This hypothesis is called (http://en.wikipedia.org/wiki/Dollo's_law_of_irreversibility) Dollo's law, after Belgian paleontologist Louis Dollo.
Scientists have differed on just how rigid Dollo's law could possibly be 6 whether it is merely an expression that it's statistically very unlikely that an animal could follow the same evolutionary path twice (regardless of direction), or just an expression of how treading down one path leaves the path not followed, and all of its branches, closed off forever.
Thus, caprellids have a complicated and interesting evolutionary history that does not follow Dollo's law on the irreversibility of evolution (Gould, 1970).
Dollo on Dollo's law: irreversibility and the status of evolutionary laws.
Three of the species live closer to the surface and are more generalized morphologically than the other six, making this family an excellent model with which to test and falsify Cope's Law of the unspecialized and Dollo's Law
of evolutionary irreversibility.
Previous discussions of evolutionary irreversibility have focused primarily on the inability to regain lost structures (sometimes termed "Dollo's Law
"; see Gould 1970).
But, over the past two decades, many biologists have challenged Dollo's Law, often by using statistical tools to reconstruct trait evolution and ancestry by looking only at existing species and their present-day traits.
They found the methods flawed, prompting them to examine the challenges to Dollo's Law.
"Fifteen years of studies have relied on these procedures to show that Dollo's Law is frequently violated.
Yet despite a century of discussion, debate, and derision of Dollo's Law (Meyrick, 1884; Dollo, 1893; Tillyard, 1919; Dacque, 1935; Gregory, 1936; Muller, 1939; Remane, 1952; Simpson, 1953; Rensch, 1960; Gould, 1970; Riedl, 1978; Lande, 1978; Macbeth, 1980; Laurent, 1983; Bull and Charnov, 1985; Brooks et al., 1988), the only consensus that has emerged is tantalizingly vague: sufficiently "complex" structures, once lost in evolution, should rarely be regained.
Biological hypotheses such as Dollo's Law may then be tested by performing statistical tests on the parameters.
This presumed irreversible adaptation to parasitic lifestyles has been offered as evidence in favor of the general application of Dollo's Law
(Carter, 1951; Inglis, 1955) even though this rule 9Dollo, 1893) has received extensive treatment only in relation to the appearance of morphological novelties (e.g., Tillyard, 1919; Sachtleben, 1951; Remane, 1952; Simpson, 1953; Gould, 1970).