Natural Selection and the Development of Complex Life


Once we get beyond the time when the first cell formed, our understanding of how life developed becomes much more detailed and precise. This understanding is due in larger measures to the work of one man, Charles Darwin (1809-1892). His Origins of Species, published in 1859, is arguably one of the most influential books in many centuries. In it he sets forward a view and developed to the point where it is fair to say that it is the single theory that unifies all biology. A biochemist working on the transport of particular molecules across the cell membrane and a zoologist working on the ecology of a tropical lake will both share the central ideas of Darwin's theory, and hence will have a common vocabulary and a common way of attacking problems. Darwin's theory was influential and controversial because it identified a simple mechanism for evolving multicelluar life forms from single–cell life. His theory is built around one central concept—the concept of natural selection.

Natural Selection

The easiest way to understand what Darwin meant by natural selection is to think about the process that he called artificial selection. Farmers have known for millennia that we can get bigger fruit, healthier plants, or animals with more meat on them is to carry out a conscious process of breeding. If you want large potatoes, you should plant only the eyes from the largest potatoes in any given crop. Over longer period of time, this practice will give you a variety of potatoes that is significantly different from the one you started. However, because of human choice, not nature, drives this process, this has the name of artificial selection. It explains how you can get animals as different from each other as longhorn and Angus cattle, or Chihuahuas, and Great Danes, from the same ancestral stock.

If humans beings can introduce such wide–ranging changes in living things. Darwin reasoned, then natural should can do the same. The mechanism he proposed natural selection, depends on two basic facts for its operation¦
  1. Every population contains genetic diversity. The individual members of any population posses a range of characteristics. Some are able to run a little faster than others, some have a slightly different color than others, etc.

  2. Many more individuals are born than can possibly survive. Therefore, those characteristics that make it more probable that a given member of the population will live long enough to reproduce will tend to be passed on to a greater percentage of the population's subsequent generations.


Think about a hypothetical example to see natural selection works. Suppose an island supports a certain number of birds, and suppose that the environment of the island is such that having a color that blends in with the local vegetation makes it easier for those birds to avoid their predators. Just by chance,some members of the bird population will have colors that match the colors of the local leaves and trees better than others. This is because of the DNA in the cells of those particular birds.

Better–camouflaged birds will be less likely to be eaten by predators, and, therefore, will be more likely to survive to adulthood and reproduce. You would expect, then the particular genes that give this advantage will be more likely passed to the next generation. In effect, the genes that are propagated in the next generation are influenced by natural forces. In this case, the selection is based on the color of the feathers.

If this process goes on for a long period of time, the entire population would eventually begin to share those advantageous genes for feather color. Natural selection works this way to modify a gene pool, just as populations of farm animals now share genes for rapid growth and meat production. Nature ”selects” those characteristics that propagated in any given species. A structure, process, or behavior that helps an organism survive and pass on its genes is an adaptation.


Natural selection, it should be remembered, is neither as controlled nor as rapid as artificial selection. It is always possible that birds who do not carry gene will, in some generations, be more successful at mating than those who do. Over the long haul, however, the selective advantage granted by color will win. Thus Darwin envisioned natural selection as a process that operates over long period to produces gradual change in populations, not a process that can be explain short–term variations in a few individual traits.

About the time that Darwin's book came out, a rather extraordinary example of the power of natural selection appeared in his native England. The Midlands area of England in the nineteen century had become a vast industrial belt. Factories poured smoke into the air (this was before the time of environmental awareness) and converted much of the countryside with dark soot. The peppered moth, a small moth living in this region at the time, featured black–and–white wing patterns that blended in well with the light–colored lichen that grew on trees in this area for centuries. At the time, lighter individuals predominated, through an occasional dark one appeared. Light coloration gave the moth protection from predators in an environment that did not include soot, but the common lighter moths, stood out clearly on trees darkness by industrial activity. Over a period of time of less than a century, the selective pressure on the moth population shifted in favor of darker individuals. By the turn of the century, most of the peppered moths to be found were almost black. Scientists point to this rapid shift in moth coloration as a modern example of natural selection in operation.

We should add that, since environmental controls are now in England. Soot emissions from Midlands factories are no longer at the nineteenth century levels. As Darwin stated the pepper–moth population is changing back to the lighter color, as the original mossy tree trunks become more common.