|MadSci Network: Evolution|
Evolution occurs through natural selection acting on individuals to modify the chance that their DNA is passed on to successive generations, all things being equal. One example of natural selection would be when males or females judge aspects of their mates to be “attractive” and this influences the number of offspring an individual produces. Research with birds and people has shown that symmetry of features is a very desirable trait in a potential mate. Another example can be found in the work of Peter and Rosemary Grant of Princeton University. They measured a number of physical characteristics of species of finches in the Galapagos over many years and many generations. By coincidence an El Nino event occurred in the midst of their work. This caused the available food for the finches to shift from seeds that were easily cracked open to a variety that was tougher to open. The Grants found that very soon after the food supply changed the beaks of the finches changed to be more robust, which would allow these birds to feed on the tough seeds. Variation in beak robustness (a function of length with respect to depth) was always present in the finches. It was when the food source changed that the mean “robustness” of beaks changed. Individual birds with beaks that could not open the tougher seeds would starve before they could reproduce. This is an example of selection acting on a physical characteristic and thereby changing this characteristic in a population. Even though the variation measured was anatomical the basis for this characteristic had a strong genetic component. This last example illustrates an important point, that individual genes exist in different varieties among individuals in a population. These varieties of a specific gene are known as alleles. If an allele has a detrimental effect on an individual’s ability to produce offspring (as in the case of Galapagos finches with slender bills) then natural selection will, over time, reduce the frequency of that allele in a population. However, this may not be strictly true under certain conditions, such as small population sizes, where an individual with a detrimental allele may produce offspring simply because others have to mate with him or her to produce any offspring at all. In general, and especially when one thinks of genes at the molecular level, most alleles aren’t detrimental at all to the production of offspring. This variation is known as neutral variation, and the persistence of neutral variation is to a very large degree a random process. In fact, the probability of neutral alleles persisting in a population is directly proportional to the probability of mutation. This leads back to another aspect of your question and that is mutation. Changes in DNA sequence from one generation to the next come about due to mutation and humans will pick up on average one new mutation to their genome each generation. The rate of mutation can be increased by environmental factors such as exposure to radiation or certain chemicals (teratogens or mutagens), but there is always a background rate of mutation. As mentioned above, much of mutation is neutral and selection will not affect these alleles' chance of being passed on to future generations. Rarely, mutation will affect how a gene product performs its task and this is when selection occurs. In humans selection often takes the form of inheritable diseases such as Tay-Sachs or cystic fibrosis which affect the patient’s ability to reproduce. However much these diseases reduce one’s ability to have children the genes themselves don’t disappear from the population, but the alleles that lead to the disease remain fairly rare. There may even be a selective advantage for having one copy of a disease allele (among the two copies of every gene that one has in his or her genome), as has been postulated for the persistence of the allele causing sickle-cell anemia in human populations subject to malaria. Indeed, if the gene responsible for sickle-cell anemia was not present then a viable embryo could not form as this gene codes for a component of hemoglobin. Likewise with cystic fibrosis as this gene codes for an ion-channel protein in cell membranes, and having one cystic fibrosis allele may reduce the effects of cholera. So, is evolution random? Sometimes it is quite random and other times less so. It all depends on the genetic and environmental context of the trait of interest.
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