evolution noun The gradual process by which species of organisms change over successive generations. Darwin's theory of evolution explains how species adapt to their environments over time. + natural selection noun The process by which organisms better suited to their environment tend to survive and pass on their traits. Darwin used natural selection to explain how species gradually change over generations. + adaptation noun The process of changing to become better suited to new conditions, or a modified version of an original work. The lizard's coloring is an adaptation that helps it hide from predators. + mutation noun A permanent change in an organism's genetic material that can be passed on to offspring. A random mutation gave the beetle population a slightly darker shell color. + gene noun A unit of hereditary material, made of DNA, that determines a particular trait in an organism. A single gene can determine whether a pea plant produces smooth or wrinkled seeds. + allele noun One of two or more alternative forms of a gene that occupy the same position on a chromosome. A child may inherit one allele for eye color from each parent. + phenotype noun The observable physical or biochemical characteristics of an organism, resulting from its genes and environment. Eye color is one visible phenotype determined by a combination of several genes. + genotype noun The specific genetic makeup of an individual organism, as distinct from its observable traits. Two plants can have the same genotype but look different because of their environment. + species noun A group of living organisms that share common characteristics and can interbreed to produce fertile offspring. Scientists discovered a new species of frog deep in the rainforest. + speciation noun The evolutionary process by which populations split and develop into distinct, separate species over time. Geographic isolation on separate islands can drive speciation among related animal populations. + fossil noun The preserved remains or traces of an ancient plant or animal, typically found in rock. Paleontologists discovered a dinosaur fossil embedded deep within the cliffside rock. + fitness noun The condition of being physically healthy and strong, especially through regular exercise. She joined a gym to improve her overall fitness before the marathon. + survival of the fittest phrase The principle that organisms best adapted to their environment are most likely to survive and reproduce. The professor explained survival of the fittest using examples of camouflage in insects. + variation noun A change or difference in amount, form, or condition from a standard, type, or previous state. There was noticeable variation in temperature between the coast and the mountains. + heredity noun The passing of physical or mental traits from parents to their offspring through genes. Heredity explains why the twins share the same curly hair and dimples. + genetic drift noun Random changes in the frequency of gene variants within a population over time. Genetic drift can cause certain traits to disappear from a small, isolated population by chance. + common ancestor noun An organism or lineage from which two or more different species or groups have both descended. Humans and chimpanzees share a common ancestor that lived millions of years ago. + extinction noun The complete disappearance of a species or type of thing from existence. Habitat loss is pushing many rainforest species toward extinction. + adaptive radiation noun The rapid evolution of many new species from a single ancestor, each adapted to a different environment. Darwin's finches are a classic example of adaptive radiation on the Galapagos Islands. + homologous adjective Having a similar structure and origin, though not necessarily the same function, especially in biology. A bat's wing and a human arm are homologous structures with the same basic bone layout. + vestigial adjective Remaining as a small, underdeveloped trace of something that had a purpose in an earlier form. The appendix is often described as a vestigial organ with little clear function today. + camouflage noun Coloring or patterns that help a person, animal, or object blend into its surroundings. The soldier's camouflage made him nearly invisible against the dense brush. + mimicry noun The act of closely imitating another's behavior, speech, or, in animals, appearance for advantage or protection. The stick insect's mimicry of a twig protects it from hungry birds. + population noun The total number of people, or of a particular species, living in a given area. The population of the small town has grown by nearly a third in ten years. + gene pool noun The complete set of genes and genetic variations present in a population of organisms. A larger gene pool generally helps a species adapt to changing environments. + dominant adjective Having the most power, control, or influence over others in a group, situation, or system. The dominant wolf in the pack ate first and decided where the group traveled. + recessive adjective In genetics, describing a gene or trait that is expressed only when no dominant version of the gene is present. Blue eyes are a recessive trait, so both parents must carry the gene for a child to have them. + descent noun The act of moving downward, or a person's family origin and lineage. The plane's descent was smooth despite the strong winds over the mountains. + trait noun A distinguishing quality or characteristic belonging to a person, animal, or thing. Patience is a trait that makes her an excellent teacher. + offspring noun The young or children produced by a person, animal, or plant. The lioness fiercely protected her offspring from the approaching hyenas. +