Evolution is both a fact and a theory, in that the change in a population between generations of species has been observed, but the explanation for how this process happens is still evolving. Darwin provided the first unified explanation for the origin of species and descent with modification of organisms, although his hypothesis has changed considerably with more fossil discoveries during the past 150 years as well as the addition of Mendelian genetics, which was further elaborated through the study of population genetics. Nevertheless, the basic tenets of Darwinian theory (natural selection through inheritable variations, overpopulation, struggle for existence, and survival of the better-adapted) are still applicable to understanding how environmental and intraspecific factors change genotype frequencies and phenotypes, causing speciation. Speciation that happened over longer periods of time and caused considerable changes within lineages (macroevolution) is attributed to either phyletic gradualism or punctuated equilibrium, depending on the timing of the changes. Evidence supporting both of these hypotheses comes from the fossil record, with possible mechanisms of allopatric and sympatric speciation, adaptive radiation, and vicariance biogeography, among others. Molecular phylogeny is an independent method used more in recent years to test phylogenetic relationships established through fossil lineages, although cladistic analyses based on characteristic traits still determine hypotheses for how dinosaurs evolved.
Dinosaur evolution can be evaluated by examining the fossil record for amniotes as a whole and archosaurs in particular. The development of a cleidoic egg from amphibian ancestors probably happened during the Carboniferous Period as a result of both genetic and environmental factors that favored this mode of reproduction. Subsequent diversification
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