Race, as a social concept, is a group of people who share similar and distinct physical characteristics. First used to refer to speakers of a common language and then to denote national affiliations, by the 17th century race began to refer to physical (i.e. phenotypical) traits. Starting from the 19th century, the term was often used, in a taxonomic sense, to denote genetically differentiated human populations defined by phenotype.Social conceptions and groupings of races vary over time, involving folk taxonomies that define essential types of individuals based on perceived traits. Scientists consider biological essentialism obsolete, and generally discourage racial explanations for collective differentiation in both physical and behavioral traits.Even though there is a broad scientific agreement that essentialist and typological conceptualizations of race are untenable, scientists around the world continue to conceptualize race in widely differing ways, some of which have essentialist implications. While some researchers sometimes use the concept of race to make distinctions among fuzzy sets of traits, others in the scientific community suggest that the idea of race often is used in a naive or simplistic way, and argue that, among humans, race has no taxonomic significance by pointing out that all living humans belong to the same species, Homo sapiens, and subspecies, Homo sapiens sapiens.Since the second half of the 20th century, the associations of race with the ideologies and theories that grew out of the work of 19th-century anthropologists and physiologists has led to the use of the word race itself becoming problematic. Although still used in general contexts, race has often been replaced by other words which are less ambiguous and emotionally charged, such as populations, people(s), ethnic groups, or communities, depending on context.