The directional selection is a type of Darwinian selection where a particular phenotype is favored in the population, thereby modifying the allelic frequencies to increase the proportion of the favored phenotype. <em>Biston betularia</em>, also known as peppered moth, is a species that was influenced by directional selection in its recent past. Before the industrial revolution, the frequency of light-colored moths was predominant compared to the darker-colored phenotypes, because this color has higher adaptive fitness in a clean, no pollution environment, thereby light-colored moths were able to avoid predatory birds. However, during the industrial revolution, the frequency of dark-colored moths increased in response to pollution (i.e. darker environment), thereby conferring a higher adaptive fitness to darker phenotypes.
What can be inferred about the evolution of the cranium and the vertebral column from examining hagfishes and lampreys? Comparison of hagfishes with lampreys shows that the cranium evolved first in early vertebrates, as it is seen in hagfishes, which evolved earlier than lampreys.
