Talk:Conservation genetics

Talk

Discussion for: Conservation genetics • View • Links • Related changes

Use this page for verification notes, disputes, and action items. Keep narrative content on the article.

Page scope

Conservation genetics covers conservation genetics as a field: the genetic consequences of small population size, the evidence on genetics and extinction, rules for how large a population must be, the move from neutral markers to genomes and the dispute over neutral and functional variation, the management of gene flow, the crop side of the field and its application to cannabis. It summarises and points to the pages that treat each part in depth: Effective population size, Genetic drift, Inbreeding depression, Population bottleneck, Minimum viable population, Genetic marker, Gene flow, Genetic erosion, Gene bank, Ex situ conservation, In situ conservation, Core collection, Germplasm sampling and Cannabis conservation.

Data verification

Record source conflicts, figures that differ between studies and sources not used here. Keep narrative content on the article.

Discussion

Sources

Ouborg, Pertoldi, Loeschcke, Bijlsma and Hedrick (2010), Trends in Genetics: the central idea of the field and the three consequences of small size, the growth of the field with neutral markers, what neutral markers established, the open questions and the expected use of genomics.
Spielman, Brook and Frankham (2004), Proceedings of the National Academy of Sciences: heterozygosity in 170 threatened taxa against related taxa not under threat, and the argument over whether genetic factors act before extinction.
Frankham, Bradshaw and Brook (2014), Biological Conservation: loss of heterozygosity in small populations, the extinction vortex, the 50/500 rule and its proposed revision to 100/1000, the ratio of effective to census size, fragmentation, genetics in population viability analysis and the Red List thresholds.
Hoban and 30 co-authors (2020), Biological Conservation: genetic diversity in the Convention on Biological Diversity, the two uses of "genetic erosion", the seed-bank indicator and the proposed indicator of populations with an effective size above 500.
Teixeira and Huber (2021), Proceedings of the National Academy of Sciences: the case that neutral genetic diversity has limited relevance for conservation.
Kardos and seven co-authors (2021), Proceedings of the National Academy of Sciences: the reply, that genome-wide variation is generally the best guide; drift load after bottlenecks; the Isle Royale wolves.
Ralls and seven co-authors (2018), Conservation Letters: genetic rescue, outbreeding depression and a decision process for fragmented populations.
Welling, Shapter, Rose, Liu, Stanger and King (2016), Frontiers in Plant Science: cannabis gene bank collections, the problems of regrowing them, cryopreservation and a proposed core collection.
Aina and six co-authors (2025), Scientific Reports: seed collected from feral cannabis in 12 states of the United States for a gene bank, its population structure and heterozygosity.

Where the sources disagree

Neutral or functional variation. Teixeira and Huber hold that neutral diversity across the genome has only very limited relevance for conservation and that conservation should map functional variation. Kardos and colleagues hold that conserving genome-wide variation is generally the best approach, and that the simulations behind the opposing view assumed rates of harmful mutation far below empirical estimates.
Red List status and diversity. Spielman and colleagues found lower heterozygosity in 77 per cent of 170 threatened taxa compared with related taxa. Teixeira and Huber describe Red List status as a poor predictor of genome-wide nucleotide diversity. Kardos and colleagues attribute the weak relation to the lag between decline and loss of variation, to uncontrolled differences between species and to the subjectivity of Red List assessments, and cite a study that found a relation after controlling for phylogeny.
Outbreeding depression. Ralls and colleagues call fears of it exaggerated and describe it as uncommon and often transient. Teixeira and Huber stress that it can appear in later hybrid generations that experiments seldom follow. Ouborg and colleagues list it among the risks of crossing populations that drift has separated.
The 50/500 rule. Frankham, Bradshaw and Brook propose effective sizes of at least 100 and 1,000. They note that Jamieson and Allendorf (2012), reviewing the same question, endorsed 50 and 500. Hoban and colleagues use 500 as the threshold of their indicator and mention that some authors advocate 1,000.
How many feral plants were collected. Aina and colleagues give 1,821 plants from 91 populations in their results and 1,815 female plants from 88 populations in their methods, where three further populations came from other collections.

How far the evidence goes

Spielman and colleagues measured heterozygosity with allozymes, microsatellites and minisatellites. Their comparison shows that threatened taxa hold less variation; it does not show that the loss caused their threatened status, which the authors state they could not determine.
The evidence linking inbreeding to extinction in the wild cited here comes from one butterfly, one plant and laboratory populations of Drosophila. The projections of extinction times with and without inbreeding depression are simulations.
The effective sizes of 50, 100, 500 and 1,000 are rules of thumb derived from theory and from laboratory and domestic populations. The ratio of effective to census size of 0.1 to 0.2 is an average across species, and Frankham and colleagues note that it varies with life history.
No study cited here estimates the effective size of a cannabis population or measures inbreeding depression or genetic rescue in cannabis. The cannabis material concerns how stored collections are regrown and how feral populations were sampled and genotyped.
The meta-analysis of 156 data sets on genetic rescue (Frankham 2015) is cited as Ralls and colleagues report it.
The heterozygosity of the feral cannabis groups comes from about 21,000 single-nucleotide polymorphisms in the 346 plants that could be assigned to a group. The north-eastern group comprised 14 plants.

Terms

Effective population size (N_e): the size of an idealised population that would lose variation or become inbred at the same rate as the real one.
Inbreeding coefficient (F): the fraction of an individual's genome that is identical by descent.
Inbreeding depression: the reduction in fitness of the offspring of related parents.
Outbreeding depression: the reduction in fitness of the offspring of a cross between distinct populations, compared with crosses within a population.
Genetic rescue: an increase in population growth or a reduction in genetic load following the arrival of individuals with new alleles.
Drift load: the reduction in mean fitness of a population caused by homozygosity for harmful alleles.
Lethal equivalent: a set of harmful alleles that would on average cause death if homozygous.
Neutral variation: variation at sites where selection does not act, measured with neutral markers.