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huelsenbeck6notes - Lecture 6 Wednesday Experimental...

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Lecture 6 Wednesday, March 11, 2009 Experimental evolution Experimental evolution is evolution carried out in the laboratory: Evolutionary biologists often carry out evolution in the laboratory, to test ideas about how evolution occurs. The usual mechanism is to select for some trait, or change the environment in some specific way. Experimental evolution can be of practical importance: Some labs evolve aptamers (short strands of DNA, RNA, or peptides) that bind to specific targets. For example, Andy Ellington’s lab (UT Austin) has evolved aptamers that bind to the Rev protein of HIV-1. There has been a long-term study at the State Agricultural Laboratory in Illinois, running since 1896, in which oil content is selected for in corn. Michael Rose selected for long life in flies: All multicellular creatures age. In mammals, risks for cancer, cardiovascular disease, and various types of reproductive failure increase with age. In insects, cancer is rare, but other types of problems increase with age, such as missing body parts and reduced physiological ability. Aging also evolves. Mice live about three years, whereas humans live about 80 years. Our closest relative, the chimpanzee, only lives about 50-60 years, even when kept in benign environments. Why do animals age? The evolutionary explanation for aging is straight-forward: Organisms deteriorate because natural selection acts more effectively in young than in the old. The argument works like this. Imagine an organism that doesn’t age. This does not mean that the organism is immortal. Far from it, as there is a chance that the organism will die each year from, say, predation. If, for example, the probability of surviving each year is 95%, this means that the probability of surviving five years is 77% but the probability of living 50 years is only 7%. A mutation that acts in early life will have many opportunities to act to promote survival or reproductive success among the many individuals that are still alive. By contrast, a late-acting mutation will find few survivors around on which to act. Natural selection, in this instance, will favor mutations that have a beneficial effect early in life, even if the same mutation has a deleterious effect late in life.
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