this definition, unlike simple "survivability", avoids being trivially true. Similarly, it is argued that evolutionary theory is circular reasoning, in that evidence is interpreted as supporting evolution, but evolution is required to interpret the evidence. An example of this is the claim that geological strata are dated through the fossils they hold, but that fossils are in turn dated by the strata they are in. However, in most cases strata are not dated by their fossils, but by their position relative to other strata and by radiometric dating, and most strata were dated before the theory of evolution was formulated. Evidence Objections to the fact that evolution occurs tend to focus on specific interpretations about the evidence. Lack of observation A common claim of creationists is that evolution has never been observed. Challenges to such objections often come down to debates over how evolution is defined (see the Defining evolution section above). Under the conventional biological definition of evolution, it is a simple matter to observe evolution occurring. Evolutionary processes, in the form of populations changing their genetic composition from generation to generation, have been observed in different scientific contexts, including the evolution of fruit flies, mice, and bacteria in the laboratory, and of tilapia in the field. Such studies on experimental evolution, particularly those using microorganisms, are now providing important insights into how evolution occurs, especially in the case of antibiotic resistance. In response to such examples, creationists say there are two major subdivisions of evolution to be considered, microevolution and macroevolution, and it is questionable if macro-evolution has been physically observed to occur. Most creationist organizations do not dispute the occurrence of short-term, relatively minor evolutionary changes, such as that observed even in dog breeding. Rather, they dispute the occurrence of major evolutionary changes over long periods of time, which by definition cannot be directly observed, only inferred from microevolutionary processes and the traces of macroevolutionary ones. As biologists define macroevolution, both microevolution and macroevolution have been observed. Speciations, for example, have been directly observed many times. Additionally, the modern evolutionary synthesis draws no distinction in the processes described by the theory of evolution when considering macroevolution and microevolution as the former is simply at the species level or above and the latter is below the species level. An example of this is ring species. Additionally, past macroevolution can be inferred from historical traces. Transitional fossils, for example,