Categories: Evolution stubs | Population genetics In biology, a species is one of the basic units of biodiversity. ... A chronospecies is a species which which changes physically, morphologically, genetically, and/or behaviorally over time on an evolutionary scale such that the originating species and the species it becomes could not be classified as the same species had they existed at the same point in time. ... Charles Darwins first sketch of an evolutionary tree from his First Notebook on Transmutation of Species (1837) Speciation is the theory of the evolutionary process by which new biological species are believed by some to arise. ... In population genetics, a cline is a gradual change of a character or feature (phenotype) in a species over a geographical area, often as a result of environmental heterogeneity. ... Allopatric speciation, also known as geographic speciation, occurs when populations physically isolated by an extrinsic barrier evolve intrinsic (genetic) reproductive isolation such that if the barrier between the populations breaks down, individuals of the two populations can no longer interbreed. ... Peripatric speciation (also known as Parapatry) is a type of speciation in the theory of natural selection. ... Parapatric speciation is a form of speciation in which the evolution of reproductive isolating mechanisms occurs when a population enters a new niche or habitat within the range of the parent species. ... Sympatry is one of three theoretical models for the phenomenon of speciation. ... Polyploidy refers to cells or organisms that contain more than two copies of each of their chromosomes. ... Illustration from The Descent of Man and Selection in Relation to Sex by Charles Darwin showing the Tufted Coquette Lophornis ornatus, female on left, ornamented male on right. ... Assortative mating (also called Assortative pairing) takes place when sexually reproducing organisms tend to mate with individuals that are like themselves in some respect (positive assortative mating) or dissimilar (negative assortative mating). ... Punctuated equilibrium (or punctuated equilibria) is a theory in evolutionary biology which states that most sexually reproducing species will show little to no evolutionary change throughout their history. ... // In biology, hybrid has two meanings. ... Haldanes rule relating to hybrids of species and to extended to speciation in evolutionary theory is easily stated: It was originally formulated in 1922 by the British evolutionary biologist J. B. S. Haldane; since it appears to be usually but not universally true, it is described as a rule... In this diagram, interbreeding populations are represented by coloured blocks. ... A phylogenetic tree of all extant organisms, based on 16S rRNA gene sequence data, showing the evolutionary history of the three domains of life, bacteria, archaea and eukaryotes. ...