As we explained in How Human Reproduction Works, higher organisms like plants, insects and other animals reproduce sexually, and this process makes the actions of evolution more interesting. Sexual reproduction can create a tremendous amount of variation within a species. For example, if two parents have multiple children, all of their children can be remarkably different. Two brothers can have different hair color, heights, blood types and so on. Here's why that happens:
- Instead of a long loop of DNA like our trusty E. coli bacterium, cells of plants and animals have chromosomes that hold the DNA strands. Humans have 23 pairs of chromosomes, for a total of 46 chromosomes. Fruit flies have five pairs. Dogs have 39 pairs, and some plants have as many as 100.
- Chromosomes come in pairs. Each chromosome is a tightly packed strand of DNA. There are two strands of DNA joined together at the centromere to form an X-shaped structure. One strand comes from the mother and one from the father.
- Because there are two strands of DNA, it means that animals have two copies of every gene, rather than one copy as with E. coli. When a female creates an egg or a male creates a sperm, the two strands of DNA must combine into a single strand. The sperm and egg from the mother and father each contribute one copy of each chromosome. They meet to give the new child two copies of each gene.
- To form the single strand in the sperm or egg, one or the other copy of each gene is randomly chosen. One or the other gene from the pair of genes in each chromosome gets passed on to the child.
Because of the random nature of gene selection, each child gets a different mix of genes from the DNA of the mother and father. This is why children from the same parents can have so many differences.
A gene is nothing but a template for creating an enzyme. This means that, in any plant or animal, there are actually two templates for every enzyme. In some cases, the two templates are the same (homozygous), but in many cases the two templates are different (heterozygous).
Here is a well-known example from pea plants. Peas can be tall or short. The difference comes, according to Carol Deppe in the book "Breed Your Own Vegetable Varieties":
...in the synthesis of a plant hormone called gibberellin. The "tall" version of the gene is normally the form that is found in the wild. The "short" version, in many cases, has a less active form of one of the enzymes involved in the synthesis of the hormone, so the plants are shorter. We refer to two genes as alleles of each other when they are inherited as alternatives to each other. In molecular terms, alleles are different forms of the same gene. There can be more than two alleles of a gene in a population of organisms. But any given organism has only two alleles at the most. ... Specific mutations or alleles are not good or bad in and of themselves, but only within a certain context. An allele that promotes better growth in hot weather may promote inferior growth in cold weather, for example.
One thing to notice in Deppe's quote is that a mutation in a single gene may have no effect on an organism, or its offspring, or its offspring's offspring. For example, imagine an animal that has two identical copies of a gene in one allele. A mutation changes one of the two genes in a harmful way. Assume that a child receives this mutant gene from the father. The mother contributes a normal gene, so it may have no effect on the child (as in the case of the "short" pea gene). The mutant gene might persist through many generations and never be noticed until, at some point, both parents of a child contribute a copy of the mutant gene. At that point, taking the example from Deppe's quote, you might get a short pea plant because the plant does not form the normal amount of gibberellin.
Another thing to notice is that many different forms of a gene can be floating around in a species.