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Hardy-Weinberg principle
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the principle that frequencies of alleles and genotypes in a population remain constant from generation to generation, provided that only Mendelian segregation and recombination of alleles are at work
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radiometric dating
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a method for determining the absolute ages of rocks and fossils, based on the half-life of radioactive isotopes
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sexual selection
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a form of natural selection in which individuals with certain inherited characteristics are more likely than other individuals to obtain mates
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average heterozygosity
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the percent, on average, of a population's loci that are heterozygous in members of the population
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homologous structures
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structures in different species that are similar because of common ancestry
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half-life
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the amount of time it takes for 50% of a sample of a radioactive isotope to decay
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vestigial structure
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a structure of marginal, if any, importance to an organism. _____ _____ are historical remnants of structures that had important functions in ancestors
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mutation
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a change in the nucleotide sequence of an organism's DNA, ultimately creating genetic diversity. ______s also can occur in the DNA or RNA of a virus
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duplication
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an aberration in chromosome structure due to fusion with a fragment from a homologous chromosome, such that a portion of a chromosome is duplicated
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cline
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a graded change in a character along a geographic axis
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neutral variation
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genetic variation that does not appear to provide a selective advantage or disadvantage
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allopatric speciation
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the formation of new species in populations that are geographically isolated from one another
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adaptive radiation
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period of evolutionary change in which groups of organisms form many new species whose adaptations allow them to fill vacant ecological roles in their communities
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polyploidy
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a chromosomal alteration in which the organism possesses more than two complete chromosome; the result of an accident during cell division
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directional selection
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natural selection in which individuals at one end of the phenotypic range survive or reproduce more successfully than do other individuals
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heterozygote advantage
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greater reproductive success of heterozygous individuals compared with homozygotes; tends to preserve variation in a gene pool
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evolution
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descent with modification; the idea that living species are descendants of ancestral species that were different from the present-day ones; also defined more narrowly as the change in the genetic composition of a population from generation to generation
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natural selection
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a process in which organisms with certain inherited characteristics are more likely to survive and reproduce than are organisms with other characteristics
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Pangaea
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supercontinent made of all the landmasses of Earth
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sympatric speciation
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the formation of new species in populations that live in the same geographic area
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disruptive selection
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natural selection in which individuals on both extremes of a phenotypic range survive or reproduce more successfully than do individuals with intermediate phenotypes
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bottleneck effect
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genetic drift that occurs when the size of a population is reduced, as by a natural disaster or human actions. Typically, the surviving population is not longer genetically representative of the original population (no longer diverse)
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endemic
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referring to a species that is confined to a specific, relatively small geographic area
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homeotic gene
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any of the master regulatory genes that control placement and spatial organization of the body parts in animals, plants, and fungi by controlling the developmental fate of groups of cells
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microevolution
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evolutionary change below the species level; change in the allele frequencies in a population over generations
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evolutionary tree
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a branching diagram that reflects a hypothesis about evolutionary relationships among groups of organisms
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phylogenetic species concept
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a definition of species as the smallest group of individuals that share a common ancestor, forming one branch on the tree of life
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hybrid
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offspring that results from the mating of individuals from two different species or two true-breeding varieties of the same species
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convergent evolution
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the evolution of similar features in independent evolutionary lineages
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continental drift
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the slow movement of the continental plates across Earth's surface
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sexual dimorphism
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marked differences between the secondary sex characteristics of males and fermales
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speciation
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an evolutionary process in which one species splits into two or more species
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species
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a population or group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring, but do not produce viable, fertile offspring with members of other such groups
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heterochrony
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evolutionary change in the timing or rate of an organism's development
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prezygotic barrier
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a reproductive barrier that impedes mating between species of hinder fertilization if interspecific mating is attempted
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homology
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similarity in characteristics resulting from a shared ancestry
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Uniformitarianism
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the principle stating that mechanisms of change are constant over time
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gene flow
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the transfer of alleles from one population to another, resulting from the movement of fertile individuals or their gametes
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adaptation
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inherited characteristic of an organism that enhances its survival and reproduction in specific environments
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balancing selection
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natural selection that maintains two or more phenotypic forms in a population
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catastrophism
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the principle that events in the past occurred suddenly and were caused by different mechanisms than those operating today
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postzygotic barrier
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a reproductive barrier that prevent hybrid zygotes produced by two different species from developing into viable, fertile adults
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geographic variation
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differences between the gene pools of geographically separate populations or population subgroups
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Hardy-Weinberg equilibrium
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the condition describing a nonevolving population (one that is in genetic equilibrium)
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Cambrian explosion
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a relatively brief time in geologic history when large, hard-bodied forms of animals with most of the major body plans known today appeared in the fossil record. This burst of evolutionary change occurred about 535-525 million years ago
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intersexual selection
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selection whereby individuals of one sex (usually females) are choosy in selecting their mates from individuals of the other sex; also called mate choice
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autopolyploid
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an individual that has more than two chromosome sets that are all derived from a single species
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frequency-dependent selection
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a decline in the reproductive success of individuals that have a phenotype that has become too common in a population
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macroevolution
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evolutionary change above the species level, including the origin of a new group of organism or a shift in the broad patter of evolutionary change over a long period of time. Examples of ___________-ary change include the appearance of major new features of organisms and the impact of mass extinctions on the diversity of life and its subsequent recovery
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biogeography
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the study of past and present distribution of species
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reproductive isolation
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the existence of biological factors (barriers) that impede members of two species from producing viable, fertile offspring
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paedomorphosis
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the retention in an adult organism of the juvenile features of its evolutionary ancestors
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relative fitness
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the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals in the population
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hybrid zone
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a geographic region in which members of different species meet and mate, producing at least some offspring of mixed ancestry
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artificial selection
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the selective breeding of domesticated plants and animals to encourage the occurrence of desirable traits
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analogous
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having characteristics that are similar because of convergent evolution, not homology
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intrasexual selection
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a direct competition among individuals of one sex (usually the males in vertebrates) for mates of the opposite sex
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reinforcement
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a process in which natural selection strengthens prezygotic barriers to reproduction, thus reducing the chances of hybrid formation; such a process is likely to occur only if hybrid offspring are less fit than members of the parent species
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mass extinction
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period of time when global environmental changes lead to the elimination of a large number of species throughout Earth
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stabilizing selection
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natural selection in which intermediate phenotypes survive or reproduce more successfully than do extreme phenotypes
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genetic drift
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a process in which chance events cause unpredictable fluctuations in allele frequencies from one generation to the next. Effects of genetic drift are most pronounced in small populations
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founder effect
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genetic drift that occurs when a few individuals become isolated from a larger population and form a new population whose gene pool composition is not reflective of that of the original population
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allopolyploid
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a fertile individual that has more than two chromosome sets as a result of two different species interbreeding and combining their chromosomes
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