Chapter 7: Producing Form: Development
A feature of living things, and not just organisms, is that they are dynamic, they change, and they do so in a predictable way that can be described and observed over and over again in a variety of biological entities. This pattern of change through time is termed development and while the term is mostly associated with organisms, it is significant to realize that organized (i.e., controlled in some manner) change through time is seen in cells (e.g., the cell cycle, apoptosis), populations (e.g., logistic and exponential population growth) and communities (e.g., succession).
TOPICS
- Organism development
- Development in unicellular organisms
- Development of cell shape
- Development in colonial organisms
- Development in coenocytic/siphonaceous organisms
- Development in multicellular organisms
- Development in plants
Organism development
While development may include changes in a variety of aspects of an organism, from structure to physiology to behavior, we will focus here on how development produces and changes the structure and form of an organism throughout its lifetime. We have defined organisms as being distinct in time and space with a starting point and an ending point. And aspects of development are seen in the alterations in size, form and structure occurring during the organism’s lifespan. For most familiar organisms the starting point is a special cell that has the ability to proliferate, develop a complex structure, and grow to produce an ‘end point’, a specific form that characterizes that particular creature. Many might think that this is the end of development, but for many organisms there are changes in the adult with time. Sometimes these changes in the ‘adult’ organism are dramatic and end its existence. That is, some organisms ‘self-destruct’ as part of its developmental process, just as some cells do. Salmon die after spawning, as do wheat plants. In both cases the endpoint of the developmental process is death. A corollary to this is that development does not just include growth and increases in complexity, it may also include, and in fact it often ends with, decreases in size and complexity and ultimately the end of the organism (Fig. 6).
The field of development has exploded in recent years and is being examined from molecular and evolutionary perspectives in great detail. We are focused at a very different level, considering the very basic developmental features that bring about the form and structure of the biological entities we describe as organisms. We have described several different forms of organisms: unicellular, simple multicellular (filamentous, sheets), coenocytic, complex multicellular. We will briefly consider how each of these forms develops. An important aspect of development for many organisms is the process of sex, involving the combining the genetic information of two organisms and subsequently halving it. Sex and reproduction are considered in Chapter 11.
Develop