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natural selection

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The process that produces adaptation. Natural selection is based on three postulates: (1) the availability of resources is limited; (2) organisms vary in the ability to survive and reproduce; and (3) traits that influence survival and reproduction are transmitted from parents to offspring. When these three postulates hold, natural selection produces adaptation.

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morphology

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The form and structure of an organism; also a field of study that focuses on the form and structure of organisms.

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equilibrium

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A steady state in which the composition of the population does not change.

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stabilizing selection

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Selection pressures that favor average phenotypes. Stabilizing selection reduces the amount of variation in the population but does not alter the mean value of the trait.

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species

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A group of organisms classified together at the lowest level of the taxonomic hierarchy. Biologists disagree about how to define a species

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fecundity

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The biological capacity to reproduce. In humans, fecundity may be greater than fertility (the actual number of children produced) when people limit family size.

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continuous variation

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Phenotypic variation in which there is a continuum of types. Height in humans is an example of continuous variation. Compare discontinuous variation.

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discontinuous variation

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Phenotypic variation in which there is a discrete number of phenotypes with no intermediate types. Pea color in Mendel's experiments is an example of discontinuous variation. Compare continuous variation.

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convergence

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The evolution of similar adaptations in unrelated species. The evolution of camera-type eyes in both vertebrates and mollusks is an example of convergence. See also analogy.

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core

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A piece of stone from which smaller flakes are removed. Cores and/or flakes may themselves be useful tools.

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alleles

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One of two or more alternative forms of a gene. For example, the A and S alleles are two forms of the gene controlling the amino acid sequence of one of the subunits of hemoglobin.

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chromosome

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A linear body in the cell nucleus that carries genes and appears during cell division. Staining cells with dyes reveals that different chromosomes are marked by different banding patterns.

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crossing over

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The exchange of genetic material between homologous chromosomes during meiosis. Crossing over causes recombination of genes carried on the same chromosome.

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deoxyribonucleic acid (DNA)

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Deoxyribonucleic acid, the molecule that carries hereditary information in almost all living organisms. DNA consists of two very long sugar-phosphate backbones (called "strands") to which the bases adenine, cytosine, guanine, and thymine are bound. Hydrogen bonds between the bases bind the two strands together.

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exons

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A segment of the DNA in eukaryotes that is translated into protein. Compare intron.

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gametes

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In animals, eggs and sperm.

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genes

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A segment of the chromosome that produces a recognizable effect on phenotype and segregates as a unit during gamete formation.

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genome

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All of the genetic information carried by an organism.

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genotype

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The combination of alleles that characterizes an individual at some set of genetic loci. For example, in populations with only the A and S alleles at the hemoglobin locus, that locus has only three possible genotypes: AA, AS, and SS. (SA is the same as AS.) Compare phenotype.

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haploid

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A cell with only one copy of each chromosome. Gametes are haploid, as are the cells of some asexual organisms. Compare diploid.

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heterozygous

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Referring to a diploid organism whose cells carry two different alleles for a particular genetic locus. Organisms that are heterozygous are called "heterozygotes." Compare homozygous.

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homologous pairs

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A pair of chromosomes in a diploid cell in which one member of the pair is derived from the father and one is derived from the mother.

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homozygous

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Referring to a diploid organism whose chromosomes carry two copies of the same allele at a single genetic locus. Organisms that are homozygous are called "homozygotes." Compare heterozygous.

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independent assortment

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The principle, discovered by Mendel, that each of the genes at a single locus on a pair of homologous chromosomes is equally likely to be transmitted when gametes (eggs and sperm) are formed. This happens because during meiosis the probability that a particular chromosome will enter a gamete is 0.5 and is independent of whether other nonhomologous chromosomes enter the same gamete. Thus, knowing that an individual received a particular chromosome from its mother (and thus a particular allele) tells you nothing about the probability that it received other, nonhomologous chromosomes from its mother.

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introns

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A segment of the DNA in eukaryotes that is not translated into protein. Compare exon.

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meiosis

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The process of cell division in which haploid gametes (eggs and sperm) are created. Compare mitosis.

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mitosis

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The process of division of somatic (normal body) cells through which new diploid cells are created. Compare meiosis.

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recombination

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The creation of novel genotypes as a result of the random segregation of chromosomes and of crossing over.

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zygote

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The cell formed by the union of an egg and a sperm.

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canalized

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Describing traits that are very insensitive to environmental conditions during development, resulting in similar phenotypes in a wide range of environments. Compare plastic.

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correlated response (positive and negative correlation)

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An evolutionary change in one character caused by selection on a second, correlated character. For example, selection favoring only long legs will also increase arm length if arm length and leg length are positively correlated.

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environmental variation

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Phenotypic differences between individuals that exist because those individuals developed in different environments. Compare genetic variation.

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fixation

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A state that occurs when all of the individuals in a population are homozygous for the same allele at a particular locus.

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gene frequency

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The fraction of the genes at a genetic locus that are a particular allele (therefore also called allele frequency). For example, a population that contains 250 AA individuals, 200 AS individuals, and 50 SS individuals has 700 copies of the A allele and 300 copies of the S allele; therefore the frequency of the S allele is 0.3.

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genetic drift

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Random change in gene frequencies due to sampling variation that occurs in any finite population. Genetic drift is more rapid in small populations than in large populations.

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Hardy-Weinberg equilibrium

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The unchanging frequency of genotypes that results from sexual reproduction and occurs in the absence of other evolutionary forces such as natural selection, mutation, or genetic drift.

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modern synthesis

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An explanation for the evolution of continuously varying traits that combines the theory and empirical evidence of both Mendelian genetics and Darwinism.

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plastic

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Describing traits that are very sensitive to environmental conditions during development, resulting in different phenotypes in different environments. Compare canalized.

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pleiotropic effects

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Phenotypic effects created by genes that influence multiple characters. See also correlated characters.

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population genetics

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The branch of biology dealing with the processes that change the genetic composition of populations through time.

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adaptive radiation

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The process in which a single lineage diversifies into a number of species, each characterized by distinctive adaptations. The diversification of the mammals at the beginning of the Cenozoic era is an example of an adaptive radiation.

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allopatric speciation

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Speciation that occurs when two or more populations of a single species are geographically isolated from each other and then diverge to form two or more new species. Compare parapatric speciation and sympatric speciation.

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analogous

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Similarity between traits that is due to convergent evolution, not common descent. For example, the fact that humans and kangaroos are both bipedal is an analogy. Compare homology.

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ancestral trait

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A trait that appears earlier in the evolution of a lineage or clade. Ancestral traits are contrasted with derived traits, which appear later in the evolution of a lineage or clade. For example, the presence of a tail is ancestral in the primate lineage, and the absence of a tail is derived. Systematists must avoid using ancestral similarities when constructing phylogenies.

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biological species concept

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The concept that species are defined as a group of organisms that cannot interbreed in nature. Adherents of the biological species concept believe that the resulting lack of gene flow is necessary to maintain differences between closely related species. Compare ecological species concept.

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character displacement

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The result of competition between two species that causes the members of different species to become morphologically or behaviorally more different from each other.

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cladistic taxonomy

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A system for classifying organisms in which patterns of descent are the only criteria used. Compare evolutionary taxonomy.

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comparative method

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A method for establishing the function of a phenotypic trait by comparing different species.

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derived trait

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A trait that appears later in the evolution of a lineage or clade. Derived traits are contrasted with ancestral traits, which appear earlier in the evolution of a lineage or clade. For example, the absence of a tail is derived in the hominin lineage, and the presence of a tail is ancestral. Systematists seek to use derived similarities when constructing phylogenies.

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ecological species concept

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The concept that natural selection plays an important role in maintaining the differences between species, and that the absence of interbreeding between two populations is not a necessary condition for defining them as separate species. Compare biological species concept.

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evolutionary taxonomy

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A system for classifying organisms that uses both patterns of descent and patterns of overall similarity. Compare cladistic taxonomy.

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genetic distance

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A measure of the overall genetic similarity of individuals or species. The best estimates of genetic distance utilize large numbers of genes.

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hominoids

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Any member of the superfamily Hominoidea, which includes humans, all the living apes, and numerous extinct apelike and human-like species from the Miocene, Pliocene, and Pleistocene epochs.

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macroevolution

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Evolution of new species, families, and higher taxa. Compare microevolution.

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microevolution

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Evolution of populations within a species. Compare macroevolution.

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molecular clocks

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The hypothesis that genetic change occurs at a constant rate and thus can be used to measure the time elapsed since two species shared a common ancestor. The molecular clock is based on observed regularities in the rate of genetic change along different phylogenetic lines.

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out-groups

answer

A taxonomic group that is related to a group of interest and can be used to determine which traits are ancestral and which are derived.

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parapatric speciation

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A two-step process of speciation in which (1) selection causes the differentiation of geographically separate, partially isolated populations of a species and (2) subsequently the populations become reproductively isolated as a result of reinforcement. Compare allopatric speciation and sympatric speciation.

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phylogeny

answer

The evolutionary relationships among a group of species, usually diagrammed as a "family tree."

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reinforcement

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The process in which selection acts against the likelihood of hybrids occurring between members of two phenotypically distinctive populations, leading to the evolution of mechanisms that prevent interbreeding.

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sympatric speciation

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A hypothesis that speciation can result from selective pressures favoring different phenotypes within a population, without positing geographic isolation as a factor. Compare allopatric speciation and parapatric speciation.

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systematics

answer

A branch of biology that is concerned with the procedures for constructing phylogenies. Compare taxonomy.

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taxonomy

answer

A branch of biology that is concerned with the use of phylogenies for naming and classifying organisms. Compare systematics.

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polyandry

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A mating system in which a single female forms a stable pair-bond with two different males at the same time. Polyandry is generally rare among mammals, but it is thought to occur in some species of marmosets and tamarins. Compare polygyny.

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polygyny

answer

A mating system in which a single male mates with many females. Polygyny is the most common mating system among primate species. Compare polyandry.

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territories

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A fixed area occupied by animals that defend the boundaries against intrusion by other individuals or groups of the same species.

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estrus

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A period during the reproductive cycle of most mammals (and most primates) when the female is receptive to mating and is capable of conceiving.

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intersexual selection

answer

A form of sexual selection in which females choose who they mate with. The result is that traits making males more attractive to females are selected for. Compare intrasexual selection.

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intrasexual selection

answer

A form of sexual selection in which males compete with other males for access to females. The result is that traits making males more successful in such competition, like large body size or large canines, are selected for. Compare intersexual selection.

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primiparous

answer

Refers to a female who has given birth for the first time.

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sexual selection infanticide hypothesis

answer

A hypothesis postulating that infanticide has been favored by sexual selection because males who kill unweaned infants are able to enhance their own reproductive prospects if they (1) kill infants whose deaths hasten their mothers' resumption of cycling, (2) do not kill their own infants, and (3) are able to mate with the mothers of the infants that they kill.

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coefficient of relatedness, r

answer

An index measuring the degree of genetic closeness between two individuals. The index ranges from 0 (for no relation) to 1 (which occurs only between an individual and itself, or between identical twins). For example, the coefficient of relatedness between an individual and its parents or its siblings is 0.5.

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Hamilton's rule

answer

A rule predicting that altruistic behavior among relatives will be favored by natural selection if rb > c, where r is the coefficient of relatedness between actor and recipient, b is the sum of the benefits of performing the behavior on the fitness of the recipient(s), and c is the cost, in decreased fitness of the donor, of performing the behavior. See also kin selection.

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kin selection

answer

A theory stating that altruistic acts will be favored by selection if the product of the benefit to the recipient and the degree of relatedness (r) between the actor and recipient exceeds the cost to the actor. See also Hamilton's rule.

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reciprocal altruism

answer

A theory that altruism can evolve if pairs of individuals take turns giving and receiving altruism over the course of many encounters.

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social intelligence hypothesis

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The hypothesis that the relatively sophisticated cognitive abilities of higher primates are the outcome of selective pressures that favored intelligence as a means to gain advantages in social groups.

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extracted foods

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Food that is embedded in a matrix, encased in a hard shell, or otherwise difficult to extract. Extracted foods require complicated, carefully coordinated techniques to process.

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third-party relationships

answer

Relationships among other individuals. For example, monkeys and apes are believed to understand something about the nature of kinship relationships among other group members.

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plesiadapiforms

answer

Any member of a group of primatelike mammals that lived during the Paleocene (65 to 55 mya). Although many paleontologists do not consider them to have been primates, the plesiadapiforms probably were similar to the earliest primates, who lived around the same time.

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foramen magnum

answer

The large hole in the bottom of the cranium through which the spinal cord passes.

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sagittal crest

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A sharp fin of bone that runs along the midline of the skull that increases the area available for the attachment of chewing muscles.

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collected foods

answer

Type of food resource, such as a leaf or fruit, that can be gathered and eaten directly.

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hunted foods

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Live animal prey captured by human foragers or nonhuman primates.

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Mode 1

answer

A category of simple stone tools made by removing flakes from cores without any systematic shaping of the core. Both the flakes and the cores were probably used as tools themselves. Tools in the Oldowan industry are Mode 1 tools.

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biface

answer

A flat stone tool made by working both sides of a core until there is an edge along the entire circumference. See also hand ax.

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hafted

answer

To attach a spear point, ax head, or similar implement to a handle. Hafting greatly increases the force that can be applied to the tool.

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Homo heidelbergensis

answer

Middle Pleistocene hominins from Africa and western Eurasia. These hominins had large brains and very robust skulls and postcrania.

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Mode 2

answer

A category of stone tools in which cores are shaped into symmetrical bifaces by the removal of flakes. The Acheulean industry is typified by Mode 2 tools.

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Mode 3

answer

A category of stone tools made by striking large symmetrical flakes from carefully prepared stone cores using the Levallois technique. The Mousterian industry in Europe and the Middle Stone Age industries in Africa are typified by Mode 3 tools.

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Neanderthals

answer

A form of archaic Homo sapiens found in western Eurasia from about 127 kya to about 30 kya. Neanderthals had large brains and elongated skulls with very large faces. They were also characterized by very robust bodies.

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sagittal keel

answer

A feature running along the midline of the skull shaped like a shallow, upside-down V. The sagittal keel is a derived characteristic of Homo erectus.

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balanced polymorphism

answer

A steady state in which two or more alleles coexist in a population. This state occurs when heterozygotes have a higher fitness than any homozygote.

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environmental covariation

answer

The effect on phenotypes that occurs when the environments of parents and offspring are similar. Because environmental covariation causes the phenotypes of parents and offspring to be similar, it can falsely increase estimates of heritability.

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environmental variation

answer

Phenotypic differences between individuals that exist because those individuals developed in different environments. Compare genetic variation.

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founder effect

answer

A form of genetic drift that occurs when a small population colonizes a new habitat and subsequently greatly increases in number. Random genetic changes due to the small size of the initial population are amplified by subsequent population growth.

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genetic variation

answer

Phenotypic differences between individuals that result from the fact that those individuals have inherited different genes from their parents. Compare environmental variation.

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selection-mutation balance

answer

An equilibrium that occurs when the rate at which selection removes a deleterious gene is balanced by the rate at which mutation introduces that gene. The frequency of genes at selection- mutation balance is typically quite low.

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selective sweep

answer

A process in which one allele increases in a population due to positive selection.

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variation among groups

answer

Differences in the average phenotype or genotype between groups.

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variation within groups

answer

Differences in phenotype or genotype between individuals in a group.

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culture

answer

Information stored in human brains that is acquired by imitation, teaching, or some other form of social learning and that is capable of affecting behavior or some other aspect of the individual's phenotype.

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environment of evolutionary adaptedness/EEA

answer

The past environment(s) in which currently observed adaptations were shaped. For example, the psychological mechanisms that cause contemporary humans to overeat were likely shaped in an environment of evolutionary adaptedness in which overeating was rarely a problem.

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observational learning

answer

A form of learning in which animals observe the behavior of other individuals and thereby learn to perform a new behavior. Compare social facilitation.

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social facilitation

answer

The situation that occurs when the performance of a behavior by older individuals increases the probability that younger individuals will acquire that behavior on their own. Social facilitation does not mean that young individuals copy the behavior of older individuals. For example, the feeding behavior of older individuals may bring younger individuals in contact with the foods that adults are eating and, therefore, increase the chance that they acquire a preference for those foods. Compare observational learning.

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discriminative parental solicitude

answer

A tendency among parents to adjust their investment in offspring according to cues that predict (or did predict in past environments) the likelihood that the offspring will survive and reproduce.

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