To understand the limits of natural selection it is helpful to compare it to artificial selection, which is also known as selective breeding.
Artificial selection and natural selection are almost identical processes because they only differ by who does the selecting. With artificial selection, people do the selecting based on the features that they prefer (color/size/shape/etc.). With natural selection, nature does the selecting based on the features that will help or hinder survival and reproduction. Either process can cause plants or animals to change by accumulating variations over time, but both processes appear to be subject to the same limitation (explained below).
It appears that people have been using artificial selection for thousands of years.1 It is widely believed that most varieties of domestic dogs were developed by people using selection to enhance or diminish various characteristics of dogs. Both Creation Scientists and Evolution Scientists believe that all domestic dogs are descended from prehistoric wolves that were similar to modern-day wolves. Even today, domestic dogs will occasionally crossbreed with wolves.2
With both artificial selection and natural selection offspring that possess preferred variations are selected to survive. This causes the frequency of some gene alleles3 in the DNA to increase or decrease in a population. Hence, selection changes the ratios of existing gene alleles. It is also possible for selection to eliminate some alleles from a population. However, neither type of selection can create new alleles.4
Anyone who has attempted to change plants or animals with selection knows that changes come easily at first, but after several generations of directed selection a limit appears to be approached. Consider the following quote attributed to Luther Burbank, who was known for his life’s work of using artificial selection to change plants, including the development of 113 new varieties of plums:
“I know from my experience that I can develop a plum half an inch long or one two and a half inches long, with every possible length in between, but I am willing to admit that it is hopeless to try to get a plum the size of a small pea, or one as big as a grapefruit…. In short, there are limits to the developments possible”5
Similarly, people have used selection to get dogs to be very big (Great Danes) and very small (Chihuahuas), but no one has succeeded in getting dogs to be as big as a large horse or as small as a tiny mouse. But why not? Consider this:
We know that horses and mice have the information in their DNA needed for those sizes, but it appears that dogs do not. And few scientists believe that the required information could be created in dogs by simply changing the frequency of existing alleles. Most would agree that selection by itself could never create the required information. In other words, more than natural selection must be required for Evolution to occur. To understand what else is required, it helps to think of DNA as an instruction book…
Evolution claims that the instruction book for building bacteria (i.e. Bacteria DNA) has changed into the instruction book for building people (i.e. Human DNA). What would that require? The same thing that would be required to change any complex instruction book into a different complex instruction book:
The writing of a large amount of new complex instructions.
According to Evolution these new complex instructions were written by random copy mistakes, usually one letter at a time. These “typos” are called mutations, and vast numbers of them are needed by Evolution to go along with natural selection. In later chapters of this e-book I discuss mutations in more detail, and whether they are capable, along with natural selection, of producing the information required for Evolution to take place.
References and Notes for Chapter 4:
1. Some researchers believe that people have used selective breeding (artificial selection) since ancient times. Breeds of dogs that appear to be obvious products of artificial selection have been around for thousands of years.
2. Dogs, wolves, and coyotes have been known to crossbreed with each other. Creation Scientists believe these species all descended from an originally created “dog kind” which likely resembled modern-day wolves. Creation Scientists also believe that all human races are descendants of an originally created “mankind,” and that all species of fruit flies are descendants of an originally created “fruit fly kind,” etc. These beliefs differ from the beliefs of Evolution Scientists who believe that all life on Earth is related, and that all life on Earth is descended from a single microscopic bacterium.
3. Alleles are alternate forms of a gene (genes make up a small part of our DNA). We each have two copies of most of our genes (one allele from each parent). The two alleles that we have for a particular gene can be the same or slightly different. Furthermore, the alleles for a gene can vary from one individual to another. When plants or animals go through selection, some alleles will be favored over others, so it is the frequency of particular alleles that is affected by selection.
4. A new allele is formed when a copy mistake (i.e. mutation) occurs while duplicating an existing allele.
Most DNA does not code for genes, but geneticists believe that much, or most of non-coding DNA affects how genes are expressed. Since non-coding DNA can vary within a population, it seems likely that selection could also favor some individuals over others based on it.
5. Luther Burbank, Partner of Nature (edited and transcribed by Wilbur Hall), Appleton-Century, New York, 1939, pp. 97-98