Here we begin a discussion of Darwin’s major contribution to evolutionary thought. The four postulates that I present here were not specifically enumerated in Origin of the Species. However, most texts break the following passage from the Introduction to Origin into four parts.

      “As many more individuals of each species are born than can possibly survive; and as, consequently, there is a frequently recurring struggle for existence, it follows that any being, if it vary however slightly in any manner profitable to itself, under the complex and sometimes varying conditions of life, will have a better chance of surviving, and thus be naturally selected. From the strong principle of inheritance, any selected variety will tend to propagate its new and modified form."

      One of the complaints often made by opponents of Darwin’s Theory of Natural Selection is that it is not falsifiable, and thus not scientific. Some go so far as to suggest that it is ultimately based on faith. This is an erroneous argument. Each of the four postulates outlined in the passage above can be tested and, potentially, proven false. That means natural selection meets the criteria for being considered scientific. Let’s take them individually in the order they were presented in the story.

1. Individuals within a species are variable.

      Just looking at the people around you is enough to demonstrate that humans are variable. The same can be said of almost every other type of living thing in the world (there are some organisms that reproduce clonally). These differences, and their potential worth, are in heren tly acknowledged in our own language with phrases and terms like “pick of the litter” and “runt.”

2. Some of an individual’s variations are passed onto its offspring.

      Most children resemble their parents. Some people say my son Max looks like me. Others say that he has Lisa’s chin. Max resembles Lisa and me because he has inherited a specific set of genes from us. Of course, Darwin never knew about genes and DNA, but his observations about the obvious process of inheritance and its implications for his Theory of Natural Selection are still valid. You may have noticed I began this particular postulate with the qualifying word “some.” Not all variations are passed on to offspring. If an organism acquires a trait during its lifetime, the trait is not passed on to its offspring. Say, for example, a lizard loses its tail to a predator, but the lizard survives and reproduces. The lizard’s offspring won’t be tailless, because the lizard will still carry, and consequently pass on, the genes required to make a tail

3. More offspring are produced than survive.

      Let’s consider the ocean sunfish (Mola mold) as an extreme example. A single female may produce as many as 300 million eggs. Such a reproductive feat has earned Mola mola bragging rights as the most fecund vertebrate. Now, if one Mola mola is capable of producing that many eggs, why aren't the oceans choked with an overabundance of sunfish? The reason is that most of the eggs and the subsequent juvenile fish are consumed by predators or don’t survive for some other reason. Few live long enough to reproduce.

4. Those offspring that survive and reproduce have inherited a variation that gives them an advantage.

      It doesn’t require seeing many National Geographic specials to know that predators like lions target young and weak members of a herd. The healthier and stronger or faster you are, the more likely you are to a) avoid being targeted by a predator or b) survive an attack. If the advantage that helps an organism survive is something controlled by its genes, then if it survives and reproduces, it may pass those genes (and consequently, the advantage) on to its offspring.

      It should be fairly clear in the descriptions I have given above that genes are pretty important when considering how natural selection works. The incorporation of genetics into Darwin's Theory of Natural Selection has led to what is known as the Modem Synthesis. In the Modern Synthesis, the four postulates are updated a touch. I present them here as they appear in Evolutionary Analysis (2nd ed.) by Freemon and Herron, 2001. You will notice that they use the term ‘allele’ in these postulates instead of ‘gene’, so a little explanation is in order.

      We are all born with two gene copies for almost every trait. We get one from our mother and one from our father. Most traits are controlled by several genes, but there are a number of single-gene traits. The ability to roll your tongue is controlled by one gene. In that gene we see two different variations, or alleles. One allele gives you the ability to roll your tongue (the dominant allele) and the other does not (the recessive allele). There are 3 possible combinations of these 2 alleles. If you have two recessive alleles, you won’t be able to roll your tongue. Folks with one or two of the dominant alleles are tongue rollers. My wife and I can both roll our tongues, and so can our eldest son. Max. The jury is still out on 7-monthold Jack. I have included a picture above of Max displaying his tongue rolling proficiency. It took 51 attempts with the digital camera to get this picture. I tried 42 before I gave up and handed the camera over to Lisa.

      As the expert picture taker in our family, she got this shot in 9 tries.

      OK, so let’s get back to the updated postulates of Natural Selection (finally). They are:

      1. As a result of mutation creating new alleles, and segregation and independent assortment shuffling alleles into new combinations, individuals within a population are variable for nearly all traits.

      2. Individuals pass their alleles on to their offspring intact.

      3. In most generations, more offspring are produced than can survive.

      4. The individuals that survive and go on to reproduce, or who reproduce the most, are those with alleles and allele combinations that best adapt them to their environment.