There was another phenotype that popped up occasionally in the population. A mutation occurs in one of the cells that changes the texture of the cell membrane from a relatively smooth surface to a highly ruffled one. There are several classes of non-synonymous mutations, which are defined by their effects on the encoded protein: missense, nonsense, and splice site mutations (Figure 4.8). There is no way to predict where the tumors will occur, or when or how quickly they will grow, although only about 15% turn malignant (cancerous). Though it might be argued that females should not be picky because it will likely reduce their number of offspring, if better males father more fit offspring, it may be beneficial. If natural selection favors an average phenotype, selecting against extreme variation, the population will undergo stabilizing selection (Figure 1a). Likewise, recessive traits that were seldom seen in the parent population may be seen frequently in the descendants of the offshoot population. Reflecting back on Case Study #1, the examination of the NF1 genetic disorder, some might find it surprising that half of the babies born with NF1 inherited it from a parent. The amoeba-like primordial cells that were used as recurring examples throughout this chapter are inspired by actual research that is truly fascinating. WebRecent human evolution refers to evolutionary adaptation, sexual and natural selection, and genetic drift within Homo sapiens populations, since their separation and dispersal in the Middle Paleolithic about 50,000 years ago. Tsunamis and hurricanes devastating island and coastal populations and forest fires and river floods wiping out populations in other areas are all too familiar. Genetic drift can result in genetic traits being lost from a population or becoming widespread in a population without respect to the survival or reproductive value of the alleles involved. Finally, when blue males become common, orange males will once again be favored. Since individuals with traits in the mid-range are selected against, disruptive selection can eventually lead to the population evolving into two separate species. Balancing selection (a.k.a., stabilizing selection) occurs when selection works against the extremes of a trait and favors the intermediate phenotype.
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