why is evolution considered a theory and not a hypothesis

Scientific Hypothesis, Theories and Laws
The principles and theories of science have been established through repeated experimentation and observation and have been refereed through peer review before general acceptance by the scientific community. Acceptance does not imply rigidity or constraint, or denote. Instead, as new data become available, previous scientific explanations are revised and improved, or rejected and replaced. Science is a way of making sense of the world, with internally-consistent methods and principles that are well described. There is a progression from a hypothesis to a theory using testable, scientific laws. Only a few scientific facts are natural laws and many hypotheses are tested to generate a theory. Find out how scientific hypotheses, theories and laws describe the natural world. What is a hypothesis? A hypothesis is an idea or proposition that can be tested by observations or experiments, about the natural world. In order to be considered scientific, hypotheses are subject to scientific evaluation and must be falsifiable, which means that they are worded in such a way that they can be proven to be incorrect. Example: When Gregor Mendel in 1865 studied the pattern of single trait inheritance of garden peas he formed a hypothesis on the manner of how these traits were inherited. The hypothesis he formed based on his observations included the following: In the organism there is a pair of factors that controls the appearance of a given characteristic. The organism inherits these factors from its parents, one from each. Each is transmitted from generation to generation as a discrete, unchanging unit. When the gametes are formed, the factors separate and are distributed as units to each gamete. (This statement is also known as Mendel's rule of segregation. ) If an organism has two unlike factors for a characteristic, one may be expressed to the total exclusion of the other.

What is a scientific theory? To scientists, a theory is a coherent explanation for a large number of facts and observations about the natural world. In popular use, a theory is often assumed to imply mere speculation, but in science, something is not called a theory until it has been confirmed over many independent experiments. Theories are more certain than hypotheses, but less certain than laws. The procedures and processes for testing a theory are well-defined within each scientific discipline. Example: Between 1856 and 1863 Mendel cultivated and tested some 28,000 pea plants which brought forth two theories of how character traits are inherited. Ironically, when Mendel's paper was published on 1866, it had little impact. It wasn't until the early 20th century that the enormity of his ideas was realized. What is a scientific law? A scientific law is a description of a natural phenomenon or principle that invariably holds true under specific conditions and will occur under certain circumstances. Example: In the early 20th century, after repeated tests and rejection of all competing theories Mendel's Laws of Heredity were accepted by the general scientific community. The law of segregation, which states that the alleles governing a trait are separated during the creation of gametes (meiosis). The law of independent assortment, which states that the genes controlling different traits are distributed separately from each other during meiosis. Lots of historical and scientific information about Gregor Mendel and his work can be found at the. Example 2: In the late 17th Century, Nicholas Steno established some natural laws relating to geology. The law of original horizontality - this states that when sediments are deposited in water, they will sink through it and deposit as horizontal layers as the result of gravity (unless acted on by other forces).

The law of stratigraphic succession - this states that in a given sequence of sediments, the oldest will be at the bottom and the youngest at the top. It does not provide ages for those sediments, simply the sequence of their deposition. For more on Nicholas Steno, try this book - a most interesting read: Cutler, Alan (2003). The seashell on the mountain top. Heinemann, London, p. 228 Is Evolution a Theory or a Fact? It is both. But that answer requires looking more deeply at the meanings of the words "theory" and "fact. " In everyday usage, "theory" often refers to a hunch or a speculation. When people say, "I have a theory about why that happened," they are often drawing a conclusion based on fragmentary or inconclusive evidence. The formal scientific definition of theory is quite different from the everyday meaning of the word. It refers to a comprehensive explanation of some aspect of nature that is supported by a vast body of evidence. Many scientific theories are so well-established that no new evidence is likely to alter them substantially. For example, no new evidence will demonstrate that the Earth does not orbit around the sun (heliocentric theory), or that living things are not made of cells (cell theory), that matter is not composed of atoms, or that the surface of the Earth is not divided into solid plates that have moved over geological timescales (the theory of plate tectonics). Like these other foundational scientific theories, the theory of evolution is supported by so many observations and confirming experiments that scientists are confident that the basic components of the theory will not be overturned by new evidence.

However, like all scientific theories, the theory of evolution is subject to continuing refinement as new areas of science emerge or as new technologies enable observations and experiments that were not possible previously. One of the most useful properties of scientific theories is that they can be used to make predictions about natural events or phenomena that have not yet been observed. For example, the theory of gravitation predicted the behavior of objects on the moon and other planets long before the activities of spacecraft and astronauts confirmed them. The evolutionary biologists who discovered Tiktaalik predicted that they would find fossils intermediate between fish and limbed terrestrial animals in sediments that were about 375 million years old. Their discovery confirmed the prediction made on the basis of evolutionary theory. In turn, confirmation of a prediction increases confidence in that theory. In science, a "fact" typically refers to an observation, measurement, or other form of evidence that can be expected to occur the same way under similar circumstances. However, scientists also use the term "fact" to refer to a scientific explanation that has been tested and confirmed so many times that there is no longer a compelling reason to keep testing it or looking for additional examples. In that respect, the past and continuing occurrence of evolution is a scientific fact. Because the evidence supporting it is so strong, scientists no longer question whether biological evolution has occurred and is continuing to occur. Instead, they investigate the mechanisms of evolution, how rapidly evolution can take place, and related questions. From Science, Evolution, and Creationism, National Academy of Sciences and Institute of Medicine. Y 2008 National Academy of Sciences

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