Daeschler, Ted: Paleontologist and associate research curator at the Academy of Natural Sciences. Discoverer of late Devonian limbed fossils Hynerpeton bassetti and Designathus rowei (tetrapods) and Sauripterus taylorii and Hyneria (lobed-finned fishes), all early examples of animals exploiting both land and water environments. Author of two books on paleontology for young people.
Dart, Raymond: Australian-born South African anatomist and anthropologist(1893-1988). In 1924 he described a fossil skull collected near Taung in South Africa, naming it Australopithecus africanus. Dart asserted that the skull was intermediate between the apes and humans, a controversial claim at the time, though later work made it clear that the Taung child, as it came to be known, was indeed a hominid.
Darwinian evolution: Evolution by the process of natural selection acting on random variation.
Darwinism: Darwin's theory that species originated by evolution from other species and that evolution is mainly driven by natural selection. Differs from neo-Darwinism mainly in that Darwin did not know about Mendelian inheritance.
Darwin, Charles: The 19th-century naturalist considered the father of evolution. His landmark work, On the Origin of Species, published in 1859, presented a wealth of facts supporting the idea of evolution and proposed a viable theory for how evolution occurs -- via the mechanism Darwin called "natural selection."In addition to his prolific work in biology, Darwin also published important works on coral reefs and on the geology of the Andes, and a popular travelogue of his five-year voyage aboard HMS Beagle.
Darwin, Erasmus: The name shared by Charles Darwin's grandfather and brother, each important in his life and work. Charles's grandfather Erasmus (1731-1802)was a glorious polymath -- physician, author, and botanist. His impact is reflected throughout a wide range of disciplines from the poetry to the technology of his day. Author of The Loves of the Plants, a 2,000-line poem detailing their sexual reproduction, and Zoonomia, or the Theory of Generations, whose themes echo throughout his grandson's work. Charles's older brother Erasmus (1805-1881), known as "Ras," used his network of social and scientific contacts to advance the theories of his shyer, more retiring sibling.
Dawkins, Richard: An evolutionary biologist who has taught zoology and is the author of several books on evolution and science, including The Selfish Gene(1976) and The Blind Watchmaker (1986). He is known for his popularization of Darwinian ideas, as well as for original thinking on evolutionary theory.
Dembski, William: A mathematician and philosopher who has written on intelligent design, attempting to establish the legitimacy and fruitfulness of design withinbiology.
Dennett, Daniel: Philosopher and director of the Center for Cognitive Studies at Tufts University, whose work unites neuroscience, computer science, and evolutionary biology. Dennett sees no basic distinction between human and machine intelligence, advocating a mechanical explanation of consciousness. He is the author of Brainchildren: Essays on Designing Minds and Darwin's Dangerous Idea: Evolution and the Meanings of Life, among many other books and publications.
derived homology: Homology that first evolved in the common ancestor of a set of species and is unique to those species. Compare with ancestral homology.
de facto: In fact; in reality. Something which exists or occurs de facto is not the result of a law, but because of circumstances.
diatom: These single-celled algae are common among the marine phytoplankton. Their glassy, two-part shells have intricate patterns and fit together like the two parts of a shirt box.
diffusion: The process by which molecules (for example, of oxygen) move passively from a region of high concentration to a region of low concentration.
dinoflagellate: Possessing two tail-like extensions called flagella that are used for movement, these single-celled algae can live freely or in other organisms such as corals. When many dinoflagellates suddenly reproduce in great numbers, they create what are known as "red tides" by making the water appearred.
diploid: Having two sets of genes and two sets of chromosomes (one from the mother, one from the father). Many common species, including humans, are diploid. Compare with haploid and polyploid.
directional selection: Selection causing a consistent directional change in the form of a population through time (e.g., selection for larger body size).disruptive selection: Selection favoring forms that deviate in either direction from the population average. Selection favors forms that are larger or smaller than average, but works against the average forms between the extremes.
distance: In taxonomy, referring to the quantitatively measured difference between the phenetic appearance of two groups of individuals, such as populations or species (phenetic distance), or the difference in their gene frequencies (genetic distance).
DNA: Deoxyribonucleic acid, the molecule that controls inheritance.DNA base sequence: A chain of repeating units of deoxyribonucleotides (adenine, guanine, cytosice, thymine) arranged in a particular pattern.
Dobzhansky, Theodosius: A geneticist and zoologist best known for his research in population genetics using the fruit fly. His study of the evolution of races led to the discovery of genetic diversity within species, and confirmed his belief that genetic variation leads to better adaptability.
dominance (genetic): An allele (A) is dominant if the phenotype of the heterozygote (Aa) is the same as the homozygote (AA). The allele (a) does not influence the heterozygote's phenotype and is called recessive. An allele may be partly, rather than fully, dominant; in that case, the heterozygous phenotype is nearer to, rather than identical with, the homozygote of the dominant allele.
drift: Synonym of genetic drift.
duplication: The occurrence of a second copy of a particular sequence of DNA. The duplicate sequence may appear next to the original or be copied elsewhere into the genome. When the duplicated sequence is a gene, the event is called gene duplication.