* species-A species is a group of similar organisms that can mate with each other and produce fertile offspring. * fossil-A fossil is the preserved remains or traces of an organism that lived in the past. * adaptation –A trait that helps an organism survive and reproduce. * evolution-The gradual change in a species over time is called evolution. * scientific theory -A scientific theory is a well-tested concept that explains a wide range of observations. * natural selection-The process by which individuals that are better adapted to their environment are more likely to survive and reproduce than other members of the same species. * variation-Any difference between individuals of the same species is called a variation. * homologous structures-Similar structures that related species have inherited from a common ancestor are known as homologous structures * branching tree-A branching tree is a diagram that shows how scientists think different groups of organisms are related. * petrified fossil-remains that become buried in sediments and are actually changed to rock. * mold-A hollow space in sediment in the shape of an organism or part of an organism is called a mold. * cast-A cast is a copy of the shape of the organism that made the mold. * relative dating-Scientists use relative dating to determine which of two fossils is older. * radioactive dating-A technique called radioactive dating allows scientists to determine the actual age of fossils. * radioactive element-The rocks that fossils are found near contain radioactive elements, which are unstable elements that decay, or break down, into different elements. * half-life- The half-life of a radioactive element is the time it takes for half of the atoms in a sample to decay. * fossil record-The millions of fossils that scientists have collected are called the fossil record. * extinct-A species is extinct if no members of that species are still alive....
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