Hardy–Weinberg Equilibrium and Evolution for AP Biology
Practice problems for these concepts can be found at: Evolution Review Questions for AP Biology
When Evolution is not Occurring: Hardy-Weinberg Equilibrium
Evolutionary change is constantly happening in humans and other species; this seems sensible because evolution is the change in allele frequencies over time. It makes sense that these frequencies are highly variable and subject to change as the environment changes. However, biologists use a theoretical concept called the Hardy–Weinberg equilibrium to describe those special cases where a population is in stasis, or not evolving.
Only if the following conditions are met can a population be in Hardy–Weinberg equilibrium:
Notice items 1–4 in this list are the four modes of evolution, which makes sense—if we are trying to establish the conditions under which evolution does not occur, we must keep these processes of evolution from occurring! The fifth condition, random mating, is included because if individuals mated nonrandomly (e.g., if individuals mated with others that looked like them), the allele frequencies could change in a certain direction, and we would no longer be in equilibrium.
Why do we ever use the Hardy–Weinberg equation if it rarely applies to real populations? This can be an excellent tool to determine whether a population is evolving or not; if we find that the allele frequencies do not add up to one, then we need to look for the reasons for this (perhaps the population is too small and genetic drift is a factor, or perhaps one of the alleles is advantageous and is therefore being selected for and increasing in the population). Therefore, although the Hardy–Weinberg equilibrium is largely theoretical, it does have some important uses in evolutionary biology.
The Evidence for Evolution
Support for the theory of evolution can be found in varied kinds of evidence:
- Homologous characters. Traits are said to be homologous if they are similar because their host organisms arose from a common ancestor (which implies that they have evolved). For example, the bone structure in bird wings is homologous in all bird species.
- Embryology. The study of embryos reveals remarkable similarities between organisms at the earliest stages of life, although as adults (or even at birth) the species look completely different. Human embryos, for example, actually have gills for a short time during early development, hinting at our aquatic ancestry. I kept my gills. I'm a good swimmer. Darwin used embryology as an important piece of evidence for the process of evolution. In 1866, the scientist Ernst Haeckel uttered the phrase, "Ontogeny recapitulates phylogeny." Ontogeny is an individual's development; phylogeny is a species' evolutionary history. What Haeckel meant was that during an organism's embryonic development, it will at some point resemble the adult form of all its ancestors before it. For example, human embyros at some point look a lot like fish embryos. The important conclusion from this is that Haeckel and others thought that embryologic similarity between developing individuals could be used to deduce phylogenetic relationships. By the end of the nineteenth century, it was clear this law rarely holds. The real development of organisms differs in several important ways from Haeckal's schemes.
- Vestigial characters. Most organisms carry characters that are no longer useful, although they once were. This should remind you of our short discussion about why organisms are not perfectly adapted to their environments (because the environment is constantly changing). Sometimes an environment changes so much that a trait is no longer needed, but is not deleterious enough to actually be selected against and eliminated. Darwin used vestigial characters as evidence in his original formulation of the process of evolution, listing the human appendix as an example.
Keep in mind that the kinds of evidence described above are often found in the fossil record–the physical manifestation of species that have gone extinct (including things like bones as well as imprints). The most important thing to remember is that adaptations are the result of natural selection.