A proposed explanation for a narrow set of phenomena. A hypothesis must be testable with evidence from the natural world. If an explanation can’t be tested with experimental results, observation, or some other means, then it is not a scientific hypothesis.

The idea that while reduced exposure to microorganisms and parasitic worms has reduced morbidity and mortality in humans, it has been associated with a rising prevalence of atopic disorders and autoimmune diseases, particularly in industrialized countries, because of insufficient or inappropriate development of the immune system in the absence of these infectious agents.

A fluid-filled cavity that supports the body of an animal because the fluid cannot be compressed into a smaller volume (hydro = liquid or water, statos = standing, unchanging).

The production of offspring from different parental forms. For example, if two recognizably different species of plant fertilized one another and produced viable, fertile offspring, the process would be called hybridization.

(1726-1797)
Scottish farmer and geologist. In his travels around Britain, he made observations which suggested to him that the geologic processes that shaped the ancient Earth could be seen operating all the time, an idea which would later form the basis of Lyell’s uniformitarianism. Hutton used his observations and hypothesis to argue that the Earth must be extremely old.

A gene that regulates the development and organization of the major body units. For a more detailed explanation, see our resource on Hox genes in Evolution 101.

Organism that serves as a habitat for another organism. A host may provide nutrition to a parasite or simply a place in which to live.

A process which results in the transfer of genetic material between members of different species. Bacteria, for example, frequently pass copies of particular genes to one another and pick up foreign genetic material from their environment, resulting in horizontal transfer.

An individual carrying two identical gene versions for a particular gene (e.g., AA as opposed to Aa).

Clade that includes all humans, human ancestors, and human relatives that are more closely related to us than chimpanzees are.

Here, humans and their extinct relatives (i.e., organisms on the “human side” of the human/chimpanzee lineage split). However, some scientists use the term hominid to refer to a larger group: humans, other great apes (gorillas, chimpanzees, bonobos, and orangutans), and their extinct relatives. However you decide to name the groups, the important thing is how all these species are related to one another and not exactly what we decide to call each lineage.

An individual carrying two different gene versions for a particular gene (e.g., Aa as opposed to AA).

An evolutionary change in the timing of a developmental event. For example, relative to the lineage’s ancestor, the early maturation of sex organs is an example of heterochrony.

An organism that eats almost entirely plants (herb = plant, vorare = to swallow up).

Individual or cell that carries one set of its chromosomes. Human eggs and sperm are haploid. They usually carry one copy of each of our 22 regular chromosomes and either an X or a Y sex chromosome.

The hypothesis that a gene version coding for an altruistic behavior will spread via kin selection when rB > C — where r is the relatedness between the altruist and recipient, B is the fitness benefit given to the recipient, and C is the fitness cost to the altruist.

Place and conditions in which an organism normally lives.

The region surrounding a star in which planets with liquid water on their surfaces may exist. The width and position of the habitable zone depend on how hot the star is.

A property of systems in which intrinsic characteristics are passed from parent(s) to offspring. This results in offspring that resemble their parent(s) more than they resemble randomly chosen individuals of the population. In biology, heredity occurs largely through transmission of the genetic materials DNA and RNA. However, epigenetic heredity also occurs.

Inherited from a common ancestor. Human eyes and mouse eyes are homologous structures because we each inherited them from our common ancestor that also had the same sort of eyes. Contrast this with homoplasious and analogous.

Similar but not because of inheritance from a common ancestor. Homoplasious characters may be explained by convergent evolution in two different organisms or character reversals.