Comparative Embryology
Homologous structures not present in adult organisms often do appear in some form during embryonic development. Species that bear little resemblance to each other in their adult forms may have strikingly similar embryonic stages. In some ways, it is almost as if the embryo passes through many evolutionary stages to produce the mature organism. For example, for a large portion of its development, the human embryo possesses a tail, much like those of our close primate relatives. This tail is usually reabsorbed before birth, but occasionally children are born with the ancestral structure intact. Even though they are not generally present in the adult organism, tails could be considered homologous traits between humans and primates.
In general, the more closely related two species are, the more their embryological processes of development resemble each other.
Molecular Evolution
Just as comparative anatomy is used to determine the anatomical relatedness of species, molecular biology can be used to determine evolutionary relationships at the molecular level. Two species that are closely related will have fewer genetic or protein differences between them than two species that are distantly related and split in evolutionary development long in the past.
Certain genes or proteins in organisms change at a constant rate over time. These genes and proteins, called molecular clocks because they are so constant in their rate of change, are especially useful in comparing the molecular evolution of different species. Scientists can use the rate of change in the gene or protein to calculate the point at which two species last shared a common ancestor. For example, ribosomal RNA has a very slow rate of change, so it is commonly used as a molecular clock to determine relationships between extremely ancient species. Cytochrome c, a protein that plays an important role in aerobic respiration, is an example of a protein commonly used as a molecular clock.
Theories of Evolution
In the nineteenth century, as increasing evidence suggested that species changed over time, scientists began to develop theories to explain how these changes arise. During this time, there were two notable theories of evolution. The first, proposed by Lamarck, turned out to be incorrect. The second, developed by Darwin, is the basis of all evolutionary theory.
Lamarck: Use and Disuse
The first notable theory of evolution was proposed by Jean-Baptiste Lamarck (1744–1829). He described a two-part mechanism by which evolutionary change was gradually introduced into the species and passed down through generations. His theory is referred to as the theory of transformation or Lamarckism.
The classic example used to explain Lamarckism is the elongated neck of the giraffe. According to Lamarck’s theory, a given giraffe could, over a lifetime of straining to reach high branches, develop an elongated neck. This vividly illustrates Lamarck’s belief that use could amplify or enhance a trait. Similarly, he believed that disuse would cause a trait to become reduced. According to Lamarck’s theory, the wings of penguins, for example, were understandably smaller than the wings of other birds because penguins did not use their wings to fly.