggs, which are deposited in her ventral brood
pouch (marsupium). Embryonic development takes several weeks (~ 35-40
days in G. minus). Around the time that the young leave the
brood pouch, the female becomes sexually attractive to males again and
is soon amplexed.
In aquatic amphipods, including G. minus, males are larger than females. Because other animals with precopula (e.g., isopods) also have larger males than females (contrary to the more general trend in animals of females being larger than males), it has been suggested that this sexual dimorphism is the result of sexual selection. Experimental studies of G. pulex have shown that males compete for access to females, and that larger males “win” (i.e., mate with females) more often than smaller males (e.g., Elwood, Gibson & Neil, 1987; Ward, 1988). Since body size is inherited (i.e., passed onto offspring), this male-male competition inevitably leads to the evolution of larger males, as is found in nature. Also larger males have an advantage over smaller males in carrying females, especially in fast water currents (Naylor & Adams, 1987). However, other factors such as predation and structural constraints have probably prevented a never-ending escalation of male body size. In particular, larger males are more conspicuous to visual predators such as fish.
In this exercise, students typically test the following four hypotheses:
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