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Topic: Small population size

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Genetic drift

Genetic variation

Population genetics

Population bottleneck

Heterozygote advantage

Frequency dependent selection

Inbreeding

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	No Need to Isolate Genetics
		Loss of genetic variation in small isolated populations is inevitable, as is an increase in the inbreeding coefficient [HN20] of surviving individuals.
		For instance, such a small population is likely to be in dire straits already because of exposure to chance environmental events (droughts, storms, disease), or simply because of demographic accidents, including those that might skew the sex ratio.
		First, the decline in demographic rates and population size occurred despite aggressive efforts (and some success) in the 1960s and 1970s to control predators and increase the quality and quantity of habitat; this suggests that the extinction vortex was set in motion by the isolation of this population during the 1970s.
	cas.bellarmine.edu /tietjen/images/no_need_to_isolate_genetics.htm (1303 words)

	Small population size - Wikipedia, the free encyclopedia
		Species with a small population size are subject to a higher chance of extinction because they are more vulnerable to genetic drift, resulting in stochastic variation in their gene pool, their demography and their environment.
		In small populations the relative importance of genetic drift (chance) is higher; deleterious alleles can become more frequent and 'fixed' in a population due to chance.
		This results in a reduction in the number of forms of alleles in a small population and in extreme cases to monomorphism where there is only one form of the allele.
	en.wikipedia.org /wiki/Small_population_size (681 words)

	Population genetics - Encyclopedia.WorldSearch (Site not responding. Last check: 2007-09-18)
		Population genetics is the study of the distribution of and change in allele frequencies under the influence of the four evolutionary forces: natural selection, genetic drift, mutation and migration.
		Population genetics was a vital ingredient in the modern evolutionary synthesis, its primary founders were Sewall Wright, J.
		In practice, there are two bodies of evolutionary theory that exist in parallel, traditional population genetics operating in the genotype space and the biometric theory used in plant and animal breeding, operating in phenotype space.
	encyclopedia.worldsearch.com /population_genetics.htm (741 words)

	Criteria for Inclusion on the ABC Green List
		One of these scores reflects vulnerability based on the size of the bird’s breeding range, and the other is for the smallest area the species occupies during its non-breeding season.
		Population trend is used because a bird whose population has been declining in past years is often more of a conservation priority than one that has been increasing.
		This threshold is a summed score of 14 or greater, or a sum of 13 with a population trend score of 5.
	www.abcbirds.org /greenlist_criteria.htm (1410 words)

	Population bottleneck Summary
		A population bottleneck is a significant reduction in the size of a population that causes the extinction of many genetic lineages within that population, thus decreasing genetic diversity.
		A population bottleneck (or genetic bottleneck) is an evolutionary event in which a significant percentage of a population or species is killed or otherwise prevented from reproducing, and the population is reduced by 50% or more, often by several orders of magnitude.
		Population bottlenecks increase genetic drift, as the rate of drift is inversely proportional to the population size.
	www.bookrags.com /Population_bottleneck (2151 words)

	Founder effect - Wikipedia, the free encyclopedia
		The result of the small number of founders is that there is a sharp loss of genetic variation compared with the parent population.
		As a result, the new population may be distinctively different, genetically and phenotypically, from the parent population it derived from.
		In addition, there is a raised probability of inbreeding, resulting in an unusual number of defects due to recessive genes.
	en.wikipedia.org /wiki/Founder_effect (167 words)

	Populations
		The rate of population growth is influenced, generally, by two opposing factors, the intrinsic tendency of a given population to increase in numbers exponentially, and the imposition of cap on their total numbers imposed by some aspect of the environment (carrying capacity).
		A population pyramid is a series of stacked bar graphs with each bar representing a given age cohort and the height of the bars representing the fraction of the population made up of members of that cohort.
		(i) species the population consists of, (ii) time the population exists, (iii) place where the population exists, (iv) the population size, (v) the density in which the population lives, (vi) the populations spatial distribution (dispersion), (vii) the populations age (etc.) demographics, (viii) the role(s) the population members play within their ecosystem.
	www.mansfield.ohio-state.edu /~sabedon/biol1540.htm (3969 words)

	Small population size -- Facts, Info, and Encyclopedia article (Site not responding. Last check: 2007-09-18)
		One demographic consequence of a small population size, the probability that all offspring in a generation are of the same sex, is easy to calculate: it is given by (The chance of all animals being females is, the same holds for all males, thus the equation).
		Continued fixation of deleterious alleles in small populations is called (Click link for more info and facts about Muller's ratchet) Muller's ratchet, and can lead to (Click link for more info and facts about mutational meltdown) mutational meltdown.
		The (Click link for more info and facts about effective population size) effective population size is commonly lower than the actual population size.
	www.absoluteastronomy.com /encyclopedia/s/sm/small_population_size.htm (757 words)

	Determining Sample Size
		In addition to the purpose of the study and population size, three criteria usually will need to be specified to determine the appropriate sample size: the level of precision, the level of confidence or risk, and the degree of variability in the attributes being measured (Miaoulis and Michener, 1976).
		The fourth approach to determining sample size is the application of one of several formulas (Equation 5 was used to calculate the sample sizes in Table 1 and Table 2).
		Where n is the sample size, N is the population size, and e is the level of precision.
	edis.ifas.ufl.edu /PD006 (2171 words)

	Population Growth (Site not responding. Last check: 2007-09-18)
		Because of the large and increasing population size, the number of people added to the global population will remain high for several decades, even as growth rates continue to decline.
		Population doubling time is useful to demonstrate the long-term effect of a growth rate, but should not be used to project population size.
		Growth rate: The number of persons added to (or subtracted from) a population in a year due to natural increase and net migration; expressed as a percentage of the population at the beginning of the time period.
	www.prb.org /Content/NavigationMenu/PRB/Educators/Human_Population/Population_Growth/Population_Growth.htm (1284 words)

	In situ conservation of livestock and poultry
		Populations should not therefore be discarded on the grounds of economic efficiency as measured at any one time, but should be considered with respect to biological efficiency operating within the context of a wide range of possible political and economic situations and a wide range of possible breed crosses (Henson, 1986).
		Effective population size is greatly affected by the ratio of males to females active within the population such that 4 males and 4 females constitute the same effective population size (Ne), as 100 females and only 2 males.
		The effects of small population size on the rate of loss of genetic variation within populations is closely linked to the number of parent animals contributing offspring to the next generation and this effective population size (Ne) must be kept as high as possible in order to prevent dramatic loss of variation in small populations.
	www.fao.org /DOCREP/004/T0559E/T0559E03.htm (4318 words)

	Washington State Department of Health - Assessment Guidelines: Small Numbers
		In general, problems with confidentiality arise when there are small denominators (population size represented in a specific cell in a table); and, problems with data reliability arise when there are small numerators (cases in a specific cell in a table).
		Caution should be exercised by the analyst if the population size is between 100 and 300, and extreme caution is warranted when the population is less than 100.
		The effect will be to increase the effective population size, since the (usually unstated) denominator is actually "person-time" in rate calculations, and the numerators are likely to rise correspondingly as well.
	www.doh.wa.gov /Data/Guidelines/SmallNumbers.htm (2822 words)

	Conservation Ecology: Population Genetic Aspects of Pollinator Decline
		The population size we need to consider in assessing heterozygosity loss is not the actual number of adults, but rather the harmonic mean of the effective population size over many generations.
		The harmonic mean size of a population that alternates between 250 and 750 individuals (these numbers were estimated for some years for the Ontario population of this species; Packer 1994) is 375 in comparison to an arithmetic mean of 500.
		The effective population size is commonly thought to approximate one order of magnitude less than the number of individuals observed; hence, more migrants per generation than may be expected based on census population sizes are required to reduce the effects of drift.
	www.ecologyandsociety.org /vol5/iss1/art4 (12069 words)

	Hoofbeats
		However, it is certain that in recent years the population size of the herd has been extremely small (.
		Small population size inevitably results in a loss of genetic variation which usually results in a loss of fitness of the population due to the increased probability of deleterious recessive genes being paired (and therefore expressed) by in breeding.
		He, TNV and Ae are strongly correlated with sample size and two is an insufficient number for meaningful estimates.
	www.arkwild.org /hoofbeats/01_articles/ukendna.html (907 words)

	Cutting Evaluation Costs by Reducing Sample Size
		Sample size is an important factor to consider in conducting Extension evaluations, because it influences the cost of evaluations and the validity of results obtained.
		If the confidence intervals for the three sample sizes overlapped, the samples were not considered to be different; whereas, if the confidence intervals did not overlap, the samples were considered to be different.
		When the sample size was reduced to 174 and 50, the discrepancy was found to also be statistically significant (p>=.0001) for all fifteen categories.
	www.joe.org /joe/1996february/a2.html (1496 words)

	Population genetics
		However, suppose the initial population size of almond trees were 10 (with the same frequency of r at 10%).
		By the 1890's the population of northern elephant seals was reduced to only 20 individuals by hunters.
		Even though the population has increased to over 30,000 there is no genetic variation in the 24 alleles sampled.
	arnica.csustan.edu /boty1050/Populationgen/population_genetics.htm (1404 words)

	BIOL115 Lecture Note Series (Site not responding. Last check: 2007-09-18)
		Small population size is often a difficult cause of evolution for students to understand.
		The small sample often does not represent the large population from which it is drawn.
		When a large population crashes, perhaps due to disease, weather, or overexploitation of resources, sometimes much genetic information is lost during that population crash.
	www.users.csbsju.edu /~dgbrown/BIOL115/evolmechanisms.html (1925 words)

	Size of population (Site not responding. Last check: 2007-09-18)
		The mathematical model based on this idea is that the population size for one generation depends on the size of the previous generation, and it is a multiple.
		Effective population size is the number of individuals in a population who contribute offspring to the next generation.
		Population size and population density in the Netherlands by municipality on January 1, 2000.
	getinfoeasy.com /q/size-of-population.html (745 words)

	Tufts Univesity - The Coffin Laboratory - People - Research Assistant Prof. - Igor Rouzine
		At relatively small N, N << 1/s, where s is the selection coefficient (relative decrease in progeny due to a mutation), selection is a small correction to random drift.
		In a small population, one of the four possible variants has to be absent from small samples of sequences, because the two sites revert one by one.
		However, small groups at the edges (in particular, the important highest-fitness group) are subject to strong stochastic effects.
	www.tufts.edu /sackler/microbiology/lab/coffin/Personnel/newigor/resinevolution.htm (1282 words)

	Discussion (Site not responding. Last check: 2007-09-18)
		The estimated effective population size using this method is only 2-3 times the number of influenza generations per year, presumably less than 100,000 virion particles.
		Deleterious mutations rising to high frequency in small populations also seems a plausible explanation for excess of `tip' replacements relative to replacements on the main trunk of the genealogy estimated by Fitch et al.
		However, it is useful to remember that the effective population size is strongly influenced by minimum sizes over a temporal sequence, and that severe reductions in population size (e.g., associated with the evolutionary innovation allowing epidemiological spread, or transfer of virulent strains to humans) seem likely from basic considerations of the life history of influenza.
	www.wsu.edu /~mmorgan/working/Fitch/node3.html (319 words)

	IIASA Publications Catalog: Search Results
		Normally, marine fish have very large population sizes, and commercial extinction is likely to occur long before populations are reduced to the level required for losses of genetic diversity due to inbreeding.
		However, when populations are very severely over-fished to small numbers, concerns associated with small population sizes and disruptions of migration between populations may become prominent.
		Fishing is expected to generate selection on life history traits such as age and size at maturation; changes in life history traits influence the dynamics of fish populations, energy flows in the ecosystem, and ultimately, sustainable yield.
	www.iiasa.ac.at /cgi-bin/pubsrch?IR03005 (277 words)

	lecture 8
		If each population starts at p = 0.5, then at the end, when all populations have lost their variation, 50% of the populations will be fixed for the A allele and 50% will be fixed for the a allele (latter = "loss" for the A allele, get it?).
		A general result as drift proceeds in small populations is a deficiency of heterozygotes, and reciprocally, an excess of homozygotes.
		With population bottlenecks where the population size drops to a small number in one generation, the effective population size is not just the average of N's for each generation.
	biomed.brown.edu /Courses/BIO48/8.GenDrift (2370 words)

	[No title]
		Estimates based on a random sample of a population are subject to error due to sampling variability.
		When it is not possible, or desirable, to create a table with no small numbers as denominators or numerators in cells, then cell suppression is used, together with complementary suppression.
		Suppress rates based on very small numbers (i.e., fewer than 5 health events), reporting only the count (numerator). HYPERLINK "" \l "foot2" 2 When rates are suppressed, tables should be constructed such that an indicator (e.g., asterisk) appears in the cell and a legend under the table explains the reason for suppression.
	www.doh.wa.gov /data/Guidelines/WordDocs/SmallNumbers.doc (2788 words)

	Lotus Artificial Life - The Character of New Species
		Here a small population is divided from the ancestral one by a physical barrier - in the form of an expanse of water.
		Due to their small size - they are less likely to be inhabited by predators - and more likely to be relatively uninhabited.
		Small population size can increases the difficulty in finding mates, problems in defending groups, problems with the division of labour - and can lead to inbreeding.
	www.alife.co.uk /essays/new_species (879 words)

	Genetics of Small Populations (Site not responding. Last check: 2007-09-18)
		Small populations are susceptible to extinction as a result of demographic, environmental and genetic factors.
		The population is forced to a small size by habitat fragmentation
		By the loss of genetic diversity, the organisms present in the population may not have the defences to sustain themselves in the face of a disease spreading through the group.
	www.science.mcmaster.ca /biology/CBCN/genetics/mac_smpop5.htm (220 words)

	Colorado Health Information Dataset: CoHID working with small numbers
		In general, problems with confidentiality arise when there are small denominators (population size represented in a specific cell in a table); and, problems with data reliability arise when there are small numerators (cases in a specific cell in a table).
		Conversely, rates based on small numbers may fluctuate dramatically from year to year, or differ considerably from one small place to another small place, even when there is no meaningful difference.
		The effect will be to increase the effective population size, since the (usually unstated) denominator is actually "person-time" in rate calculations, and the numerators are likely to rise correspondingly as well.
	www.cdphe.state.co.us /cohid/smnumguidelines.html (2340 words)

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