A characteristic of an organism that makes it fit for its environment or for its particular way of life. Adaptation is a key biological process in organisms and can take many forms. Typically, adaptation refers to two mechanisms: (1) an adjustment to new or altered environmental conditions by changes in genotype (natural selection) or phenotype; and (2) the occurrence of physiological changes in an individual exposed to changed conditions. For example, the Arctic fox (Vulpes lagopus; see illustration) is well adapted for living in a very cold climate. Appropriately, it has much thicker fur than similar-sized mammals from warmer places; measurement of heat flow through fur samples demonstrates that the Arctic fox and other Arctic mammals have much better heat insulation than tropical species. Consequently, Arctic foxes do not have to raise their metabolic rates as much as tropical mammals do at low temperatures. This is demonstrated by the coati (Nasua narica), which lives in Panama and has a body mass similar to Arctic foxes and about the same metabolic rate at comfortable temperatures. When both animals are cooled, however, the coati's metabolic rate starts to rise steeply as soon as the temperature falls below 20°C (68°F), whereas that of the Arctic fox begins to rise only below −30°C (−22°F). The insulation of Arctic foxes is so effective that they can maintain their normal deep-body temperatures of 38°C (100°F) even when the temperature of the environment falls to −80°C (−112°F). Thus, thick fur is obviously an adaptation to life in a cold environment. See also: Adaptive responses in animals to climate change; Environment; Physiological ecology (animal); Physiological ecology (plant); Temperature adaptation in animals; Thermoregulation

The processes of biological and organic change in animals by which descendants come to differ from their ancestors. Animal evolution encompasses the theory that modern animals are the modified descendants of animals that formerly existed and that these earlier forms descended from still earlier and different organisms. The concepts of evolutionary biology and their relationship to phylogeny (the genealogical history of organisms, both living and extinct), adaptation, and speciation were formulated by Charles Darwin. Much of Darwin's conclusions emanated from his investigative study of animals. Animals are multicellular organisms that feed by ingestion of other organisms or their products, being unable to derive energy through photosynthesis or chemosynthesis. Animals are currently classed into approximately 36 phyla, and each of these phyla has evolved a distinctive body plan or architecture. Representatives of the major animal groups are depicted in Fig. 1. See also: Adaptation (biology); Animal; Animal kingdom; Charles Darwin and his theory of evolution; Evolution; Macroevolution; Phylogeny; Speciation; Species concept

An ongoing current event in which a large number of living species are threatened with extinction or are going extinct because of environmentally destructive human activities. The Earth is presently in the midst of a mass extinction event. In the past, there have been five other similar large extinctions. Because of this, the current Anthropocene extinction is often referred to as the sixth extinction or the sixth great mass extinction; another term is the Holocene extinction because this event is taking place in the current Holocene Epoch, which covers the last 10,000 years of the Earth's history. However, as the present-day extinction event is anthropogenic (that is, related to or resulting from the influence of humans on nature), the favored term is Anthropocene extinction. This article presents an overview of the five previous large extinctions, the major human activities causing the Anthropocene extinction, and the trends in species extinction related to this present-day extinction event (see illustration). See also: Biodiversity; Endangered species; Extinction; Holocene; Mass extinctions; Paleobiodiversity; Paleoecology; Species concept

The observation, measurement, and explanation of human variability in time and space. Anthropology is a science concerned with the interrelations of the biological, cultural, geographical, and historical aspects of humankind, including modern human beings and their ancestors (Fig. 1). It also includes the study of biological variability and the study of cultural, or learned, behavior among contemporary human societies. In general, anthropological studies are closely allied with the fields of archeology and linguistics. Studies range from rigorously scientific approaches, including investigations into the physiology, demography, and ecology of hunter-gatherers, to more humanistic research on symbolism and ritual behavior. See also: Archeology; Bioarcheology; Dental anthropology; Forensic anthropology; Human biological variation; Human ecology; Molecular anthropology; Origins of symbolic behavior; Physical anthropology; Sociobiology

The ongoing lower level of species extinction intensity occurring between episodes of mass extinctions. Extinction is a normal part of the history of life. Out of every 100 species that have ever lived on Earth, 99 are no longer in existence. However, occurrences of species extinction can be split into two categories: mass extinctions, in which substantial percentages of the global diversity of life disappear within a geologically brief interval; and background extinctions, which represent a much lower rate of species loss that occurs routinely over geologically long intervals between mass extinctions (see illustration). Background extinctions are ongoing consequences of normal environmental changes, local catastrophes, or interspecies competition. On the basis of the fossil record, evolutionary paleontologists have generally estimated that the baseline level of background extinction is approximately 1 extinction per million species per year (however, other analyses indicate that the rate might be as low as a tenth of that figure, so further investigations are necessary). Background extinction is mostly a local phenomenon. It befalls only one or a few species at any time, usually within a particular area. In contrast, hundreds or thousands of species can be affected worldwide during mass extinctions. Five mass extinctions have occurred during the past 550 million years on Earth, and investigators believe that the Earth is currently amid a sixth mass extinction, termed the Anthropocene extinction, caused predominantly by environmentally destructive human activities. See also: Anthropocene extinction; Biodiversity; Extinction; Extinction and the fossil record; Fossil; Geologic time scale; Macroevolution; Mass extinctions; Speciation; Species concept

The collective measures and actions concerned with the preservation and restoration of animal and plant species and their habitats. To conserve an endangered species, an adequate number of populations must be maintained within a sufficient amount of protected habitat, with the goal of minimizing the likelihood of future extinction. In fact, of the thousands of animal and plant species listed as threatened or endangered, the majority are declining as a result of the loss of habitat (Fig. 1). The most direct approach for ensuring that a species will continue to exist is to conserve its environment. The primary elements that determine the overall effectiveness of a habitat conservation measure are (1) the number and type of factors causing species decline; (2) the condition of the habitat and the surrounding land matrix; and (3) the specific regulatory and management approaches used on the land, which may not always directly relate to species conservation. Although a variety of land protection strategies are available for conserving endangered species, the most common is federal land management. Government agencies often have direct responsibility for ownership and management of lands containing endangered species, but there are a growing number of private initiatives that can also be successful in habitat protection. See also: Biodiversity; Conservation of resources; Endangered species; Environment; Environmental management; Extinction; Forestry; Land-use planning; Population ecology; Speciation; Species concept

The process by which unrelated or distantly related organisms independently evolve similar traits and features as they adapt to comparable environments. Convergent evolution, or evolutionary convergence, is a term used to describe instances in which species from different taxonomic groups have evolved a similar form or body plan (see illustration). In these cases, separate and unrelated organisms have independently acquired almost identical adaptations and characteristics under similar environmental conditions or selection pressures. Convergent evolution thus stands in contrast to resemblances among related species, which may share similar adapted features because of their shared origins. See also: Adaptation (biology); Evolution; Macroevolution; Species concept; Taxonomic categories

The science of restoring an extinct animal or plant using advanced genetic and reproductive technologies. The concept of de-extinction was science fiction until the recent advent of advanced genome sequencing platforms. The technological breakthroughs provided by these platforms opened the door for the sequencing of even highly damaged deoxyribonucleic acid (DNA) fragments from very old specimens and enabled the sequencing of the Neandertal and mammoth genomes. Both sequencing projects provided enormous insights into the biology of these species and sparked numerous ancient-DNA explorations in samples of other extinct species. Some of the most highly discussed candidates for de-extinction include the mammoth (Fig. 1), Tasmanian tiger (thylacine; Fig. 2), bucardo (Pyrenean ibex), passenger pigeon, dusky seaside sparrow (Fig. 3), and Australian gastric-brooding frog. Although sequencing the genomes of each of these species has been done or is an achievable goal, there is a huge gap—and one that science has yet to bridge—between defining the genome of an extinct animal or plant and restoring that species to the realm of living organisms. See also: Ancient DNA; Biodiversity; Biotechnology; Deoxyribonucleic acid (DNA); DNA microarray; DNA sequencing; Extinction; Extinction (paleontology); Genetic mapping; Genomics; Mammoth cloning and de-extinction; Neandertal genome; Neandertals; Speciation; Species concept; Thylacine genetics; Zooarcheology

The modification of living organisms during their descent, generation by generation, from common ancestors. Evolution, also known as biological or organic evolution, encompasses the processes of change in organisms by which descendants come to differ from their ancestors, and it includes the history of the sequence of such changes (Fig. 1). Evolution includes two major processes: anagenesis, the alteration of the genetic properties of a single lineage over time; and cladogenesis, or branching, whereby a single lineage splits into two or more distinct lineages that continue to change anagenetically. Most importantly, there is no controversy about the reality of evolution as a historical event. That organisms have descended from common ancestors is accepted by knowledgeable biologists as fact. Molecular and other similarities point to the fact that all living things are related to each other by common ancestry. See also: Animal evolution; Macroevolution; Plant evolution

An interdisciplinary science area that is concerned with modeling aspects of natural evolution in order to solve real-world problems of optimization. Living organisms, as well as those that are long extinct, demonstrate optimized complex behavior at all levels: cells, organs, individuals, and populations. The English naturalist Charles Darwin wrote of “organs of extreme perfection” when describing the ability of evolution to craft ingenious solutions to complex problems such as vision. Evolution is the great unifying principle of biology, but it extends beyond biology and can be used as an engineering principle where individuals in a population of candidate solutions to some particular problem undergo random variation (mutation and recombination) and face competition and selection based on their appropriateness for the task at hand. See also: Biology; Evolution; Mutation; Recombination (genetics)