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{{Short description|Group of individuals of a species, separated from other groups by in some manner}}
{{About|[[biology]]|[[human biology]]|Population (human biology)|other uses|Population (disambiguation)}}
{{pp-move}}
{{Use dmy dates|date=November 2024}}

In [[biology]], a '''population''' of [[Organism|organisms]] is a group of individuals of the same [[species]], defined by a discontinuity or [[wikt:disjunction|disjunction]] from other groups of individuals in certain characteristics, such as living area, genetic attributes, demographic structure.<ref name=":1">{{Cite journal |last1=Wells |first1=Jeffrey V. |last2=Richmond |first2=Milo E. |date=1995 |title=Populations, Metapopulations, and Species Populations: What Are They and Who Should Care? |url=https://www.researchgate.net/publication/281243816 |journal=Wildlife Society Bulletin |volume=23 |issue=3 |pages=458–462 |issn=0091-7648 |jstor=3782955}}</ref> Among biologists, the term definition varies, in some cases significantly, and sometimes those variations can be confusing.<ref name=":1" /> There are also plenty of other terms to describe groups of individuals if no clear [[wikt:disjunction|disjunction]] is present.<ref name=":1" /> Commonly, a population can be described by what individuals constitute the population, its size, a geographical area it occupies, and the time within which the population is examined.<ref>{{Cite book |last1=Rastogi |first1=V. B. |url=https://books.google.com/books?id=XnM4q8nBeWUC |title=A Complete Course in ISC Biology |last2=Kishore |first2=B. |date=1997 |publisher=Pitambar Publishing |isbn=978-81-209-0822-2 |pages=19–21 |language=en}}</ref> In [[wikt:qualitative|qualitative]] terms, it is usually defined like "a group of organisms of the same species occupying a particular space at a particular time".<ref>{{Cite book |last=Krebs |first=Charles J. |url=https://books.google.com/books?id=WjGupYHvthUC |title=Ecology: The Experimental Analysis of Distribution and Abundance |date=1972 |publisher=Harper & Row |isbn=978-0-06-043770-1 |language=en}}</ref><ref name=":3" />

The two main approaches to define a population are ecological and evolutionary. From the ecological perspective, individuals are considered interacting and competing in a certain geographic area. From the evolutionary (genetic) perspective, genes and reproduction are considered the driving forces of a population.<ref name=":3">{{Cite journal |last1=Waples |first1=Robin S. |last2=Gaggiotti |first2=Oscar |date=2006 |title=INVITED REVIEW: What is a population? An empirical evaluation of some genetic methods for identifying the number of gene pools and their degree of connectivity |url=https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1365-294X.2006.02890.x |journal=Molecular Ecology |language=en |volume=15 |issue=6 |pages=1419–1439 |doi=10.1111/j.1365-294X.2006.02890.x |pmid=16629801 |bibcode=2006MolEc..15.1419W |issn=1365-294X}}</ref> Since each population has its own gene pool that changes and adapts to the environment over time, the population is considered to be the main organizational unit in biology.<ref>{{Cite book |last=Young |first=Allen M. |url=https://books.google.com/books?id=yGH1BwAAQBAJ&dq=%22population+is%22+census+biology&pg=PA22 |title=Population Biology of Tropical Insects |date=2012-12-06 |publisher=Springer Science & Business Media |isbn=978-1-4684-1113-3 |pages=22 |language=en}}</ref>

== Etymology ==
The word ''population'' is derived from the [[Late Latin]] ''populatio'' (a people, a multitude), which itself is derived from the [[Latin]] word ''populus'' (a people).<ref>{{Cite web |title=population {{!}} Etymology, origin and meaning of population by etymonline |url=https://www.etymonline.com/word/population |access-date=2023-06-08 |website=etymonline.com |language=en}}</ref>

== Terminology ==
There is no standard terminology to define a population, but there are many definitions of this term.<ref name=":1" /> Aside from biology, a population can be commonly defined as a group of units.<ref name=":1" /> In biology, those units are individuals. And such a group is separated from other groups by some characteristics (i.e. disjunct from other groups).<ref name=":1" /> Populations can be demographically, spatially, or genetically disjunct from each other.<ref name=":1" /> Before the 1940s–1950s,<ref>{{Cite book |last1=Yablokov |first1=A. V. |url=https://books.google.com/books?id=8kovt-ZOhuMC&pg=PA356 |title=Population Principles in Research Into Natural Focality of Zoonoses |last2=Korenberg |first2=Ėduard Isaevich |date=1989 |publisher=CRC Press |isbn=978-3-7186-4978-5 |pages=255–256 |language=en}}</ref> the population term was used by biologists and ecologists to describe multiple species of a region, like a population of birds,<ref>{{Cite book |last=McIntosh |first=Robert P. |url=https://books.google.com/books?id=1bYSnG7RITAC&pg=PA150 |title=The Background of Ecology: Concept and Theory |date=1986-09-26 |publisher=Cambridge University Press |isbn=978-0-521-27087-8 |pages=150 |language=en}}</ref> but the term evolved and is not used this way anymore, describing only members of the same species.<ref name=":1" /> Although the term polyspecific population was proposed by some ecologists, but in ecology, the term [[Community (ecology)|community]] is generally used to describe multiple species of a region instead.<ref>{{Cite book |last=Manglik |first=Mr Rohit |url=https://books.google.com/books?id=_odREQAAQBAJ&pg=PT107 |title=Fundamentals of Environmental Sciences |date=2024-03-24 |publisher=EduGorilla Publication |isbn=978-93-7093-345-3 |language=en}}</ref><ref>{{Cite book |last1=Sharma |first1=P. D. |url=https://books.google.com/books?id=fjmhn4g5VHkC&pg=PA112 |title=Ecology And Environment |last2=P.D |first2=Sharma |date=2009 |publisher=Rastogi Publications |isbn=978-81-7133-905-1 |pages=112 |language=en}}</ref>

Also, there are at least 30 other terms to describe a group of individuals.<ref name=":1" /> Some of these terms were introduced because the definitions of the population term were not clear.<ref name=":1" /> If individuals of a group are semi-isolated from other groups, or they are more genetically similar, then the term [[Deme (biology)|deme]] can be used.<ref name=":1" /> Groups that are not disjunct from other individuals can simply be called groups, or they also can be called subpopulations or local populations.<ref name=":1" /> For individuals that are spatially closer to each other than to other individuals, the terms aggregation and cluster can be used.<ref name=":1" /> Spatially separated populations of the same species can be called a [[metapopulation]].<ref name=":1" /> And all the populations or metapopulations of a species can be called a species population.<ref name=":1" />

In biology (particularly, in [[evolutionary biology]]<ref>{{Cite journal |last1=Walton |first1=Andy |last2=Aylward |first2=Alex |last3=Thomas |first3=Mark G. |last4=Rutherford |first4=Adam |date=September 2025 |title=The History of the Panmictic Population Concept and Its Legacy in Contemporary Population Genetics |journal=Annals of Human Genetics |volume=89 |issue=5 |pages=274–284 |doi=10.1111/ahg.70015 |issn=1469-1809 |pmc=12336970 |pmid=40719227}}</ref>), a population is sometimes defined as a set of individuals that interbreed or that are able to interbreed.<ref name=":1" /> In this context, interbreeding individuals are those individuals that can produce an offspring with combined genetic material from both parents.<ref name=":2">{{Cite book |last1=Minkoff |first1=Eli |url=https://books.google.com/books?id=Ab69EAAAQBAJ&pg=PA238 |title=Biology Trending: A Contemporary Issues Approach |last2=Hood-DeGrenier |first2=Jennifer K. |date=2023-07-24 |publisher=CRC Press |isbn=978-1-000-87794-6 |language=en}}</ref> Different populations of the same species are also able to interbreed.<ref name=":2" /> But if there are some barriers that prevent interbreeding between different populations, such barriers are called [[reproductive isolation]].<ref name=":2" /> In context of these terms, a species can be defined as a group of one or more interbreeding populations when such a group is reproductively isolated.<ref name=":2" />

== Use of the term ==
=== Ecology ===
{{main|Population ecology}}
In [[ecology]], a population is a group of [[organism]]s of the same [[species]] which inhabit the same [[geographical area]] and are capable of [[Sexual reproduction|interbreeding]].<ref>{{cite web |title=Population |url=http://www.biology-online.org/dictionary/Population|publisher=Biology Online |access-date=5 December 2012}}</ref><ref>{{cite web |title=Definition of population (biology) |url=http://oxforddictionaries.com/definition/english/population?q=population |archive-url=https://web.archive.org/web/20130510042807/http://oxforddictionaries.com/definition/english/population?q=population |url-status=dead |archive-date=10 May 2013 |work=Oxford Dictionaries |publisher=Oxford University Press |access-date=5 December 2012 |quote=a community of animals, plants, or humans among whose members interbreeding occurs}}</ref> The area of a [[sex]]ual population is the area where [[interbreeding]] is possible between any opposite-sex pair within the area and more probable than [[Crossbreed|cross-breeding]] with individuals from other areas.<ref>{{cite book | last=Hartl | first=Daniel | title=Principles of Population Genetics | publisher=[[Sinauer Associates]] | year=2007 | isbn=978-0-87893-308-2 | page=45}}</ref> Ecology studies populations of different species rather than populations of a single species. Such populations as a whole constitute a [[Community (ecology)|community]]. Each community consists of populations of animals and plants that live together, interacting and being dependent on each other. So in ecology, a population of a single species is studied in the context of a community.<ref>{{Cite journal |last=Serrano Sanchez |first=C. |date=1996-04-01 |title=Introduction: The concept of population |url=https://www.researchgate.net/publication/226825761 |journal=International Journal of Anthropology |language=en |volume=11 |issue=2 |pages=15–18 |doi=10.1007/BF02441407 |issn=1824-3096|doi-access=free }}</ref>

In ecology, the population of a certain species in a certain area can be estimated using the [[Lincoln index]] to calculate the total population of an area based on the number of individuals observed.

Census, being defined as a complete count of individuals, is rarely possible to conduct in large populations, but it can be useful in small population that are easy to count, particularly, in [[applied ecology]].<ref>{{Cite book |last1=Conroy |first1=Michael J. |url=https://books.google.com/books?id=mcp2D4IH8vIC&pg=PA74 |title=Quantitative Conservation of Vertebrates |last2=Carroll |first2=John P. |date=2011-09-20 |publisher=John Wiley & Sons |isbn=978-1-4443-5649-6 |pages=74 |language=en}}</ref>

==== Dynamics ====
{{excerpt|Population dynamics}}

=== Genetics ===
{{main|Population genetics}}

In genetics, a ''population'' is often defined as a set of organisms in which any pair of members can [[Breeding in the wild|breed]] together. They can thus routinely exchange gametes in order to have usually fertile progeny, and such a [[Sexual reproduction|breeding]] group is also known therefore as a gamodeme. This also implies that all members belong to the same species.<ref>{{cite book | last=Hartl | first=Daniel | title=Principles of Population Genetics | publisher=[[Sinauer Associates]] | year=2007 | isbn=978-0-87893-308-2 | page=95}}</ref>
If the gamodeme is very large (theoretically, approaching infinity), and all gene alleles are uniformly distributed by the gametes within it, the gamodeme is said to be panmictic. Under this state, [[allele]] ([[gamete]]) frequencies can be converted to genotype ([[zygote]]) frequencies by expanding an appropriate [[quadratic equation]], as shown by Sir [[Ronald Fisher]] in his establishment of quantitative genetics.<ref>{{cite book | last=Fisher | first=R. A. | title=The Genetical Theory of Natural Selection | publisher=[[Oxford University Press]] (OUP) | year=1999 | isbn=978-0-19-850440-5}}</ref>

This seldom occurs in nature: localization of gamete exchange – through dispersal limitations, preferential mating, cataclysm, or other cause – may lead to small actual gamodemes which exchange gametes reasonably uniformly within themselves but are virtually separated from their neighboring gamodemes. However, there may be low frequencies of exchange with these neighbors. This may be viewed as the breaking up of a large sexual population (panmictic) into smaller overlapping sexual populations. This failure of [[panmixia]] leads to two important changes in overall population structure: (1) the component gamodemes vary (through gamete sampling) in their allele frequencies when compared with each other and with the theoretical panmictic original (this is known as dispersion, and its details can be estimated using expansion of an appropriate [[binomial equation]]); and (2) the level of homozygosity rises in the entire collection of gamodemes. The overall rise in homozygosity is quantified by the inbreeding coefficient (f or φ). All homozygotes are increased in frequency – both the deleterious and the desirable. The mean phenotype of the gamodemes collection is lower than that of the panmictic original – which is known as inbreeding depression. It is most important to note, however, that some dispersion lines will be superior to the panmictic original, while some will be about the same, and some will be inferior. The probabilities of each can be estimated from those binomial equations. In [[plant breeding|plant]] and [[animal breeding]], procedures have been developed which deliberately utilize the effects of dispersion (such as line breeding, pure-line breeding, backcrossing). Dispersion-assisted selection leads to the greatest genetic advance (ΔG=change in the phenotypic mean), and is much more powerful than selection acting without attendant dispersion. This is so for both allogamous (random fertilization)<ref>{{cite journal | last=Gordon  | first=Ian L. | title=Quantitative genetics of allogamous F2 : an origin of randomly fertilized populations | journal=[[Heredity (journal)|Heredity]]| year=2000  | volume=85  | issue=1 | pages=43–52  | doi=10.1046/j.1365-2540.2000.00716.x  | pmid=10971690| doi-access=free  | bibcode=2000Hered..85...43G }}</ref> and autogamous (self-fertilization) gamodemes.<ref>{{cite journal | last=Gordon  | first=Ian L. | title=Quantitative genetics of autogamous F2 | journal=[[Hereditas]] | year=2001 | volume=134 | pages=255–262 | doi=10.1111/j.1601-5223.2001.00255.x | pmid=11833289 | issue=3| doi-access=free }}</ref>

== World human population ==
{{Main|World population|Demographics of the world}}
{{Undue weight section|date=September 2025}}

According to the UN, the world's population surpassed 8 billion on 15 November 2022,<ref>{{Cite web |title=Population Clock |url=https://www.worldometers.info/world-population/ |access-date=15 November 2022 |website=www.worldometers.info |url-status=live |archive-url=https://archive.today/20221115125002/https://www.worldometers.info/world-population/ |archive-date=15 November 2022}}</ref> an increase of 1 billion since 12 March 2012. According to a separate estimate by the United Nations, Earth's population exceeded seven billion in October 2011. According to the [[United Nations Population Fund]] (UNFPA), growth to such an extent offers unprecedented challenges and opportunities to all of humanity.<ref>[http://www.unfpa.org/public/home/news/pid/8328Countdown to a World of Seven Billion People] ({{webarchive|url=https://web.archive.org/web/20120113192804/http://www.unfpa.org/public/home/news/pid/8328Countdown |date=13 January 2012 }}). [[UNFPA]], 12 September 2011.</ref>

According to papers published by the United States Census Bureau, the world population hit 6.5 billion on 24 February 2006. The United Nations Population Fund designated 12 October 1999 as the approximate day on which world population reached six billion. This was about 12 years after the world population reached five billion in 1987, and six years after the world population reached 5.5 billion in 1993. The population of countries such as [[Nigeria]] is not even known to the nearest million,<ref>{{cite web |url=http://www.mongabay.com/igapo/2005_world_city_populations/Nigeria.html | title=Cities in Nigeria: 2005 Population Estimates&nbsp;– MongaBay.com |access-date=1 July 2008}}</ref> so there is a considerable margin of error in such estimates.<ref>{{cite news |url=http://news.bbc.co.uk/2/hi/africa/country_profiles/1064557.stm |title=Country Profile: Nigeria |access-date=1 July 2008 |publisher=BBC News | date=24 December 2009}}</ref>

Researcher Carl Haub calculated that a total of over 100 billion people have probably been born in the last 2000 years.<ref>Haub, C. 1995/2004. "How Many People Have Ever Lived on Earth?" Population Today, {{cite web |url=http://www.prb.org/Articles/2002/HowManyPeopleHaveEverLivedonEarth.aspx |title=How Many People Have Ever Lived on Earth? - Population Reference Bureau |access-date=29 April 2013 |url-status=dead |archive-url=https://web.archive.org/web/20130424014209/http://www.prb.org/Articles/2002/HowManyPeopleHaveEverLivedonEarth.aspx |archive-date=24 April 2013  }}</ref>

=== Predicted growth and decline ===
{{Main|Population growth}}
[[File:World population growth - time between each billion-person growth.svg|thumb|upright=1.8|The years taken for every billion people to be added to the world's population, and the years that population was reached (with future estimates).]]

[[Population growth]] increased significantly as the [[Industrial Revolution]] gathered pace from 1700 onwards.<ref name="historic population graphs">As graphically illustrated by [[:File:Poulation-since-10000BC.jpg|population since 10,000BC]] and [[:File:Poulation-since-1000AD.jpg|population since 1000AD]]</ref> The last 50 years have seen a yet more rapid increase in the [[Population growth#Population growth rate|rate]] of population growth<ref name="historic population graphs"/> due to [[History of medicine#Contemporary medicine|medical advances]] and substantial increases in agricultural productivity, particularly beginning in the 1960s,<ref>{{Cite news |url=http://news.bbc.co.uk/1/hi/world/south_asia/4994590.stm |publisher=BBC News |title=The end of India's green revolution? |date=29 May 2006 |access-date=29 November 2009 }}</ref> made by the [[Green Revolution]].<ref>[http://www.foodfirst.org/media/opeds/2000/4-greenrev.html Food First/Institute for Food and Development Policy] ({{webarchive|url=https://web.archive.org/web/20090714215036/http://www.foodfirst.org/media/opeds/2000/4-greenrev.html |date=14 July 2009 }})</ref> In 2017 the [[United Nations Population Division]] projected that the world's population would reach about 11.6 billion in 2050 and 24.8 billion in 2100.<ref>{{Cite web |url=https://population.un.org/wpp/Publications/Files/WPP2017_KeyFindings.pdf |title=UN Population Prospects 2017}}</ref>

[[File:PRB 2017 Data Sheet Largest Populations.jpg|thumb|left|upright=0.9|PRB 2017 Data Sheet Largest Populations]]

In the future, the world's population is expected to peak at some point,<ref>[http://www.statista.com/statistics/1717/forecast-about-the-development-of-the-world-population/ World Population Development Statistics: Forecast], United Nations, 2011.</ref> after which it will decline due to economic reasons, health concerns, land exhaustion and environmental hazards. According to one report, it is very likely that the world's population will stop growing before the end of the 21st century. Further, there is some likelihood that population will actually decline before 2100.{{Fix|text=The date at which it stops growing is the exact same date when it starts to decline.}}<ref>{{cite journal |title=The End of World Population Growth |volume=412 |issue=6846 |doi=10.1038/35087589 |pmid=11484054 |journal=Nature |pages=543–545 |year=2001 |last1= Lutz |first1= Wolfgang |last2= Sanderson |first2= Warren |last3= Scherbov |first3= Sergei | bibcode=2001Natur.412..543L| s2cid=4425080| url=http://pure.iiasa.ac.at/id/eprint/6524/1/RR-01-12.pdf}}</ref><ref>{{cite journal | last1 = Ojovan | first1 = M.I. | last2 = Loshchinin | first2 = M.B. | year = 2015 | title = Heuristic Paradoxes of S.P. Kapitza Theoretical Demography | journal = European Researcher | volume = 92 | issue = 3| pages = 237–248 | doi=10.13187/er.2015.92.237| doi-access = free }}</ref> Population has already declined in the last decade or two in Eastern Europe, the Baltics and in the former Commonwealth of Independent States.<ref>{{Cite web|url=http://gsociology.icaap.org/report/demsum.html|title=world demographic trends|website=gsociology.icaap.org|access-date=18 July 2019}}</ref>

The population pattern of less-developed regions of the world in recent years has been marked by gradually declining birth rates. These followed an earlier sharp reduction in death rates.<ref name="users.rcn.com">{{cite web |url=http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/Populations.html |title=Human Population Growth |access-date=7 April 2009 |url-status=dead |archive-url=https://web.archive.org/web/20090330191122/http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/Populations.html |archive-date=30 March 2009  }}</ref> This transition from high birth and death rates to low birth and death rates is often referred to as the [[demographic transition]].<ref name="users.rcn.com" />

=== Population planning ===
{{Main|Human population planning}}

Human population planning is the practice of changing the rate of growth of a human population. Historically, human population control has been implemented with the goal of limiting the rate of population growth. In the time from the 1950s to the 1980s, concerns about global population growth and its effects on poverty, [[environmental degradation]], and political stability led to efforts to reduce population growth rates. While population control can involve measures that make people's lives better by giving them greater control of their reproduction, a few programs, most notably the Chinese government's one-child per family policy, have resorted to coercive measures.

In the 1970s, tension grew between population control advocates and women's health activists who advanced women's [[reproductive rights]] as part of a [[human rights]]-based approach.<ref name=":0">{{cite book |title=Reproductive Rights in a Global Context |last= Knudsen |first=Lara |year=2006 |publisher= Vanderbilt University Press |isbn=978-0-8265-1528-5 |pages=[https://archive.org/details/reproductiverigh0000knud/page/2 2] |url=https://archive.org/details/reproductiverigh0000knud |url-access=registration |quote=reproductive rights. }}</ref> Growing opposition to the narrow population control focus led to a significant change in population control policies in the early 1980s.<ref name=":0" />

== See also ==
* [[Community (ecology)]]
* [[Disjunctive population]]
* [[Human overpopulation]]
* [[List of countries by population]]
* [[Lists of organisms by population]]
* [[Population ethics]]
*[[Population geography]]

== References ==
{{Reflist|2}}

==Further reading==
* Hopfenberg, Russell. "[https://ia802907.us.archive.org/4/items/3anexpansionofthedemographictransitionmodel/3%20An%20expansion%20of%20the%20demographic%20transition%20model.pdf An expansion of the demographic transition model: the dynamic link between agricultural productivity and population]". ''Biodiversity'' 15.4 (2014): 246–254.

== External links ==
* [https://www.un.org/esa/population/unpop.htm United Nations Population Division]
* [http://www.cicred.org/Eng/Publications/IndexPublications.htm CICRED homepage] a platform for interaction between research centres and international organizations, such as the United Nations Population Division, [[UNFPA]], [[WHO]] and [[FAO]].
* [http://www.prb.org/ Population Reference Bureau] analyzes demographic data and research providing objective, accurate, and up-to-date population information in accessible formats.
* {{cite EB1911|wstitle = Population |volume=22 |first=Jervoise Athelstane |last= Baines | pages=92-100 |short=1 }}
* {{cite EB9 |wstitle = Population |volume= XIX  |last= Hooper |first= Wynnard  |author-link= | pages=513-517 |short=1 }}
* [https://web.archive.org/web/20080205020959/http://www.census.gov/ipc/www/popclockworld.html Current World Population]
* {{oweb|http://www.unece.org/pau}} of the United Nations Economic Commission for Europe
* [http://www.populationsdumonde.com/ Populations du monde]. {{In lang|fr}}
* [http://gsociology.icaap.org/  Understanding the World Today] Reports about world and regional population trends
* {{cite encyclopedia |last1=Lee |first1=Ronald Demos |editor=David R. Henderson |editor-link=David R. Henderson |encyclopedia=[[Concise Encyclopedia of Economics]] |title=Population |url=http://www.econlib.org/library/Enc/Population.html |year=2008 |edition= 2nd |publisher=[[Library of Economics and Liberty]] |location=Indianapolis |isbn=978-0-86597-665-8 |oclc=237794267}}

{{Population}}
{{Population country lists}}
{{biological organisation}}
{{Sustainability}}
{{Ethnicity}}
{{Evolution}}
{{Evolutionary psychology}}
{{Authority control}}

[[Category:Population| ]]
[[Category:Demography]]
[[Category:Population ecology]]
[[Category:Population models]]