What Natural Selection Is (and Isn’t)

Animals have structures that come in a variety of shapes and sizes and often are suited to perform a specific function. Some animals, however, seem to take this to the extreme. One of the characteristics of life is that it is organized at all levels. Animals are multicellular organisms that have specialized cells grouped into tissues. Combinations of certain tissues make up units called organs, and groups of organs that work together are called organ systems. The complimentary relationship between anatomy, or structure, and physiology, or function, is seen at each level. Here are a few examples of how certain structures that make up an animal’s body plan are related to a specific function:

Mule deer have ears that are very large to enhance their ability to hear.





Northern flickers have a slightly curved bill that make it easy for them to dig for ants and beetles.





The grey wolf has a thick undercoat that is oily and waterproof which provides good insulation against cold weather.






Pronghorn have a large windpipe, heart, and lungs with an extremely light bone structure which make it the fastest land mammal in the western hemisphere.





Black bears are built for strength and power.







Bison have large shoulder muscles that aids them in “plowing” snow to get to the vegetation.






Charles Darwin explained this complimentary relationship between structure and function with his idea of natural selection. To understand how Darwin’s proposed mechanism for evolution, natural selection, underlies the great diversity in the world, it is important to understand what natural selection is and what it is not.
The following shows the five main points of natural selection:

1. Individuals within a species are variable
2. Some of these variations are passed on to offspring
3. In every generation, more offspring are produced than can survive
4. Survival and reproduction are not random. The individuals that survive and go on to reproduce, or who reproduce the most, are those with the most favorable variations. They are naturally selected.
5. Species multiplication– As species move through time, in a manner described by the previous four points, species split and new ones are formed. 99% of all species that have ever lived are extinct.

The model may help with understandings Darwin’s thoughts:

-Observation 1- Organisms have great potential fertility, which permits exponential growth of populations.

-Observation 2- Natural populations normally do not increase exponentially, but remain fairly constant in size.

-Observation 3- Natural resources are limited.
-Inference 1- A struggle for existence occurs among organisms within populations because resources are limited

-Observation 4- Variation occurs among organisms within populations

-Observation 5- Variation is heritable
-Inference 2- Varying organisms show differential survival and reproduction, favoring advantageous traits
-Inference 3- Natural selection, acting over many generations, gradually produces new species

Natural selection leads to adaptations of species over time. This process, however, does not involve a species trying , wanting, or needing a particular adaptation.  Populations evolve. Individuals do not. Natural selection results from genetic variation in a population (see Observations #4 and #5 above). This genetic variation is generated by random mutations, which is independent of what an individual within a population “needs.”

So, natural selection is the underlying factor in the “structure compliments function” idea that we see in living organisms.