The main question being asked and analyzed in the research is how selection occurs, how it contributes to evolution processes, and three different modes of selection. Means and methods of measuring selection have also been discussed in the paper and the challenges associated with them are outlined. Phenotypic selection occurs when individuals in a species produce more surviving offspring than others of the same species but with different characteristics. Selection is regarded as the main driving force behind evolution. Scientists had however not measured its impact in the wild until recently. Selection, through studies, has been found to have a serious impact on species in the wild and could even lead to the development of new species. Strong questions have now been raised on matters concerning selection. Questions about how strong selection is, whether different traits experience different patterns of selection and if selection on traits that affect mating success is as strong as a selection on traits that affect survival (Endler, 1986) have been raised. Questions on whether selection favors a larger body size and the consequences have also been raised. All these combined concerns, therefore, justify the research as it discusses these questions and explores future options for measuring and studying selection. Selection is the nonrandom differential survival or reproduction of phenotypically different individuals. Selection, therefore, requires variation with individuals with different characteristics. Some of these individuals have more surviving offspring due to their characteristics. Factors that bring about selection (Agents of selection) have been highlighted in the research. Biological factors include competitors, predators, and parasites while the nonbiological ones may include weather. The three modes of selection (Directional, stabilizing, and Disruptive selections) are discussed and analyzed in the paper. They are defined according to the shapes of their fitness functions. Directional selection is characterized by a linear fitness function. Fitness here is directly proportional to the value of the trait. For positive directional selection, fitness increases with increasing trait values and vice versa. In stabilizing selection, a nonlinear fitness function is observed; individuals with intermediate trait values have the highest fitness. Disruptive selection on the other hand is characterized by a nonlinear fitness function only that individuals with extreme trait values have the highest fitness. Different ways and means of measuring Selection are also discussed. Measuring could be done by measuring the body size or reproductive conditions of individuals of different trait values or by following individuals over time and measuring their components of fitness like survival, mating success, and fecundity. For both cases, the fitness value for the individual under observation should be standardized as the mean fitness for all members of the population because evolutionary consequences of selection depend on relative and not absolute fitness. This method is simple and easy to use but has one complication; there is an assumption that variat