The reason why the blood is clear is because it lacks red blood cells, which carry the iron-rich, oxygen-absorbing protein called hemoglobin. This fish has blood that’s almost completely clear-not red like your blood, or the blood of other fish. The biologist Sean Carroll has discussed this in relationship to a type of ice fish that lives close to Antarctica. In response, over time, allele frequencies will shift to match this selective regime. In other words, in the same way that during the process of artificial selection, breeders select the most desired trait, the birds are selecting the best adapted trait. What are they selecting them for? For survival and reproduction. Dark and light colored moths on a light colored tree: Source: Wikipedia In terms of natural selection, the birds select the best camouflaged moths. The best camouflaged moths get eaten less. Birds more frequently eat the moths that are easier to see. Let’s return to the peppered moth, which we discussed in an earlier tutorial in this series. Natural Selection in Action: Peppered Moths, Ice Fish, and Pocket Mice The key idea is that through selection of phenotypes, the allele frequencies in a gene pool change. Over time, the gene pool of that population will have more copies of alleles that code for beneficial phenotypes, and less copies of alleles for harmful traits. The individuals that survive will get to pass their alleles on to the next generation. Individuals who have phenotypes with some type of advantage will be selected for survival by the environment. Natural selection is frequently summarized as “survival of the fittest.” In a population genetics context, you can think of natural selection in terms of phenotypes and genotypes. Natural Selection Increases the Frequency of Beneficial Alleles, and Decreases the Frequency of Harmful Alleles If you inherited two alleles for cystic fibrosis (an inherited lung disease), your phenotype would involve all of the symptoms connected with cystic fibrosis (build-up of mucus in the lungs, salty sweat, problems with digestion, etc.) Your genotype would be homozygous recessive, and could be represented as “cc.” If you inherited one allele, your phenotype would be “normal,” and your genotype would be heterozygous normal, represented by “Cc.” Genotypes are the underlying genes or alleles. The length of an elk’s antlers, a wolf’s sense of smell, the height of a redwood tree, or the shape of an enzyme are all phenotypes A phenotype could be an organism’s color, size, speed, form, or behavior. A phenotype is an observable trait in an organism. 2. Natural Selection Directly Acts on Phenotypes What you see is phenotype Now, we’ll use what we’ve learned about alleles in populations to look again at natural selection, this time from a population genetics perspective. It may metaphorically be said that natural selection is daily and hourly scrutinizing, throughout the world, the slightest variations rejecting those that are bad, preserving and adding up all that are good silently and insensibly working, whenever and wherever opportunity offers, at the improvement of each organic being in relation to its organic and inorganic conditions of life.ĭarwin was able to develop his theory despite being unaware about how genes are transmitted from one generation to the next. In the first tutorial in this unit, we saw how Darwin (and Wallace) proposed a theory of natural selection that explains how adaptation could arise through natural processes. Introduction: Natural Selection in Gene Pools
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