A compound that produces an excess of hydrogen ions (H+) when dissolved in water. Using quantitative hydrogen ion measurements, such solutions have a pH value less than 7.0. (See ph scale.)
This notion derived from the typical relationship between structure and function: that an organism's structures seem suitable ("adapted") for their tasks. Until Darwin, the cause for adaptation was commonly ascribed to intelligent (divine) guidance. Darwinians replaced this view by proposing that an adaptation is any trait that replaces other variants because of selection for greater reproductive success (See Fitness). An adaptation is a trait whose presence enhances survival or fertility. It is selection rather than intelligent design that produces and/or maintains the correlation between structure and function. The complexities of evolution, however, shroud the Darwinian concept with many qualifications. For example, should selection cease or reverse its direction, as occurs for traits that become vestigial, then the trait is no longer an adaptation, although it may have been in the past. Traits that are not maintained by selection (that is, not related to reproductive success) are generally considered "nonadaptive." Such traits may be introduced or persist in a population through mutation, random genetic drift, the accidental extinction of adaptive varieties, developmental constraints that now impede their elimination, close linkage with genes selected for other functions (See Hitchhiking), or as one of the multiple phenotypic effects of a selected gene (See Pleiotropy). Also, not all selected traits are necessarily beneficial to a population, since some may increase the reproductive success of genes or individuals but not benefit (or even decrease) populationfitness (See Segregation distortion, Sexual selection). Even when selected traits are unquestionably adaptive, they often involve "trade-offs" in other traits that can lose adaptive advantages. (For example, trees that grow competitively taller put more resources into wood production than seed production.) In addition, earlier selected stages of an adaptation may have been for a function different from that of a later stage (See Preadaptation). In general, since it is quite difficult to examine historical circumstances leading to a particular trait, it can be difficult to determine how or to what extent it is an adaptation. Mostly, such determinations depend on evaluating functional utility ("optimality") for reproductive success, based on the reasonable assumption that a useful trait generally replaces or has replaced less useful variants. Unfortunately, since it can also be challenging to establish functional utility—to uncover a trait's many possible variations, and to compare their relative reproductive success—identifying adaptations can be controversial. Although selection may not be obvious, it is difficult to accept that any prominent nonadaptive trait can long persist without being affected by selection in some way and to some degree. The term is also frequently used for the process that produces adaptations (natural selection). However adaptations are defined, it is the genetic transmission of traits whose structure and function let their carriers interact successfully with the environment that drives evolution and makes biology unique and historical.
A model originally devised by Sewall Wright that describes a topography in which high fitnesses correspond to peaks and low fitnesses to valleys; each position potentially occupied by a population bearing a unique and frequent genotype.
The diversification of a single species or group of related species into new ecological or geographical zones to produce a large variety of species and groups. Such events may include the following: 1. Survivors of a catastrophe (for example, mammals) invade the adaptive zones that were abandoned by extinct species (for example, dinosaurs). 2. One or a few colonizers enter a new habitat in which competing species are absent (for example, the Hawaiian Drosophilidae). 3. One group of species (for example, pollinating insects) evolves in step with the adaptive radiation of another group (for example, angiosperms), or parasites (for example, viruses and bacteria) evolve new strains in concert with proliferation of their hosts (for example, humans). 4. A preadaptive feature (for example, the shelled reptilian amniotic egg) allows invasion into a previously inaccessible ecological zone (for example, terrestrialhabitats). 5. A new morphological or physiological character (for example, pharyngeal jaw innovations in cichlid fishes) causes divergent evolution by partitioning the environment into different niches. These various historical contingencies promote genetic diversity by letting selection—the primary biological force changing organisms through time—channel evolution in new directions.
Movement of an appendage or body part toward the midline (median sagittal) plane, for example, bringing a laterally extended arm to the side of the body.
An organic compound commonly involved in the transfer of phosphate bond energy, composed of adenosine (an adenine base + a D-ribose sugar) and one phosphate group.
An organic compound commonly involved in the transfer of phosphate bond energy, composed of adenosine (an adenine base + a D-ribose sugar) and two phosphate groups.
An organic compound commonly involved in the transfer of phosphate bond energy, composed of adenosine (an adenine base + a D-ribose sugar) and three phosphate groups.
The generalization that warm-blooded animals (mammals) tend to have shorter extremities (for example, ears and tail) in colder climates than they have in warmer climates.
Differential growth rates of different body parts; during development one feature may change at a rate different from that of another feature, resulting in achange of shape. (For a change in developmental timing, see Heterochrony.)
An organism or species that has more than two sets (2n) of chromosomes (that is, 3n, 4n, and so on) that derive from two or more different ancestral groups.
The particular form (amino acid sequence) of an enzyme produced by a particular allele at a gene locus when there are different possible forms of the enzyme (different possible amino acid sequences), each produced by a different allele.
Organic molecules of the general formula R-CH(NH,)COOH, possessing both basic (NH2) and acidic (COOH) groups, as well as a side group (R) specific for each type of amino acid. Normally 20 different types of amino acids are used in cellularly synthesized proteins.
The type of egg produced by reptiles, birds, and mammals (Amniota), in which the embryo is enveloped in a series of membranes (amnion, allantois, chorion) that help sustain its development.
The possession of a similar character by two or more quite different species or groups that arises from a developmental pathway unique to each group; that is, the similarity is caused by factors other than their distant common genetic ancestry. (See also Convergence.)
The gain or loss of chromosomes leading to a number that is not an exact multiple of the basic haploidchromosome set (n) (for example, n+1, 2n+1, 2n-1, 2n-2, 2n+3, and so on).
Reproduction without fertilization; offspring produced from unfertilized eggs in which meiosis has been partially or completely suppressed. (See also Parthenogenesis.)
Conspicuous warning coloration in potential prey species that advertises their toxicity or distastefulness to predators. Aposematic patterns usually contain bright colors or shades such as those found among wasps, monarch butterflies, coral snakes, skunks, and poisonous salamanders.
Prokaryotes that, unlike eubacteria, do not incorporate muramic acid into their cell walls and possess other distinguishing characteristics. They are considered to represent one of the early cell forms.
The concept of an ideal primitive plan ("Bauplan") on which organisms, such as vertebrates, are presumably based. Called by Richard Owen the "primal pattern" and "divine idea."
Mating among individuals on the basis of their phenotypic or genotypic similarities (positive assortative) or differences (negative assortative) rather than mating among all individuals on a random basis.
A chromosome whose presence or absence is ordinarily not associated with determining the difference in sex (that is, a chromosome other than a sex chromosome).
An organism capable of synthesizing complex organic compounds needed for growth from simple inorganic environmental substrates: photoautotroph, an organism that can use light as an energy source and carbon dioxide as a carbon source; chemoautotroph (chemolithotroph), an organism that obtains energy for growth by oxidizing inorganic compounds such as hydrogen sulfide.
The persistence of two or more different genetic forms through selection (for example, heterozygote advantage) rather than because of mutation or other evolutionary forces.
A compound that produces an excess of hydroxyl (OH ) ions when dissolved in water. Using quantitative hydrogen ion measurements, such solutions have a pH value greater than 7.0. (See pH scale.)
The similarity in appearance of a harmless species (the mimic) to a species that is harmful or distasteful to predators (the model), maintained because of selective advantage to the relatively rare mimic.
Holy book of Christianity. Its canon has been fixed for over 1700 years, although the rise of Protestantism and the Protestant Bible has led to minor variations. Comprised of two smaller books called the Old Testament and the New Testament.
Instances in which the left and right sides of a longitudinal (sagittal) plane that runs through an organism's midline are approximately mirror images of each other.
A term used mostly to describe terrestrialtetrapod locomotion that is restricted to the hind limbs when these two limbs move alternately (for example, human walking) rather than together (for example, kangaroo jumping).
The opening formed by the invagination of cells in the embryonic gastrula, connecting its cavity (archenteron) to the outside. In protostome phyla the blastopore is the site of the future mouth, whereas in deuterostomes the blastopore becomes the anus and the mouth is formed elsewhere.
Apelike locomotion through trees: hanging from branches and swinging alternate arms (left, right, left,...) from branch to branch, accompanied by a rotation of the body during each swing.
Water whose salt content (salinity) is intermediate between fresh water and sea water; usually at the mouths of rivers that empty into the ocean (estuaries).
The interval between about 545 and 505 million years before the present, marking the plentiful appearance of fossilized organisms with hardened skeletons. It is considered the beginning of the Phanerozoic time scale (eon) and is the first period in the Paleozoicera.
A compound in which the hydrogen and oxygen atoms bonded to carbons are commonly in a ratio of 2/1 (for example, glucose (C6H12O6), starch (C6H12O6)n, and cellulose, (C6H10O5)n).
An organic compound that has an acidic group consisting of a carbon with a double-bond attachment to an oxygen atom and a single-bond attachment to a hydroxyl group (0=C-OH).
The eighteenth- and nineteenth-century concept that fossilized organisms and changes in geological strata were produced by periodic, violent, and widespread catastrophic events (presumably caused by capricious supernatural forces) rather than by naturally explainable events based on laws that act uniformly through time. (See also Uniformitarianism.)
The period from 65 million years ago to the present, marked by the absence of dinosaurs and the radiation of mammals. This is the third and most recent era of the Phanerozoic eon and is divided into two major periods, the Tertiary and Quaternary.
A feature, trait, or property of an organism or population. If possible, the description of a character should include the conditions under which it is observed.
Divergence in the appearance or measurement of a character between two species when their distributions overlap in the same geographical zone, compared to the similarity of the character in the two species when they are geographically separated. When common resources are limited, it is presumed that competition between overlapping species leads to divergent specializations and therefore to divergence in characters that were formerly similar.
Linkage between a chemical process (electron transport chain) and a proton pump that causes protons (H+) to be transferred across a membrane. This creates a proton gradient that drives a membrane-bound enzyme, ATP synthetase, to catalyze the reaction ADP + Pi->ATP.
A chlorophyll-containing, membrane-bound organelle that is the site of photosynthesis in the cells of plants and some protistans. These organelles contain their own genetic material (circular DNA without histones) and are believed to be descendants of cyanobacteria that entered eukaryotic cells via endosymbiosis.
One of the two sister products of a eukaryotic chromosome replication, marked by an attachment between the sister chromatids at the centromere region. When this attachment is broken during the mitotic anaphase stage, each sister chromatid becomes an independent chromosome.
A length of nucleic acid comprising a linear sequence of genes that is unconnected to other chromosomes. In eukaryotes, histone proteins are bound to nuclear chromosomes, and this protein-nucleic acid complex can be made microscopically visible as deeply staining filaments.
A mode of classification based principally on grouping taxa by their shared possession of similar ("derived") characters that differ from the ancestral condition.
The grouping of organisms into a hierarchy of categories commonly ranging from species to genera, families, orders, classes, phyla, and kingdoms, each category reflecting one or more significant features. In practice, the decision as to the species in which to place an organism, or the genus in which to place a species, and so forth, is most often based on phenotypic similarity to other members of the group: organisms in a species are more similar to each other than they are to organisms in other species of the same genus, species in a genus are more similar to each other than they are to species in other genera of the same family, and so forth.
A gradient of phenotypic or genotypic change in a population or species correlated with the direction or orientation of some environmental feature, such as a river, mountain range, north-south transect, or altitude.
Techniques for producing identical copies of a section of genetic material by inserting a DNA sequence into a cell, such as a bacterium, where it can be replicated.
A statistical term used to describe relationships among different gene sequences that are all descended from a common ancestral sequence (the "coalescent"). Instead of a genealogical tree expanding each generation from its apex toward its base, the tree is conceived as collapsing (coalescing) by proceeding in reverse, from base to apex.
The independent phenotypic expression of two different alleles in a heterozygote (for example, genotypes carrying both Mand N alleles of the MN blood group show the MN blood type).
Nonprotein enzyme-associated organic molecules (for example, NAD, FAD, and coenzyme A), that participate in enzymatic reactions by acting as intermediate carriers of electrons, atoms, or groups of atoms.
An association between organisms of different species in which one species is benefited by the relationship but the other species is not significantly affected.
Relationship between organismic units (for example, individuals, groups, species) attempting to exploit a limited common resource in which each unit inhibits, to varying degrees, the survival or proliferation of another unit by means other than predation.
A state of intricate organization caused by arrangement or interaction among different component parts or processes: presumably, the greater the number of interacting parts, the greater the complexity. The term levels of complexity describes gradations in which complex organizations are included (nested) within others. (See also Hierarchy.) Attempts to compare the degrees of complexity among organisms have used numbers of their different kinds of structures, organs, tissues, cells, genes, and proteins. However, such numbers do not always change in a consistent fashion, and MeShea points out, "Something maybe increasing [in evolution]. But is it complexity?"
A mammalian order that became extinct during the Miocene period of the Cenozoic era but whose first occurrences are in the late Cretaceous period of the Mesozoic. It includes a diversity of early herbivorous placental mammals and the ancestors of all later herbivores.
Constraint has been used in biology to describe factors that limit charactervariation or evolutionary direction. According to some authors, the term applies to traits molded primarily by physical agents and laws, such as crystallization, friction, gravity, and surface tension, but unaffected by historical contingencies, such as selection. Others argue that excluding biological factors does not help explain distinctions among phenotypic variants, nor the causes for differences among lineages. A biological concept of constraint therefore seems more useful to evolutionists - phenotypic channeling and evolutionary trends caused mostly by processes involving adaptation. That is, because adaptation depends on the availability of appropriate genes, adaptive constraints are mostly tied to organismic histories: constraints are affected by genes that evolved and were selected previously. Among such constraining forces are directional selection, stabilizing selection, canalization, and factors that may limit or direct genetic mutability, as well as developmental innovation. Some constraints may focus phenotypic trends in adaptive directions, but others (limited genetic variation and restricted developmental ability) may also limit response to new environmental challenges, leading even to extinction. Although constraints can provide reasonable explanations for trends and attributes, postulating which particular constraints were in force seems highly conjectural in the absence of detailed historical-phylogenetic information. For a term that can apply to so many different phiiiomena, constraint's usage in specific instances needs to be defined.
Charactervariations (such as height in humans) whose distribution follows a series of small nondiscrete quantitative steps from one extreme to the other. (See also Quantitative character.)
The evolution of similar characters in genetically unrelated or distantly related species, mostly because they have been subjected to similar environmental selective pressures. (See also Analogy.)
The degree to which two measured characters tend to vary in the same quantitative direction (positive correlation) or in opposite directions (negative correlation).
Creationist term for the beginning when God created the earth and the universe. For YECs creation was only some thousands of years ago and is literally described in Genesis. For OECs creation was billions of years ago and Genesis is interpreted allegorically.
The belief that each different kind of organism was individually created by one or more supernatural beings whose activities are not controlled by known physical, chemical, or biological laws. (See also Young Earth Creationism (YEC), Old Earth Creationism (OEC))
Elevations on the crowns of premolars and molars. The number, shapes, and positions of cusps are inherited characters that can provide useful phylogenetic information.
Photosynthetic prokaryotes possessing chlorophyll a but not chlorophyll b. Many are photosynthetic aerobes (oxygen producing) and some are anaerobes (not oxygen producing). Formerly called blue- green algae, their color caused by a bluish pigment masking the chlorophyll.
The type of genetic code used by existing terrestrialorganisms, for which there is more than one triplet codon for a particular amino acid but a specific codon cannot code for more than one amino acid. Thus, the 20 different amino acids translated in protein synthesis are coded by 61 of the 64 possible different triplet codons, some by as many as six different "synonymous" codons. (See also Genetic code.)
An allele whose effect reduces the adaptive value of its carrier when present in homozygous condition (recessive allele) or in heterozygous condition (dominant or partially dominantallele).
The dependence of population growth and size on factors directly related to the numbers of individuals in a particular locality (for example, competition for food, accumulation of waste products).
The dependence of population growth on factors (climatic changes, meteorite impacts, and so on) unrelated to the numbers of individuals in a particular locality.
Changes that occur in the structure and function of cells and tissues as the development of the organism proceeds. Generally, the change from an immature embryo to a more complex mature organism.
Presence in a population or species of two morphologically distinctive types of individuals (for example, differences between males and females, pigmented and nonpigmented forms).
A nucleic acid that serves as the genetic material of all cells and many viruses; composed of nucleotides that are usually polymerized into long chains, each nucleotide characterized by the presence of a deoxyribose sugar.
In molecular biology, an amino acid sequence within a polypeptide chain that performs a particular subfunction in the protein. The term has also been used in systematics to provide a tripartite division of organisms - Archaea, Bacteria, Eucarya - as a substitute for the rank of superkingdom, which commonly designates prokaryotes and eukaryotes.
Instances in which the phenotypic effect of a particular allele (for example, A, the dominant) is expressed in both the heterozygote (Aa) and hornozygote (AA), but the phenotypic effect of the other allele (for example, a, the recessive) is not expressed in heterozygotes but only in homozygotes (aa).
Relations within a group in which one or more individuals, sustained by aggression or other behaviors, rank higher than others in controlling the conduct of group members.
The shift in wavelength of light or sound that is perceived as the emitting body moves toward us (shorter wavelengths, for example, blue-shifted) or away from us (longer wavelengths, for example, red-shifted).
A mechanism that compensates for the difference in number of X chromosomes (or Z chromosomes) between males and females so the metabolic activities (gene expression) of their X-linked genes are equalized. Although dosage compensation is widespread among animals, the mechanism by which it is accomplished varies. In species with XY males and XX females such as Drosophila, male X-linked genes show increased gene expression, whereas in mammals only one X chromosome in each sex is metabolically active and any additional X chromosomes are inactivated.
A distinctive feature of angiosperm plants in which two nuclei from a male pollen tube fertilize the female gametophyte, one producing a diploid embryo and the other producing polyploid (usually triploid) nutritional endosperm.
The environmental habitat of a population or species, including the resources it uses and its interactions with other organisms. Since resources and interactions are rarely constant, populations remain continually subject to selective pressures for adaptational change. A particular organism's ecological niche is commonly reflected in its adaptations when these can be specified. (See Adaptation.)
Geological formations containing soft- bodied invertebrate fossils found in South Australia and other places, dating to a Precambrian period lasting about 60 or more million years.
A technique that separates dissolved particles subjected to an electrical field according to their mobility. Given a particular medium through which a particle moves, electrophoretic mobility depends on the size of the particle, its geometry, and electrical charge.
The layer of cells that lines the primitive gut (archenteron) during the early stages of development in animals, and later forms the epithelial lining of the intestinal tract and internal organs such as the liver, lung, and urinary bladder.
A relationship between two different organisms in which one (the endosymbiont) lives within the tissues or cell of the other, benefiting one or both. It is now generally thought that some eukaryotic organelles, such as mitochondria and chloroplasts, had an endosymbiotic prokaryotic origin.
The measure of disorder of a physical system. In a closed system, to which energy is not added, the second law of thermodynamics essentially states that entropy, or energy unavailable for work, will remain constant or increase but never decrease. Living systems, however, are open systems, to which energy is added from sunlight and other sources, and order can therefore arise from disorder in such systems, that is, energy available for work can increase and entropy can decrease.
The complex of external conditions, abiotic and biotic, that affects organisms or populations. It provides the facilities and resources that enable hereditary data (genotypes) to produce organismic features (phenotypes).
A major division of the geological time scale, often divided into two eons beginning from the origin of the earth 4.5 billion years ago: the Precambrian or Cryptozoic (rarity of life forms) and the Phanerozoic (abundance of life forms).
The concept that tissues and organs are formed by interaction between cells and substances that appear during development, rather than being initially present in the zygote (preformed). (See also Preformationism. )
Interactions between two or more gene loci that produce phenotypes different from those expected if each locus were considered individually. In statistical population studies that evaluate the causes for phenotypic differences, the term epistasis is commonly used for all phenotypic variation caused by interaction between nonallelic genes.
One of the categories into which geological time is divided; a subdivision of a geological period. For periods divided into three epochs, they are often named Early, Middle, and Late; for example, Early Cambrian period, . . .
Prokaryotes, other than archaebacteria, marked by sensitivity to particular antibiotics and by the incorporation of muramic acid into their cell walls.
Organisms whose cells contain nuclear membranes, mitochondrialorganelles, and other characteristics that distinguish them from prokaryotes. Eukaryotes may be unicellular or multicellular and include protistans, fungi, plants, and animals.
The process in which an aquatic system becomes overloaded with nutrients, thereby increasing its organic productivity and causing an accumulation of debris.
The concept that the rate at which mutational changes accumulate is constant over time. To which genes or genomes this clock may apply, and whether it is really constant, are disputed.
A nucleotide sequence in a gene that is transcribed into messenger RNA and spliced together with the transcribed sequences of other exons from the same gene. The continuous RNA molecule formed is then transferred to the ribosome and forms the template used in polypeptide synthesis. Exons ("expressed sequences") are separated from other exons in the same gene by intervening nontranslated sequences (See Intron) that are removed from the rnRNA. Such intron-exon split genes are commonly found in eukaryotes but are almost entirely absent in prokaryotes,
Central to evolutionary concepts evaluating genotypes and populations, fitness has had many definitions, ranging from comparing growth rates to comparing long-term survival rates. The basic fitness concept that population geneticists commonly use is relative reproductive success, as governed by selection in a particular environment; that is, the ability of an organism (genotype) to transmit its genes to the next reproductively fertile generation, relative to this ability in other genotypes in the same environment ("relative fitness"). Since there are forces other than selection that influence genotype frequencies (for example, mutation, random genetic drift, migration), fitness is not the only way of characterizing short-term populational genetic changes. Nevertheless, because reproductive success, sooner or later, affects most variation, fitness and selection enter into practically all enduring organismic-environmental interactions, with adaptations their phenotypic manifestations.
A concept held by Linnaeus and others that members of a species could only produce progeny like themselves, and therefore each species was fixed in its particular form(s) at the time of its creation.
The effect caused by a sampling accident in which only a few "founders" derived from a large population begin a new colony. Since these founders carry only a small fraction of the parental population's genetic variability, radically different gene frequencies can become established in the new colony. (See also Bottleneck effect.)
The concept that an accidental event in the distant past was responsible for the presence of a universal feature in living organisms. Such events may include an accident in which the present genetic code was used by a group of early organisms that managed to survive some populational bottleneck, thereby conferring this particular code on later organisms.
The belief that creation stories and the many events and rules given in religious documents (for example, the Judeo-Christian Bible, the Moslem Koran) are to be taken literally.
A system of numerous stars such as the Milky Way (150 billion stars, 100,000 light-years across) held together by mutual gravitational effects. Galaxies, in turn, are grouped into clusters and superclusters. Our own supercluster, centered on Virgo, contains many thousands of galaxies and is more than 100 million light-years across.
A cuplike embryonic stage in multicellular animals that follows the blastula stage. Its hollow cavity (archenteron) is lined with endoderm and opens to the outside through a blastopore. (See also Haeckel's gastrula hypothesis.)
Two or more gene loci in an organism whose similarities in nucleotide sequences indicate they have been derived by duplication from a common ancestral gene (for example, the p-globin gene family, which includes beta, gamma, delta and epsilon genes).
Human-directed repair or replacement of genes that cause inherited diseases. When confined to somatic (body) cells rather than to sex cells (sperm or eggs), such gene repairs are not passed on to future generations.
