Developmental Processes

Organisms generally increase in size during development. Growth is usually allometric, in that it occurs simultaneously with cellular differentiation and changes in overall body pattern. Allometry is a discipline of biology which specifically studies the relationships between the size and morphology of an organism as it develops and the size and morphology of different species.

A developing organism generally increases in complexity as it increases in size. Moreover, in an evolutionary line, larger species are generally more complex that the smaller species. The reason for this correlation is that the volume (or weight) of an organism varies with the cube of its length, whereas gas exchange and food assimilation, which generally occur on surfaces, vary with the square of its length. Thus, an increase in size requires an increase in cellular specialization and morphological complexity so that the larger organism can breathe and eat.

Depending on the circumstances, natural selection may favor an increase in size, a decrease in size, or no change in size. Large size is often favored because it generally makes organisms faster, giving them better protection against predators, and making them better at dispersal and food gathering. In addition, larger organisms have a higher ratio of volume to surface area, so they are less affected by environmental variations, such as temperature variation. Large organisms tend to have a prolonged development, presumably so they have more time to develop the morphological complexities needed to support their large size. Thus, evolutionary selection for large size leads to a prolongation of development as well as morphological complexity.

Sometimes the coordination between growth and differentiation goes awry, resulting in a developmental abnormality. One such abnormality is an undifferentiated mass of cells called a tumor. A tumor may be benign, in which case it does not invade adjacent cells; alternatively, it may be malignant, or cancerous, in which case the proliferating cells invade their neighbors. Cancers often send colonies of tumor cells throughout the body of an individual, a process called metastasis.

Cancers can be caused by damaging the DNA, the molecular information carrier, of a single cell. This damage may be elicited by a variety of factors such as carcinogenic chemicals, viral infection, or ultraviolet radiation. In addition, some cancers may arise from unprovoked and spontaneous damage to DNA. Basic studies of the different developmental processes may lead to a better understanding of cancer and how it might be prevented or cured.