Tuesday, March 31, 2009
Independent Convergence
Carroll believes that the strongest evidence for the process of natural selection is the idea behind independent convergence. On pages 155-163, Carroll describes more of the mathematics of mutations and why it is very credible that species can independently produce the exact same mutations. What is independent convergence and why does it happen? What is the difference between two species that have independently evolved a trait and two species that both received it from the same ancestor. What are two methods that scientists can use to determine which of these two cases happened between different species? Provide an example from the text. Finally, and most importantly, why is independent convergence among species the most convincing evidence for natural selection? (In other words, why is it more convincing to show that two species found the same solution to the same problem rather than showing one species found a solution to its problem). To answer the last part, it's necessary to explain the three recurring "ingredients" of evolution. Explaining the two independent convergences as two trials of an experiment might make it much easier to explain your argument.
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I am only going to answer part of this question. Convergent evolution is when two totally unrelated species acquire the same trait or gene. The species don’t have shared ancestry, and yet through evolution developed the same trait when faced with similar environments and situations. Speaking in terms of evolution, this phenomenon occurs because certain environmental circumstances require certain traits for survival. Animals that live in those environments, in order to adequately hunt for food and reproduce, must harbor certain adaptations to survive. If not, the species will either move or go extinct. All adaptations come from mutations, which are totally random. However, somewhere along the line (maybe after a million generations) the right mutation will pop up by chance, and, if given the chance, might spread. Natural selection ensures that the beneficial traits survive over time, so when a new adaptation comes along that is beneficial, it has a great chance of surviving. This happens to every species on earth. Species that live in similar environments or niches have a chance of developing similar characteristics because the environment and natural selection dictates it.
ReplyDeleteA good example of convergent evolution is the fact that birds, bats and pterosaurs (which all evolved along different lineages at different times, proven by DNA analysis) acquired the ability to fly. They each developed wings independently of each other. The development of the ability to fly was heavily influenced by similar environmental conditions, albeit at different points in time.
Why is it significant that the species came from different ancestors? Traits are passed on from parent to child. Genes are passed on through reproduction. It is no surprise when a mother is similar to her baby. It is also not a surprise when two related species are similar, because they share some similar traits. Two species that branched off recently will share similar characteristics, because genes were passed on in the recent past. Humans share many similar characteristics with chimpanzees even though they are two different species. Why? They are similar species, from the same ancestor. They branched off fairly recently, so physically they are quite similar. When two totally unrelated species like birds and bats share a similar characteristic, it is a big deal, because it means that no genes were passed on; natural selection had to be the force at work. There is no other explanation of the same trait appearing in two unrelated species. With no genes being transferred through reproduction, it is only possible that through chance mutations over time, the two species retained similar adaptations because of similar environmental queues.
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ReplyDeleteTo answer the last part of his question Eric hinted that it is necessary to analyze how chance, selection, and time each played key roles in independent convergence, thus proving that evolution really is the process of the making of the fittest. As proof, Carroll provided us with the enticing example of the four separate incidents where organisms developed Potassium channel blocking polypeptides to aid in their survival. I would like to analyze why this development would have been crucial in the survival of each organism, and hopefully this will shed some light on Eric’s inquiry.
ReplyDeleteI would first like to look at the Black Mamba. The history of venom in snakes has been a controversial debate in the world of medicine. It was always thought that snakes in different regions evolved venom producing glands separately. This goes along with the theory of independent divergence. However, recent research done by Dr. Bryan Fry, biologist from the University of Melbourne, has refuted this opinion based on the false assumptions that relatives of venomous snakes were non-venomous. Dr. Fry has found, in accordance with the Genome Project, that snakes previously thought to be non-venomous, such as the Gartner snake, are actually nominally venomous. This has led to his theory that all snakes have inherited their venom producing glands from one common, venomous ancestor. This ancestor probably only produced very small amounts of venom, like the Gartner, with the purpose of slowing down small creatures like mice and other lizards. This inhibiting of prey’s ability to flee was a crucial development in snake evolution for it expanded the snake’s possible niches. Previously, snakes were only able to constrict and strangle their prey. This tactic would require the snakes of old (some survive today such as the anaconda and other constrictors) to be big muscled and slow moving. However, in what realm would chance serve to evolve venom producing glands in snakes? Dr. Fry has found the answer. He has discovered that venom glands originally were modified saliva glands, only capable of rapidly breaking down food. This became a selective advantage as snakes that had turned this venomous saliva into a “lethal weapon” had a higher chance of killing prey and sustaining life. This resulted from a process called accidental duplication, in which a gene encoding a protein from another organ got copied into the region of the saliva glands. Some of these “accidental” proteins actually had the chance of being harmful in snake victims, and over time, the ones that aided in predation were selected to stay. “Natural selection then favors mutations that make these proteins more lethal”. Thus we have the Black Mamba, a snake that has developed proteins capable of killing a victim in 30 minutes (Zimmer). One such protein is the 60 amino-acid long polypeptide known as Calciseptine. This protein blocks the Calcium channels in cardiovascular tissue. It has two effects: one is the relaxation of the smooth muscle and the other is its ability to inhibit aortic contractions. This, coupled with the numerous other neurotoxins in Mamba venom has given this snake a weapon unparalleled by any other in the snake family. It allows the snake to strike once and then wait only a short period of time for its victim to collapse into a tasty, and fresh meal (2). This is ideal for the Mamba, because in its native areas on the Savannahs, it can not afford the energy to chase its prey for any great distance. Thus, developing a neurotoxin capable of quickly paralyzing its prey is a selective advantage for this venomous snake.
The immobile Sea Anemone has developed these calcium channel blocking toxins independently of the mamba. However, its similar need is what necessitated the similar toxin. Anemones are cnidarians that are rooted to the ocean floor, thus they are unable to move in search of food. As a result, they have developed a toxin very similar to that of the mamba, which slows down the muscles of its prey. However, the sea anemone also contains toxins which shut down the nervous system of its prey by blocking Potassium and Sodium channels which are crucial in synaptic junctions (3). These toxins are administered by cells called cnidocytes, which function as a defense as well as a means to capture prey. Cnidocytes contain nematocysts, capsule-like organelles which shoot out of the tentacles. Each nematocyst contains a small vesicle filled with the toxins and an inner filament and an external sensory hair. When the hair is touched, it mechanically triggers the cell explosion, a barbed, hook-like structure which attaches to the prey, and injects a dose of poison in the flesh of the aggressor or prey. After the toxins have done their job and paralyzed the prey, the anemone’s tentacles are then able to bring the food into its gastrovascular cavity where it can be digested.
1. Zimmer, Carl. "Open Wide: Decoding The Secrets of Venom. " New York Times [New York, N.Y.] 5 Apr. 2005, Late Edition (East Coast): F.1. New York Times. ProQuest. 5 Apr. 2009 http://www.proquest.com/
2. Monteiro R. Q., Foguel D., Castro H. C., Zingali R. B. Subunit dissociation, unfolding, and inactivation of Calciseptine from snake venom. Biochemistry. 2003;42:509–515. [PubMed]
3. Diochot, Sam. “Sea Anemone Peptides with a Specific Blocking Activity against the Fast Inactivating Potassium Channel Kv3.4J”. Biol. Chem. 1998 273:6744-6749 [PubMed]
Since this question is pretty well covered, I am going to give an example of independent convergence, and use it to show that natural selection is at work. My example has to do with the evolution of hinged jaws in phylum arthropoda and chordate. Neither of these phyla originally had hinged jaws. However, as years passed, both developed hinged jaws. Although hinged jaws are slightly different in the two phyla, they serve the same purpose: mechanical digestion. Namely, chewing. Chewing is a selective advantage because it increases the surface area of food. This allows the enzymes of the digestive system to work more efficiently in breaking down food into monomer units. This leads to a higher level of cellular respiration, leading to higher energy levels. This ensures that the organism has a higher chance at survival and reproduction. The hinged jaw also allows an organism to control what it puts in its mouth. This prevents the risk of accidental digestion of a dangerous substance. Digesting a toxin is obviously a disadvantage to an organism, but so is digesting an unnecessary substance. Energy has to be spent in the digestion, and ingesting an unnecessary substance only takes away energy that could be used in other activities. So the hinged jaw actually prevents the animal from putting dangerous and unnecessary substances in its mouth.
ReplyDeleteThe point of all this is that similar selective pressures resulted in similar adaptations in two totally different phyla. Both phyla needed a way to chew food for better digestion, and both phyla needed a way to control ingestion. Hinged jaws solve both problems. This particular example shows that natural selection was a force at work because there is little variation in the hinged jaws of Phylum Chordata and Arthropoda. Obviously, other mouth mutations were passed up along the way; the hinged jaw was eventually chosen for both phyla because it fit both their needs. Natural selection eliminated most other mutations to the point that animals of both phyla had extremely similar hinged jaws. The hinged jaw mutations were favored so much that they were developed separately by two totally different phyla of animals.
http://en.wikipedia.org/wiki/Chordate
http://en.wikipedia.org/wiki/Arthropod
http://en.wikipedia.org/wiki/Jaw