A Beginner's Guide
Natural Selection is a simple concept. Yet it explains so much about what we observe in nature. Scientists like simple theories that explain a lot about what we observe because such theories open up new questions for study and suggest new ways to conduct experiments and research programs.
Natural Selection rests on 3 principles. We can call them the 3 S's:
1) Strong Principle of Inheritance
Offspring look more like their parents and grandparents than like more distant relations.
Experiments in this area of evolution research resulted in the theory of genetics.
Some of these similarities are due to genes, and some are not. For example, speaking French is not genetic, even though Frenchmen tend to have relatives that also speak French. Evolution only concerns traits with a genetic basis.
Basic science research may seem not to be very useful. But evolutionary theory led to genetics. Genetic theory led Watson & Crick to discover the double helix structure of DNA. Evolutionary relationships remain the basis for understanding the results of experiments in genetics. Thus, the theory of evolution is the foundation of the new Bio-Tech industry that is creating new jobs for many Americans.
2) Small-Scale Variation
Even close relatives are not identical in every respect.
Each father passes on only some of his genes to each of his offspring, not all of his genes. And each mother passes on only some of her genes. Each baby plant or animal gets some genes from its mother & some from its father. So each baby's combination of genes is one-of a-kind (except for identical twins).
Two human parents could produce billions upon billions of genetically unique offspring. Two salamander parents could produce many times more than that, because salamanders have more DNA than humans. Flies have less DNA. But even critters with only a couple dozen different genes can still produce millions of uniquely different offspring.
A plant or animal's traits are the result of many genes working together. Even without mutation, different combinations of two parents' genes make different offspring taller or shorter than their parents, or darker or lighter in color, or more or less hairy, etc. This is why children in the same family differ in what color hair they have, what shape their noses are, or what they are allergic to.
Such variations are very, very common. As near as scientists can tell, they affect every aspect of an animal or plant's anatomy, physiology, and behavior.
It is also interesting that the genes produce their varying combinations with no regard to what would work best in the environment. Take a mama & papa polar bear to Florida, and they will have the same offspring as they would at the North Pole. They will have just as many baby bears with more hair than mommy & daddy, and they will have just as many baby bears with less hair than mom & dad.
Investigations in this area led to the mathematical theory of population genetics.
Parents produce more offspring than can possibly survive.
Examining the implications of this aspect of evolutionary theory has led to the science of ecology. This tendency to produce many, many offspring creates the complex food chain, and contributes to forest succession.
So, what happens when we put these three things together?
Since parents produce more offspring than can survive, we can look into what
happens to the ones that die. And in many cases we can find out why certain ones die and certain others live. In some of these cases, the reasons that certain ones die have to do with their genetic make-up. For example, a green bug might be a darker shade of green than his parents and sisters and brothers. That might make him stand out more on the leaf of an oak tree, which might mean that birds see him first. So he gets eaten while his sister doesn't.
When the dark green bug gets eaten, this keeps the number of genes like his from increasing in the population. This is called stabilizing selection. That means that natural selection has the effect of keeping the gene frequencies about the same as they have been.
But what if the dark green color allows him to blend in better to the oak leaves? Well, in that case, he is more likely to survive and produce offspring. When he produces offspring, some of them will be his color, some will be lighter, and some will be darker. If the ones that survive are his color or darker, this will result in an increase in the dark green genes within the population. Over a hundred generations or so, this can result in a noticeable shift in color in that population. This is called directional selection.
Notice that the change in gene frequency is due to a shift in the proportion of genes. You do not need new mutations for natural selection to work on variation that already exists within a population. Mutation is a different force in evolution, and should not be confused with selection.
Contrary to some popular claims, Natural Selection is never a random process. It is not due to chance.