Pinniped Evolution • The Debate o Through most of the last hundred years, there has been a debate over whether the pinnipeds (seals, sea lions and walruses) are monophyletic or paraphyletic (specifically diphyletic) § Monophyletic- all members of a taxon evolved from a common ancestor § Paraphyletic- not all members of a taxon evolved from a common ancestor • Diphyletic means that there are two points of origin for the taxon. § Monophyletic Argument • All pinnipeds evolved from an ursid (bear-like) ancestor • In particular, the phocids (true seals) evolved from the otariids (sea lions) § Diphyletic Argument • The otariids evolved from an ursid ancestor • The phocids evolved from a mustelid (weasel-like) ancestor. o How to resolve the issue? § Zoogeography • Otariids are found mostly in the Pacific • Phocids are found mostly in the Atlantic • Odobenids (walruses) are found exclusively in the Arctic • Fossil Record o Otariid fossils are found strictly in the Pacific, while Phocid fossils are found only in the Atlantic • Diphyletic Argument o Suggests two separate origins of evolution § Otariids evolved in the Pacific § Phocids evolved in the Atlantic • Monophyletic Argument o Otariids, which evolved first, moved easily from the Pacific to the Atlantic, through the Arctic because of the geography during the Pliocene. o This lead to speciation of different groups § Morphology • Otariids and Phocids have similar flippers • However, Phocids use their hind flippers for propulsion and cannot rotate them forward. • Otariids use their front flippers from propulsion and can rotate their hind flippers forward • Diphyletic Argument o Otariids and Phocids have similar flippers because of convergent evolution (§ Convergent Evolution- when the same trait appears in two different lineages as an adaptation to the same environment) • Ex: Bird wings and Bat wings o The differences between the flippers indicates different origins • Monophyletic Argument o Convergence is overrated o The flippers of phocids, otariids and odobenids are more alike than they are like the flippers of any other group of marine mammals. o Assuming parsimony the development of structures so anatomically identical, yet functionally completely dissimilar, could only be explained by descent from a common ancestor § Parsimony-the simplest of two or more competing theories is preferable. § DNA • With the advent of molecular techniques it has been possible to see how genetically similar the different pinnipeds are and thus construct a cladogram • Definitively show that phocids are more closely related to otariids than they are to weasels and otters • This gives strong evidence in support of the monophyletic argument o How did phocids evolve from Otariids? § Let’s look at maternal feeding strategies • While nursing their young, all mammals face the tradeoff of feeding their offspring and feed themselves. • Because all pinnipeds must return to a solid substrate in order to give birth, yet feed in the ocean, this tradeoff can be particularly problematic. • How do they manage this? o Otariids feed while nursing § Being smaller, the otariids must forage while nursing § This leads to longer lactation periods o Almost all phocids fast while nursing § Phocids tend to be large animals with large fat reserves and can therefore withstand not eating for a few weeks while their offspring mature § The lactation period is also shortened § The harbor seal, however, is a small phocid and thus forages during lactationo Size, not phylogeny, seems to play an important role in determining maternal feeding strategy. o Dan Costa at UCSC suggested a scheme in which life history trait and feedback looks may be important in the evolution of phocid and otariid breeding patterns and therefore their overall evolution § Phocids • Being bigger has some benefits o The larger you are, the longer you can go without eating, making it possible for you to use more disparate food sources § Remember the Albatross? o Larger body mass also leads to a low, at-sea metabolic rate o Low metabolic rate, makes for better diving abilities o Better diving means better utilization of patchy resources § Otariids • Being smaller limits you o Tend to live near areas of abundant resource, such as upwelling zones o Reduced transit time while feeding leads to rapid surface swimming. o Rapid swimming leads to elevated metabolic rate, which reduces absolute food requirements and allows to maintain smaller body size § Field study to support this • During the 1983 El Nino, California Sea Lions living in Baja California were found to spend longer during their feeding trips because nearshore resources were scarce. • Demonstrates selective pressure on an otariid for larger body size in order to utilize patchy
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