Wednesday, March 12, 2008

Natural Selection

Natural Selection

When I first heard about the Channel Islands mammoth (now called the Pygmy mammoth), I was quite intrigued--rather like what you might have in the back yard, if you wanted a mammoth, but couldn't afford the space or the food supply for the version that stood 14 feet high at the shoulder:
Found only on the California Channel Islands and nowhere else in the world, the pygmy mammoth was probably a small form of the Columbian mammoth found on the mainland. Pygmy mammoths varied from 4.5 to 7 feet high at the shoulders and may have weighed only about 2,000 pounds, compared to the 14-foot tall, 20,000 pound Columbian mammoth. In other respects, they were probably similar, with short fur, a typical mammoth body form, and a relatively large head.
The first remains of "elephants" on Santa Rosa Island were reported in 1873. Additional excavations over the years have given a basic understanding of a population of mammoths on the islands which became smaller in body size through time and which perished as the Pleistocene ended. Paleontological excavations on Santa Rosa Island in 1927 and 1928 resulted in the retrieval of a significant collection of a new species described as Mammuthus exilis. Philip Orr of the Santa Barbara Museum of Natural History recovered additional materials during archeological and geological work on Santa Rosa Island during the 1940s and 1950s.
Remains of mammoths are most common on Santa Rosa and San Miguel Islands, with smaller numbers recovered from Santa Cruz Island and from San Nicolas Island, outside the park. The recovered specimens range in size from 4 to 8 feet at the shoulder, compared to the 12 to 14 foot height of mainland mammoths.
This was apparently a population that became isolated from the mainland mammoths at the end of the last Ice Age:
Approximately 40,000 to 20,000 years ago, a small group of 14-foot tall, 20,000-pound Columbian mammoths embarked on a journey that would eventually end in the development of a new species—the Channel Islands pygmy mammoth. Leaving the heavily grazed mainland behind, these Columbian mammoths swam towards the scents of abundant vegetation from the huge, mountainous island of Santarosae.
Approximately 20,000 years ago when sea level was about 300 feet lower than it is today, the four northern islands joined together to form an Ice Age “superisland” known as Santarosae. This island was only 6 miles from the mainland at its closest distance. As the ice sheets and glaciers melted and the sea level rose, only the highest parts of Santarosae remained as modern islands.
But how did they reach the island? With their snorkel-like trunk and buoyant mass, elephants, living relatives of mammoths, are considered excellent distance swimmers, among the best of all land mammals, and skilled at crossing watergaps. Documented accounts demonstrate that Asian elephant swim to islands they cannot even see – some up to 23 miles away—guided by the odor of ripening fruit and vegetation. There is no reason that Pleistocene mammoths were not just as seaworthy, and just as good at swimming.
Where I think they may be stretching a bit is when they make this claim about a different species:
Once on the island, the population of mammoths increased and, eventually, the food supply became scarce as the island decreased in size due to climatic changes – glaciers and ice sheets melting, and sea levels rising. Those mammoths that were smaller and could survive with less food and water were at an advantage, especially in times of seasonal shortages. Over time, smaller animals that required fewer resources became the norm. In addition, the absence of predators on the islands also may have contributed to this downsizing: large size was no longer needed for predator avoidance and defense.
Within less than 20,000 years, natural selection favored smaller-sized mammoths that stood less than 6.5 feet tall at the shoulder, less than half the height of their mainland ancestor. Thus, the small mammoths became a new species, the Channel Islands pygmy mammoth.
Unless they have enough DNA to do a comparison to the mainland mammoths, saying that it became a new species is a stretch. Pygmy humans are substantially smaller than other races of humans--not quite as dramatic a disparity as the Pygmy mammoth is to the Columbian mammoth, but close. And yet we don't call them a different species. As I mentioned a while back, DNA analysis of the remains of 50 species and subspecies of North American horse revealed that there were just two species. There was a lot of natural selection that caused significant differences in different regions, but just two species. I wonder if someone is making a similar mistake in assuming that the Pygmy mammoth was a different species from the Columbian mammoth.

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