Measuring Natural Selection Natural selection is investigated in two ways: by use of indirect measurements and direct measurements. The indirect methods involve observing the outcome of natural selection and inferring its presence. The direct methods involve measuring the three parts of the process and following the course of evolution. Although the direct methods are preferred, as they provide direct proof of natural selection, in most instances, only indirect methods can be used. Indirect methods involve three kinds of observations. First, comparisons are made of trait similarities or differences among populations or species living in the same or different areas. For example, many species of animals living in colder climates have larger bodies than those living in warmer climates. It is inferred, therefore, that colder climates result in natural selection for larger bodies. Second, long-term studies are done of traits, in particular changes in a group in the fossil record. For example, during the evolution of horses, their food, grasses, became tougher and horses' teeth became thicker. It is inferred, therefore, that tough grass resulted in natural selection for thicker teeth. Third, comparisons are made of gene frequencies of natural populations, with predictions from mathematical models. Gene frequencies are measured using various techniques, including scoring differences in appearance, as with light-colored and dark-colored moths; using electrophoresis to observe differences in proteins; and determining the sequence of base pairs of deoxyribonucleic acid (DNA). The models make predictions about expected frequencies in the presence or absence of selection. Indirect methods are best at revealing long-term responses to evolution and general processes of natural selection that affect many species. The indirect methods suffer fromthe problem that oftenmanyprocesses will result in similar patterns. So, it must be assumed that other processes were not operating, or other predictions must be made to separate the processes. Direct methods involve two kinds of observation. First, there is observation of changes in a population following some change in the environment. There are many types of environmental changes, including man-made changes, natural disasters, seasonal changes, and introductions of species into new environments. For example, from the changes in the peppered moth following a change in pollution levels, one can measure the effects of natural selection. The second type of observation is the direct measurement of fitness differences among individuals with trait differences. For example, individual animals are tagged at an early age and survival and reproduction are monitored. Then, statistical techniques are used to find a relationship between fitness and variation among individuals in some trait. Alternatively, comparisons of traits are made between groups of individuals, such as breeding and nonbreeding, adults and juveniles, or live and dead individuals, again using statistical techniques. For example, lions that breed are larger than lions that do not breed. Direct methods are best at revealing the relative importance to natural selection of the three factors (variation, fitness differences, and heritability). The direct methods suffer from two limitations. It takes a long time for evolution to occur. So, although one can measure natural selection, it is often not known if it results in evolution. Also, for many species, it is impossible or impractical to mark individuals and follow them through their lives. Many methods can be used to study natural selection and evolution. Each method provides information about different parts of the process. Only through the integration of these methods can the entire process of evolution be revealed.
Pecking Orders in Domestic Fowl Among common barnyard chickens, social behavior is a relatively simple system based on the dominance hierarchy. Apower struggle begins soon after a new flock is established. The chickens quickly form a hierarchy that is quite literally a pecking order. The chickens establish their status by pecking or by threatening actions toward an opponent with the obvious intention of attacking in this manner. Superior genetic fitness is attained by the high-ranking birds. They enjoy more freedom of movement, and they have priority of access to food, better nesting sites, and favored roosting places. While the dominant males mate far more frequently than the subordinates, the dominant females actually mate less, because the subordinate females more readily display submissive and receptive postures to the males. The fitness of the dominant females is still enhanced due to the advantages gained in access to food and nesting sites. Males establish a separate hierarchy above that of the females. The adaptive advantage to this behavior lies in the fact that males who are subordinate to females will not be able to mate. It has been well documented that fighting ability among fowl is an inherited trait, and significant genetic variation in this trait can be observed both between and within the various species of fowl. It is advantageous for chickens to live in a stable hierarchy. If the hierarchy of the flock is intentionally disrupted, the chickens will eat less food, lose more weight, and lay fewer eggs.
