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ecosystem: A community of organisms interacting with a particular environment.
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embryo: An early stage of animal development that begins after division of the zygote (the earliest stage, in which joined egg and sperm have not yet divided).
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empirical: Determined by observation or experimentation.
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enzyme: A protein that acts as a catalyst for chemical reactions.
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eukaryote: Any organism made up of eukaryotic cells. Eukaryotes are generally larger and have more DNA than prokaryotes (whose cells do not have a nucleus to contain their DNA). Almost all multicellular organisms are eukaryotes.
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eukaryotic cell: A cell with a distinct nucleus.
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evolution: Darwin defined this term as "descent with modification." It is the change in a lineage of populations between generations. In general terms, biological evolution is the process of change by which new species develop from preexisting species over time; in genetic terms, evolution can be defined as any change in the frequency of alleles in populations of organisms from generation to generation.
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extinction: The disappearance of a species or a population.
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fact: A natural phenomenon repeatedly confirmed by direct observation.
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fitness: The success of an individual (or allele or genotype in a population) in surviving and reproducing, measured by that individual's (or allele's or genotype's) genetic contribution to the next generation and subsequent generations.
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fossil: Most commonly, an organism, a physical part of an organism, or an imprint of an organism that has been preserved from ancient times in rock, amber, or by some other means. New techniques have also revealed the existence of cellular and molecular fossils.
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