It's nice to see a topic that relates to mollusks and to paleontology!
Phyletic aging has largely been abandoned with the discounting of some of the most popular examples, e.g. Irish elk or ammonites. In many cases, the apparent aging is only apparent, based on our perception and, in some cases, sampling biases. For example, among the ammonites (extinct cephalopods, not the northern neighbors of the Moabites), the appearance of assorted weird-looking forms in the late Cretaceous was cited as evidence for senescence in the group. However, these heteromorph ammonites were actually a very successful and diversifying group, not degenerate, malformed failures. The extinction of the ammonites seems to have been caused by the Chixulub bolide impact, which took out all sorts of organisms regardless of the age of the lineage. They were perceived as a case of phyletic aging merely because they look weird.
Another factor explaining some purported senescence is allometry. The giant antlers of the Irish elk were thought to be so big as to be a problem, contributing to its extinction. However, the Irish elk is big, too, and the antler size is not out of proportion when compared with the body size:antler size trends of other deer. It's just that really big things catch our attention.
Another pattern that has misled workers into thinking that there was a directional trend in a lineage is random [i.e., mathematically probabilistic]variation with a boundary. Mollusks again provide a good example. Dan Miller has looked at evolutionary trends in muricid gastropods. The earliest fossils have no spines; today, there are lots of big spiny forms. Has there been direction in evolution towards more spines? Not necessarily. Spininess may show random variation, but you cannot have fewer than zero spines. Thus, the average number of spines will tend to increase as the range of variation increases, but it may merely be flatter and flatter bell curves with one end cut off. This also occurs in organisms size. Clay Kelley (possibly as a co-author) looked at foraminifera around the K/T boundary. Judging by the literature, there is a significant trend to large size over time after the extinction (i.e., the lineage appears to obey Cope's rule, which could be a fo!
rm of lineage senescence). However, it turns out that this is another case of random variation. The surviving forms were all small. Usually foram workers do not pay much attention to the smaller sizes, but with nothing else to look at, the small ones get studied in the immediate post-Cretaceous. As larger sizes start to appear in later deposits, the small ones get ignored again and so published records suggest an increase in size as an artifact of sampling. Small species may have a greater chance of surviving mass extinctions (easier to find spots to hide, less food required, etc.) and may also include more generalized forms, so the founder of a lineage is likely to be towards the small end of the range for the lineage.
The influence of developmental patterns on evolution is actually quite a hot topic at present, often called evo-devo for short. This is something that is poorly accounted for at present. There are several ideas that genetic factors related to development may indeed have played an important role in large-scale evolutionary patterns. For example, there are several studies done on the difference in patterns between closely related invertebrates with either long planktonic larval stages or no planktonic stage. Several workers have suggested that greater genetic flexibility may have been present in the early Cambrian than now. If there are novel niches available, a high level of flexibility may be advantageous, allowing occupation of novel niches. However, once a niche is occupied, it becomes more advantageous to stick with the niche and defend it (unless the niche disappears)-if it ain't broke, don't fix it.
Decline of a lineage as a whole can also occur if the lineage has specialized for a niche that disappears. An example of this comes from the late Precambrian-early Cambrian, at which point bioturbation greatly increased, destroying the algal-bacterial mats that had carpeted the shallow seafloor. As a result, organisms that had depended on that habitat either moved (and usually declined), evolved novel habits, or went extinct. Monoplacophorans and helioplacoids are two taxa that had trouble adjusting.
Decline of a lineage is also not entirely meaningful if the lineage is paraphyletic. Monoplacophorans are represented today by about three genera, in contrast to the many early Paleozoic forms. However, their descendants include the Bivalvia, Gastropoda, Cephalopoda, Scaphopoda, and Rostroconchia (with some impressive transitional forms). The diversity of this group as a whole has generally increased (except at mass extinctions) throughout the Phanerozoic. Thus, in another sense, there are many thousands of living genera descended from the early monoplacophorans.
Mivart's dilema overlooks two important factors. First, there is exaptation-the use of existing features for novel functions. Whales did not lose legs and then grow fins; rather, they modified legs formerly used for walking into steering paddles. Secondly, it is the fit enough, not just the fittest, that survive. This is particularly important in the case of transitional forms. Transitional forms are generally leading up to the occupation of a new niche. As a result, they face little competition. An otter or a primitive whale can swim well with legs, so they are no impediment. The large marine reptiles had just died out, so the niche was open for whales to exploit. Even though otter-like forms do not swim as well as whales, they can catch fish. Sea lions have modified the limbs into paddles, but can still move them around to sort of function as feet. Even seals, which can only scoot the body around, can be on land enough to give birth. Thus, several intermediate !
stages are fully functional.
Dr. David Campbell
Saint Mary's College of Maryland
18952 E. Fisher Road
St. Mary's City, MD 20686-3001 USA
firstname.lastname@example.org, 301 862-0372 Fax: 301 862-0996
"Mollusks murmured 'Morning!'. And salmon chanted 'Evening!'."-Frank Muir, Oh My Word!
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