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phenotype. However, environmental factors acting upon the phenotype could produce a radically different physical form or behavior than anticipated from the original genotype of an organism. For example, tailless mice that acquired their physical trait through severing of their tails (Chapter 5) still have a genotype for a tailed condition, although their phenotype shows otherwise. Their offspring still have tails when they are born, regardless of the environmentally-caused features possessed by the parents. Similarly, a bipedal theropod trackway that consistently shows three digits on one foot and only two on the other foot can be concluded as representing an environmentally-induced condition (probably from an injury or other pathological cause) that was not passed on to any of the dinosaur's offspring (Chapter 14). Behavior also can be greatly influenced by environmental conditions, rather than inherited predispositions. For example, certain breeds of dogs can be bred selectively for aggressive behavior, but breeds that are ordinarily passive also can be taught to attack and be threatening.

Modifications of phenotypes encourage the argument of "nature versus nurture" (inherited characteristics versus acquired characteristics) in examining the physical appearance of an organism. Acquired characteristics cannot be inherited. This hypothesis that promoted the contrary view, articulated by French naturalist Jean-Baptiste Lamarck (1744-1829), was critically examined and effectively falsified by the end of the nineteenth century.

Related to the phenotype of an organism is another fundamental property of any individual in a population: it shows adaptations to its environment. An adaptation is a physical attribute of an organism that can help it to survive at least long enough to reproduce successfully. Accordingly, a lack of this attribute will decrease an organism's chances of surviving to reproductive age. For example, hadrosaurs had impressive rows of teeth (dental batteries) that were well-adapted for the processing of vegetative material, presumably for better digestion (Chapter 11). A lack of these teeth would have considerably decreased their life spans, perhaps to the extent that they would not have reached reproductive age. In evolution, adaptations such as these must be inheritable from one generation to the next. An acquired adaptation is meaningless in the change of a population over generations. An example of an acquired adaptation is the development of a suntan in a normally light-skinned person. Melanin is produced in response to an environmental stimulus (sunlight over time), but this suntan is not inherited by any successive generations coming from this individual. Likewise, a human adult's lifetime habit of dyeing ordinarily dark-brown hair to blonde is perhaps an adaptation used for social enhancement and subsequent reproductive advantage in some societies. However, this adaptation does not necessarily affect whether any offspring of a chronic colorist will also have the same conferred reproductive benefit of blonde hair.

Natural selection, a hypothesis proposed conjointly by Darwin and one of his contemporaries, Alfred Russel Wallace (1823-1913; Fig. 6.1), helped explain why populations change through time and organisms composing these populations have inheritable adaptations. This explanation was proposed with the following tenets, based on numerous observations in natural settings by Darwin and Wallace:

The preceding background information, especially regarding adaptations, is necessary to understand natural selection, the central hypothesis of modern evolutionary theory.

Species have variations within their populations that are inheritable.

Species tend to overpopulate, producing more individuals than will actually survive to reproductive age. A struggle for existence occurs within the population, perhaps through competition over resources, habitat, or mates.

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