Mr. Coyne vividly describes the critical importance of finding "missing link" species, the common ancestors between two groups of organisms that split and formed different branches on the phylogenetic tree. Coyne discusses the impact of Neil Shubin's research and discovery of one "missing link," Tiktaalik roseae, beginning on page 35.
Darwin called the lack of fossils of transitional species "the most obvious and gravest objection which can be urged against my theory." There have been numerous "missing links" found since Darwin's time. Research one of these species and describe the link it solidified. Include the similarities in phenotype and how they suggest genetic likeness, along with information on the discovery (location, images, etc.) of fossil evidence.
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Another transitional species that Coyne discusses is Archaeopteryx lithographica, a link providing evidence for the evolutionary origins of birds from dinosaurs. Discovered in 1860, Archaeopteryx was found in a limestone quarry in Germany with an age of about 145 million years. (http://www.ucmp.berkeley.edu/diapsids/birds/archaeopteryx.html) Some features that Archaeopteryx shares with modern birds is feathers and an opposable toe; however, there are dinosaur features as well. For example, skeleton-wise, Archaeopteryx is similar to theropod dinosaurs because of the “teeth, reptilian pelvis, and a long bony tail.” (Coyne 41) Because Archaeopteryx is more reptilian than bird-like, it represents a transitional species that was on the earlier end of the evolution of birds. More bird-like organisms such as Sinornithasaurus millenii came later, closing the gap between reptiles and birds even more. (Coyne 40)
ReplyDeleteThis relates to the theme continuity and change because as the COB and 1.1 states, changes in gene pools over time are partly explained by natural selection. (Campbell 8/COB) Coyne gives many examples of the intermediate traits that come before flying being useful, like in the case of the gliding lemur, colugo, from Southeast Asia. (39) If these traits were useful, then it follows that natural selection would favor the evolution of Archaeopteryx. As subsequent mutations allowed for full wings and other features of birds, Archaeopteryx continued to evolve because these new traits provided a selective advantage.
Coyne also mentions the multiple transitional forms from the Indohyus to the Balaena, today's modern whale.
ReplyDeleteAccording to Coyne, the Indohyus was only a close relative to the ancestors of the modern whale, but the traits of the Indohynus can be applied as those of the direct ancestors of modern whales. The Indohyus was a semi-aquatic creature, spending part of its life on land, and part of its life in the water, which could have been the precursor to the full-time aquatic life of the modern whale.
What most scientists consider the first in line in the evolutionary sequence of the modern whale is the Pakicetus. The Pakicetus lived roughly 52 million years ago. The teeth of the Pakicetus were the beginnings of the transition from the flatter shaped teeth of land animals to the triangular shaped teeth (similar to those of sharks)used for hunting fish. This shows an increased affinity for a fully aquatic life.
Next in the evolutionary ancestry of the Balaena is the Ambulocetus, also known as the 'walking whale' that lived about 50 million years ago. The Ambulocetus was still capable of walking on land, albeit awkwardly with the largely increased size of the forelimbs and the decreased size of the hindlimbs, similar to the waddling sea lions today. The forelimbs of the Ambulocetus were intermediary in that they were adapted to both supporting their weight on land and waddling as well as powerful strokes for swimming.
The Rodhocetus lived 47 million years ago, and continued the trend of an elongating skull as well as a deteriorating usefulness of the hindlimbs and an increased affinity for water over land.
The Basilosaurus from 40 million years ago completed the move to a permanent residency in the water, unable to walk on land any longer. The pelvis of the Basilosaurus was completely removed from the spine, and the hind legs were quite nearly useless.
The Dorudon, also from 40 million yeras ago, is the closest ancestor yet discovered to modern whales. The Dorudon, similar to the Basilosaurus, was a completely aquatic creature and could no longer veenture upon land.
All the fossils that have been found show the overall flattening and elongating of the skull, along with the near disappearance of the hindlimbs and pelvis (thus leading to the vestigial pelvis and hindlimbs in the modern whale today), and the transformation of the forelimbs into paddle-like fins.
The Pakicetus were discovered in areas that showed signs of being near shallow water, and as evolution progressed, the different transitional creatures were found in areas that were deeper into the water.
(Coyne 49-51)
http://www.talkorigins.org/features/whales/
Another transitional species is Maiacetus inuus, which solidified the link between land mammals and whales, while Maiacetus inuus has a similar skull shape to early whales, the fetal position of the baby found in the mother whale indicates that Maiacetus inuus gave birth on land. The limbs of Maiacetus inuus are also more akin to those of land mammals than to those of whales, though they have been altered to be optimal in an aquatic setting. Because phenotype is dependent on genotype as stated under the theme of interdependence in nature, where phenotype is a synergistic product of both environmental conditions and genotype, the phenotypical similarities that Maiacetus inuus has with both early whales and land mammals indicates that Maiacetus inuus could be a genetic bridge between land mammals and whales. The two specimens of Maiacetus inuus were found in Pakistan in 2000 and 2004, and are estimated to be 47.5 million years old. (http://www.nsf.gov/news/news_summ.jsp?cntn_id=114086&govDel=USNSF_57)
ReplyDeleteIn addition to the “found” link between reptiles and birds mentioned by Radhe (don’t worry, I won’t judge you), as well as the links between modern and ancient whales mentioned by Samantha and Ray, a transitional species between strepsirrhines (lemurs, bush babies, etc.) and haplorhines (apes, monkeys, humans, etc.) has recently been discovered. The 47 million-year-old fossil of Darwinius masillae [italic], also called “Ida,” shares the fingernails, opposable thumbs, and forward-facing eyes of all primates (http://www.guardian.co.uk/science/2009/may/19/fossil-ida-at-a-glance). Unlike lemurs, Ida does not have a grooming claw on the second toe or fused teeth in the bottom jaw. The shape of Ida’s talus, or ankle bone, also links the species to apes and monkeys, rather than lemurs.
ReplyDeleteA good diagram showing the relationship between Ida, lemurs, and other primates can be seen here: http://image.guardian.co.uk/sys-files/Guardian/documents/2009/05/20/Primate_tree.pdf.
Although Ida’s status as a “missing link” has been disputed (mostly by creationists), she does match Coyne’s definition of transitional species because she clearly demonstrates a “mixture of traits from organisms that lived both before and after” (35), sharing the general traits of primates, but having some traits more in common with haplorhines (monkeys) than with strepsirrhines (lemurs).
Ida was found in Messel Pit in Germany, and is remarkable in that she is so well-preserved. (Ida is about 95% intact, whereas the famous Lucy skeleton in only 40% intact).
The finding of Darwinius masillae [italic] is especially pertinent to the theme of “science, technology, and society.” Ida represents a link between primates; creationists draw the line at believing that humans could have evolved from apes (Coyne 193). The ongoing struggle between belief in creationism and acceptance of evolution hinges on the poor fossil record of primates, which makes it difficult to tangibly link humans to other apes. Hopefully, if more missing links like Ida could be found, evolution could become a more widely accepted fact, thus changing the way humans view themselves and society.