to frequent misunderstandings. For example, the idea of devolution ("backwards" evolution) is a result of erroneously assuming that evolution is directional or has a specific goal in mind (cf. orthogenesis). In reality, the evolution of a biological organism has no "objective" and is only showing increasing ability of successive generations to survive and reproduce in their environment; and increased suitability is only defined in relation to this environment. Biologists do not regard any one species (such as humans) as more highly evolved or advanced than another. Certain sources have been criticized for indicating otherwise due to a tendency to evaluate nonhuman organisms according to anthropocentric standards rather than according to more objective ones. Evolution also does not require that organisms become more complex. Although the biological development of different forms of life shows an apparent trend towards the evolution of biological complexity, there is a question as to whether this appearance of increased complexity is real, or whether it comes from neglecting the fact that the majority of life on Earth has always consisted of prokaryotes. In this view, complexity is not a necessary consequence of evolution, but specific circumstances of evolution on Earth frequently made greater complexity advantageous and thus naturally selected for. Depending on the situation, organisms' complexity can either increase, decrease, or stay the same, and all three of these trends have been observed in studies of evolution. Creationist sources frequently define evolution according to a colloquial, rather than a scientific, meaning. As a result, many attempts to rebut evolution do not address the findings of evolutionary biology (see straw-man argument). This also means that advocates of creationism and evolutionary biologists often simply speak past each other. Scientific acceptance Status as a theory Critics of evolution assert that evolution is "just a theory", which emphasizes that scientific theories are never absolute, or misleadingly presents it as a matter of opinion rather than of fact or evidence. This reflects a difference of the meaning of theory in a scientific context: whereas in colloquial speech a theory is a conjecture or guess, in science, a theory is an explanation whose predictions have been verified by experiments or other evidence. Evolutionary theory refers to an explanation for the diversity of species and their ancestry which has met extremely high standards of scientific evidence. An example of evolution as theory is the modern synthesis of Darwinian natural selection and Mendelian inheritance.