Tuesday, April 7, 2009
Fossil Genes: Cause or Effect?
On page 133, Carroll explains that the fossilization of genes can be a "cause of evolutionary change" or an "effect." Explain what is meant by these two statements. In your response, provide examples of the fossilization of genes in both instances. Please use examples from outside of the text.
Subscribe to:
Post Comments (Atom)
Carroll is saying whether the fossilization of genes is a cause of evolutionary change brought about by natural selection or a by-product of natural selection on other features. He states that it cannot be exactly stated but it could be either. In the morning glory species Ipomoea quamoclit, the species adapted to produce red flowers instead of blue or purple that the ancestral flowers produced. Carroll believes that since the red color attracts hummingbirds, the species’ pollinators, the mutation in the genes that produce the enzymes are naturally selected for the increased attraction to the pollinators, increasing their chance of pollination and reproduction. The different sets of enzymes that produce either the blue and purple or the red pigments differ between the Ipomoea quamoclit and other species of the same genus. The pathway for the production of blue and purple pigments have degenerated due to random mutations, and this became a selective advantage to the flowers that had to rely on the enzymes that produce red pigments to attract their pollinators. The organisms that produce the blue and purple pigments have had less successful rates of pollination, resulting in the decrease in the gene inheritance to future generations. The natural selection of the inactivation of the blue/purple-promoting enzymes may have led to the evolution of the red pigmentation of the flowers (Carroll 133-134). However, Carroll’s statement can be construed to be both the cause AND effect. The enzymes could have been naturally selected to become inactivated to give rise to the red pigmentation-promoting enzymes, but the fossilization of the blue/purple-promoting enzymes could have been a by-product of the natural selection on the red-promoting enzymes that continuously decreased the gene inheritance of the blue/purple-promoting enzymes. It cannot be exactly stated whether the fossilization of the blue/purple-promoting enzymes were naturally selected to become fossilized, or the continued disuse of the enzymes due to the selective advantage of the red-promoting enzymes caused the fossilization of the gene.
ReplyDeleteThe human gene MYH16 is a fossilized gene that Carroll explains to show how a fossilized gene can be an effect of evolutionary change. The fossilization of the MYH16 gene has caused the decrease in large fibers in the muscles in the jaw, reducing muscle strength. The decreased strength in the jaw limits the movement of the human jaws, which would not necessarily be a selective advantage. This fossilization of the gene can be seen as an effect of another selective advantage in a different feature that caused the gradual degeneration and disuse of the MYH16 gene, leading to its fossilization.
The fossilization of red blood cells in ice fish can be seen as a cause of natural selection, not a by-product. When the temperature in the southern ocean dropped from 68°F to less than 30°F and the ice fish were isolated in Antarctica due to changes in the ocean currents and continental drift, the Antarctic ice fish were forced to either adapt or die. Therefore, the ice fish’s genes mutated to decrease red blood cells and hemoglobin, and this caused a selective advantage in the freezing water. The ice fish further adapted by developing larger gills, losing scales for greater gas exchange, developing vascular skins, and developing larger hearts.
student.ccbcmd.edu/~sschmit4/BIOL%20111/Evolution/The%20Ice%20Fish%20of%20Anarctica.ppt
When Carroll states that the fossilization can be an effect of evolutionary change, it is meant that changes in the organism’s or microbe’s environment have occurred in a manner that changes the selective pressures on the population. In other words, the environment changes, and as a result, a certain gene is no longer needed. Carroll invokes the example of Japanese yeast, S. kudriavzevii. This yeast has lost the ability to breakdown galactose. This is because the yeast, as some point in evolutionary history, located itself on dying leaves as opposed to the sugar rich areas where most other yeasts are found. The Japanese yeast was therefore not exposed to galactose, and the ability to break down galactose was no longer needed. As such, over time, the galactose genes were allowed to degrade, and became fossil genes. Another example in which environmental changes caused genes to become unneeded and dye off can be seen in the microbes recently found in extremely old ice in the Dry Valleys of the Transantarctic Mountains. The microbes found were isolated for incredible periods, and as a result, did not have a constant supply of resources. As resources were used up in these small pockets of ice, the microbes living there no longer needed certain genes. This led to these unneeded genes to no longer be chosen for by the principles of selective advantage. It was found that genes were becoming fossilized at an “exponential decline in the average community DNA size with a half-life of approximately 1.1 million years”. Another example of the fossilization of genes as an effect of evolution can be seen in the loss of olfactory receptor genes in primates. As primates rely more on their eyes for hunting and navigation than their sense of smell, many of their OR genes have become useless to them. As natural selection has not weeded out harmful mutations to the genes, the number of OR in primates has steadily declined to far below that of other animals
ReplyDeletehttp://www.plosone.org/article/info:doi%2F10.1371%2Fjournal.pone.0000708
http://www.ncbi.nlm.nih.gov/pubmed/17686983