The Way of Science

These two steps (variation and selection) form the basis of most evolutionary changes. To increase diversity (i.e., speciation), additional factors need to be added: geographic isolation and/or polyploidy. We will return to speciation soon; in the meantime, let us consider the mechanisms for both variation and selection in some detail.

1. Step One: Genetic Variation

Darwin knew nothing of modern genetics, which had its beginning with the publication of Gregor Mendel's work in 1865-66. Alas, its publication was in an obscure journal, and it effectively disappeared until the 20th century, when Mendelism and Darwinism were fused into the beginnings of the MODERN SYNTHETIC THEORY. See the handout.

All sexually-reproducing individuals form populations with enormous genetic variability. In humans, for example, no two individuals are genetically identical; you are unique. Unless you have an identical twin, there is no "double" out there. Why is this so? To follow the answer, you must be comfortable with some basic biology. For organisms with nuclei, excluding viruses and bacteria, we have included below a quick review of terms. Consult your text for additional background.

The cells of nucleated organisms have DNA packaged into chromosomes. Each chromosome has regions that are recipes, or codes, called genes. There are thousands of genes per chromosome. Genes code for molecules, usually proteins, that constitute both building blocks and controllers of physiology. The sum total of your "code" is your genotype. The product of genes interacting with the environment during development is your phenotype: height, weight, blood type, etc. The distinction between genotype and phenotype is very important. If you have any doubts about these two concepts, clear them up in the classroom discussion.

In almost all adult plant and animal cells, chromosomes occur in pairs (the diploid or 2n state). In humans, 2n = 46 (23 pairs). Each member of a pair contains genes that are either identical to, or a small variation of, the genes on the other chromosome. For example, you might have a gene that codes for type B blood on one chromosome, and type O on the other. These variants are called alleles. In any population, there may be scores of alleles for a given gene.

To form gametes (sperm, eggs) in animals, the entire DNA code is copied, and one of each chromosome pair enters the gamete. Which one? They are "chosen" at random. In humans, sperm or egg contains n = 23 chromosomes (no pairs). This state (n) is called the haploid condition. Fertilization restores 2n = 46, or 23 pairs.

Now, finally, we can begin to understand the sources of step one (genetic variation). Why are no two humans (or cockroaches, or sunflowers) genetically alike?

FIRST: every time a gene is copied, there is a small chance of an error. This chemical change in the DNA produced is one kind of mutation, and is the source of new alleles. Note that mutation is rare, in the sense that any given gene has about one in a million to one in a billion chance of being copied incorrectly. However, since there are millions of cells dividing in most organisms at any moment, many different mutations occur. If the error happens while producing gametes (as opposed to, say, skin cells) then there is a good chance that this mutation will appear in future generations.

SECOND: mutation has much too low a rate to account for the many differences between you and your siblings, and you and your parents. Recombination is the second (and major) source of variation in populations. Every time a sperm or egg is formed in animals, only one of each chromosome pair goes into the gamete. Since they are "picked" at random, we have the possibility of 223 (over 8 million) different sperm (or eggs). In addition, chromosomes break and exchange segments during this "reduction division" (properly called "meiosis"). This process adds enormous variation on top of the first source (random picks). An analogy might be dealing cards (gametes) from a deck of a huge number of different kinds of cards. Although Darwin could see much variation of offspring, he did not know about genes, chromosomes, etc. Since then, modern laboratory techniques have revealed that populations are enormously more variable that anyone suspected before the mid- 20th century.

The major sources of Darwin's first step ("inheritable variation") should now be familiar to you. Never forget that both mutation and recombination are essentially random processes. Note also that for variation to be inherited, the mutations and/or new combinations must be present in gametes. Skin cancer is a mutation. Can it be inherited? Can the environment produce mutations? Before going to step two (natural selection), be sure that you are comfortable with mutation and recombination!