By chance, some individuals will have more offspring than others-not due to an advantage conferred by some genetically-encoded trait, but just because one male happened to be in the right place at the right time (when the receptive female walked by) or because the other one happened to be in the wrong place at the wrong time (when a fox was hunting).įigure 19.4 Genetic drift in a population can lead to eliminating an allele from a population by chance. In other examples, better camouflage or a stronger resistance to drought might pose a selection pressure.Īnother way a population’s allele and genotype frequencies can change is genetic drift ( Figure 19.4), which is simply the effect of chance. That is, this would occur if this particular selection pressure, or driving selective force, were the only one acting on the population. Over time, the genes for bigger size will increase in frequency in the population, and the population will, as a result, grow larger on average. The pack leader will father more offspring, who share half of his genes, and are likely to also grow bigger and stronger like their father. A big, powerful male gorilla, for example, is much more likely than a smaller, weaker one to become the population’s silverback, the pack’s leader who mates far more than the other males of the group. The theory of natural selection stems from the observation that some individuals in a population are more likely to survive longer and have more offspring than others thus, they will pass on more of their genes to the next generation. In addition to natural selection, there are other evolutionary forces that could be in play: genetic drift, gene flow, mutation, nonrandom mating, and environmental variation. However, if a family of carriers begins to interbreed with each other, this will dramatically increase the likelihood of two carriers mating and eventually producing diseased offspring, a phenomenon that scientists call inbreeding depression.Ĭhanges in allele frequencies that we identify in a population can shed light on how it is evolving. While it is likely to happen at some point, it will not happen frequently enough for natural selection to be able to swiftly eliminate the allele from the population, and as a result, the allele maintains itself at low levels in the gene pool. Because the allele is rare in a normal, healthy population with unrestricted habitat, the chance that two carriers will mate is low, and even then, only 25 percent of their offspring will inherit the disease allele from both parents. For example, a disease that is caused by a rare, recessive allele might exist in a population, but it will only manifest itself when an individual carries two copies of the allele. This also helps reduce associated risks of inbreeding, the mating of closely related individuals, which can have the undesirable effect of bringing together deleterious recessive mutations that can cause abnormalities and susceptibility to disease. When scientists are involved in the breeding of a species, such as with animals in zoos and nature preserves, they try to increase a population’s genetic variability to preserve as much of the phenotypic diversity as possible. We call the diversity of alleles and genotypes within a population genetic variability. The greater the heritability of a population’s phenotypic variation, the more susceptible it is to the evolutionary forces that act on heritable variation. Heritability is the fraction of phenotype variation that we can attribute to genetic differences, or genetic variability, among individuals in a population. While the majority of scientists have not supported this hypothesis, some have recently begun to realize that Lamarck was not completely wrong. Before Darwinian evolution became the prevailing theory of the field, French naturalist Jean-Baptiste Lamarck theorized that organisms could inherit acquired traits.
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