1. analogous structure Body parts that once differed in evolutionarily distant lineages but converged in structure and function as responses to similar environmental pressures.
2. asteroid Rocky, metallic body, a few to 1,000 kilometers across, hurtling through space.
3. comparative morphology Scientific study of comparable body parts of adults or embryonic stages of major lineages.
4. derived trait A novel feature that evolved but once and is shared only by the descendants of the ancestral species in which it evolved.
5. fossil Recognizable, physical evidence of an organism that lived in the distant past.
6. fossilization How fossils form. An organism or evidence of it gets buried in sediments or volcanic ash; water and dissolved inorganic compounds infiltrate it; then chemical changes and pressure from accumulating sediments above transform it to stony hardness.
7. geologic time scale Time scale for the Earth's history with major subdivisions corresponding to mass extinctions. Now radiometrically dated.
8. Gondwana Paleozoic supercontinent; with other land masses, it formed Pangea.
9. half-life The time it takes for half of a given quantity of any radioisotope to decay into a different, and less unstable, daughter isotope.
10. homologous structure Of separate lineages, comparable body parts that show underlying similarity even when they may differ in size, shape, or function; outcome of morphological divergence from a shared ancestor.
11. lineage Line of descent.
12. morphological convergence Macroevolutionary pattern. In response to similar environmental pressures over time, evolutionarily distant lineages evolve in similar ways and end up being alike in appearance, functions, or both.
13. morphological divergence Macroevolutionary pattern; genetically diverging lineages undergo change from body form of a common ancestor.
14. Pangea Paleozoic supercontinent upon which the first terrestrial plants and animals evolved.
15. plate tectonics theory Theory that great slabs (plates) of the Earth's outer layer float on a hot, plastic mantle. All plates are slowly moving and have rafted continents to new positions over time.
16. stratification Stacked layers of sedimentary rock, built up by gradual deposition of volcanic ash, silt, and other materials over time.
17. theory of uniformity Early theory that the earth's surface changes in gradual, uniformly repetitive ways (major floods, earthquakes, and other infrequent catastrophes also occur every year and were not considered unusual). Has since been replaced by plate tectonics theory.
18. adaptive radiation Macroevolutionary pattern; burst of genetic divergences from a lineage that gives rise to many species, each using a novel resource or a new (or newly vacated) habitat.
19. adaptive zone Minimum amount of energy required to get a specific reaction going, with or without the help of an enzyme. Reactions differ in the amount required.
20. allopatric speciation Speciation model. A physical barrier arises, separates populations or subpopulations of a species, ends gene flow, and favors divergences that end in speciation.
21. anagenesis Speciation pattern; changes in allele frequencies and morphology accumulate within an unbranched line of descent.
22. archipelago Island chain some distance away from a continent.
23. biological species concept Defines a species as one or more populations of individuals that are interbreeding under natural conditions, producing fertile offspring, and are isolated reproductively from other such populations. Applies to sexually reproducing species only.
24. cladogenesis Speciation pattern in which a lineage splits and isolated populations undergo genetic divergence.
25. dosage compensation Any mechanism that balances gene expression between the sexes during critical early stages of development.
26. evolutionary tree Treelike diagram; a branch point means divergence from a shared ancestor and branches signify separate lines of descent.
27. extinction Irrevocable loss of a species.
28. gene flow Microevolutionary process; alleles enter and leave a population as an outcome of immigration and emigration, respectively.
29. genetic divergence Gradual accumulation of differences in gene pools of populations or subpopulations of a species after a geographic barrier arises and separates them; thereafter, microevolution occurs independently in each.
30. gradual model of speciation Idea that species arise by many small morphological changes that accumulate over great spans of time.
31. hybrid zone Where adjoining populations are interbreeding and producing hybrid offspring.
32. mass extinction Catastrophic event or phase in geologic time when entire families or other major groups are irrevocably lost.
33. parapatric speciation Idea that neighboring populations can become distinct species while maintaining contact along a common border.
34. punctuation model of speciation Idea that most morphological changes occur in a brief span when populations start to diverge; speciation is rapid, and the daughter species change little for the next 2-6 million years or so.
35. reproductive isolating mechanism Heritable feature of body form, functioning, or behavior that prevents interbreeding between two or more genetically divergent populations.
36. speciation The formation of a daughter species from a population or subpopulation of a parent species by way of microevolutionary processes. Routes vary in their details and duration.
37. species One kind of organism. Of sexually reproducing organisms, one or more natural populations in which individuals are interbreeding and are reproductively isolated from other such groups.
38. sympatric speciation A speciation event within the home range of an existing species, in the absence of a physical barrier. Such species may form instantaneously, as by polyploidy.