Genetics Joins Evolutionary Theory

How is evolution defined in genetic terms?

After Mendel's work was rediscovered around 1900, genetics took off like a rocket. Researchers discovered that heritable traits are controlled by genes that are carried on chromosomes. They learned how changes in genes and chromosomes generate variation.

All these discoveries in genetics fit perfectly into evolutionary theory. Variation is the raw material for natural selection, and finally scientists could study how and why variation occurs. Today, techniques of molecular genetics are used to form and test many hypotheses about heritable variation and natural selection. Modern genetics enables us to understand, better than Darwin ever could, how evolution works.

Genotype and Phenotype in Evolution Typical plants and animals contain two sets of genes, one contributed by each parent. Specific forms of a gene, called alleles, may vary from individual to individual. An organism's genotype is the particular combination of alleles it carries. An individual's genotype, together with environmental conditions, produces its phenotype. Phenotype includes all physical, physiological, and behavioral characteristics of an organism, such as eye color or height. Natural selection acts directly on phenotype, not genotype. In other words, natural selection acts on an organism's characteristics, not directly on its alleles.

FIGURE 17–1 Genes and Variation Why do biological family members resemble each other, yet also look so different? Similarities come from shared genes. Most differences come from gene shuffling during reproduction and environmental influences. A few differences may be caused by random mutations.