A population is a group of individuals of the same species living in a specific geographic area at a given time. A species is a group of organisms that share similar characteristics and genetic makeup and can interbreed to produce fertile offspring. While all members of a population belong to the same species, a species can have multiple populations inhabiting different locations.
If a population splits into parts which become isolated from each other, such as through habitat fragmentation, the cessation of gene flow between them will cause natural selection, genetic drift and mutation to act independently on each of them, causing them to diverge genetically, physically and behaviorally. A new species is formed when these differences become sufficiently large to prevent successful interbreeding (i.e., reproductive isolation).
A population can range from as small as a single individual to any larger size, with no upper limit. While a population with just one individual can exist temporarily, it is usually not sustainable because it cannot reproduce. The exception is organisms that can reproduce asexually (i.e., without the need for a mate), such as some plants (that can reproduce through fragmentation, budding or sending out shoots) and some single-celled organisms such as bacteria.
The minimum viable population is the smallest population size required for a species' long-term survival, preventing extinction due to inbreeding, genetic drift, demographic fluctuations and environmental changes. Studies suggest that a minimum of about 50 individuals is needed in the short term to prevent inbreeding depression, and at least 500 to several thousand may be necessary to maintain genetic diversity for long-term viability.
Possibly the largest temporary animal population ever recorded was a swarm of Rocky Mountain locusts, estimated to be about 3.5 trillion individuals, which occurred in the summer of 1875 and covered about 198,000 square miles (512,818 square kilometers) in the U.S. Midwest. This compares with a global total number of insects estimated at about 10 quintillion, although this consists of a vast number of separate populations.
The most abundant vertebrate may be the cyclothone fish, a genus of 14 deep-sea species, with total numbers estimated near a quadrillion. Other fish such as the Atlantic herring can form schools of several billion individuals.
Among birds, the passenger pigeon had flocks exceeding three billion individuals. Despite this massive number, it became extinct in less than 70 years due to large-scale hunting and habitat destruction.
Among mammals, bats are the most numerous, with estimates ranging from 10 to 100 billion individuals across more than 1,400 species. The most populous large mammal by far is humans, with a current population of approximately eight billion and continuing to grow rapidly.
Rats follow closely, with around seven billion worldwide, comprising numerous populations across multiple species that generally cannot interbreed. Other highly populous mammals include farm animals and pets, led by sheep at about 1.2 billion, followed by cattle, then goats, dogs, pigs, and cats. Among wild mammals, the white tailed deer number around 45 million, followed by wild boar and kangaroos.
The numbers of large, iconic mammals are much smaller. For example, the number of African elephants, the largest land animal and one of the most iconic, is estimated to have dropped to a maximum of 350,000. Giraffes, divided into several subspecies, are estimated at slightly more than 97,000. The blue whale, the largest animal that has ever lived, has recovered from near-extinction to a few tens of thousands, making it the most numerous among the largest marine animals.