In general, evolution (or organismic evolution) is about changes in populations, species, or groups of species. More specifically, evolution occurs because populations vary by the frequency of heritable traits that appear from one generation to the next. These traits are represented by alleles for genes that modify morphology (form or structure), physiology, or behavior. Thus, evolution is changes in allele frequencies over time.

Evidence for evolution is provided by the following five scientific disciplines:

• Paleontology provides fossils that reveal the prehistoric existence of extinct species. As a result, changes in species and the formation of new species can be studied.

  • Fossil deposits are often found among sediment layers, where the deepest fossils represent the oldest specimens. For example, fossil oysters removed from successive layers of sediment show gradual changes in the size of the oyster shell alternating with rapid changes in shell size. Large, rapid changes produced new species.

• Biogeography uses geography to describe the distribution of species. This information has revealed that unrelated species in different regions of the world look alike when found in similar environments. This provides strong evidence for the role of natural selection in evolution.

  • Rabbits did not exist in Australia until introduced by humans. A native Australian wallaby resembles a rabbit both in structure and habit. As similar as these two animals appear, they are not that closely related. The rabbit is a placental mammal, while the wallaby is a marsupial mammal. The fetus of a placental mammal develops in the female uterus, obtaining nourishment from the mother through the placenta. The fetus of a marsupial leaves the mother's uterus at an early stage of development and completes the remaining development while attached to a teat in the abdominal pouch. The great similarity of the rabbit and the wallaby is the result of natural selection.

• Embryology reveals similar stages in development (ontogeny) among related species. The similarities help establish evolutionary relationships (phylogeny).

  • Gill slits and tails are found in fish, chicken, pig, and human embryos.

• Comparative anatomy describes two kinds of structures that contribute to the identification of evolutionary relationships among species.

  • Homologous structures are body parts that resemble one another in different species because they have evolved from a common ancestor. Because anatomy may be modified for survival in specific environments, homologous structures may look different, but will resemble one another in pattern (how they are put together). The forelimbs of cats, bats, whales, and humans are homologous because they have all evolved from a common ancestral mammal.

  • Analogous structures are body parts that resemble one another in different species, not because they have evolved from a common ancestor, but because they evolved independently as adaptations to their environments. The fins and body shapes of sharks, penguins, and porpoises are analogous because they are adaptations to swimming.

• Molecular biology examines the nucleotide and amino acid sequences of DNA and proteins from different species. Closely related species share higher percentages of sequences than species distantly related. In addition, all living things share the same genetic code. This data strongly favors evolution of different species through modification of ancestral genetic information.

  • More than 98% of the nucleotide sequences in humans and chimpanzees are identical.