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Literature analysis of the impact of drivers of evolutionary resilience

Tolerance to selfing

Literature search based on the following keyword combinations:

• extinction AND "tolerance to selfing"
• extinction AND "inbreeding depression"
• "population size" AND "inbreeding depression"
• "population size" AND "tolerance to selfing" 

Frankham (1995) and Saccheri et al. (1998) showed a positive relationship between levels of inbreeding and risk of extinction in a limited number of cases, but could not investigate the effect of inbreeding depression or tolerance to selfing per se. Charlesworth and Charlesworth (1987) showed that inbreeding populations suffer less fitness loss from further inbreeding than do outcrossing populations, but this still does not prove that tolerance to selfing allows smaller populations to be viable. This would be true if it could be proven that smaller (conspecific) populations have a larger tolerance to selfing.

Data are contradictory: Ouborg and Van Treuren (1994), working on the wild mint Salvia pratensis, concluded to no relationship between census population size and tolerance to selfing. Yet, the most comprehensive study to date on the relationship between inbreeding depression and population size (Angeloni et al. 2011), a meta-analysis based on close to 2500 data sets, showed that the relationship is significant, with smaller populations suffering less from inbreeding depression. This only proves, of course, that smaller populations have smaller inbreeding depression, but does not prove that they avoid extinction thanks to their better tolerance of inbreeding, nor does it prove that better tolerance of selfing is the causal explanation of the capacity of populations to persist at small sizes. Nevertheless, this is a sort of "smoking gun" in favour of the idea that better tolerance to selfing may extend the domain of persistence.

See also the talk by Richard Frankham at this conference, Tuesday 20 August 10:00

GxE effects on fitness

Literature search based on the following keyword combinations:

• "genotype x environment interaction" "fitness"
• "genotype-environment interaction" "fitness"
• "population collapse" AND fitness

Literature on this specific topic is hard to come by. However, some Authors have related temporal variation of fitness-related traits to risk of extinction in experimental conditions. In particular, Clements and Ozgul (2016) have shown that body size (directly proportional to fitness in their experimental system) is strongly influenced by environmental conditions, and that a decrease in such trait predicts population collapse. This kind of result lays the foundation of the link between changes in average population fitness and risk of extinction. The second part of the argument requires showing that some populations present GxE interactions for fitness, such that there are multiple "genetic optima" depending on environmental conditions. This is a much less hard piece of information to obtain, and can be tracked back to classical studies on temporal variation of allelic frequencies in desert fruit flies (e.g., Dobzhansky & Ayala 1973). Indirectly, results such as those of Dobzhansky and Ayala - as well as the numerous papers showing that meta-populations display local adaptive optima - suggest that populations can steer away from collapse by the effect of fluctuating selection favouring different genotypes in different environmental conditions. Starrfelt & Kokko (2012) have modelled GxE interactions on fitness as a form of "bet hedging" and separated correlation and variance components for fitness, highlighting the different roles of maximisation of fitness and increased fitness variance (to the expense of fitness mean) in population persistence. Direct empirical proof of the link between variation in such form of adaptive flexibility and risk of extinction is lacking, though. [this section requires further validation through a more thorough leterature search]

Fitness variance

Literature search based on the following keyword combinations:

• "hard selection" extinction
• "fitness variation" extinction
• "fitness variance" "population collapse"

The question of whether disturbance depresses population dynamics through selective effects, and in which conditions populations can cope with it, boils down to asking two questions:

1. how much variance for fitness is there in wild populations?
2. how "hard" is selection (i.e., how much mortality does it produce above baseline mortality rates)?

Adaptability

"Adaptability" is a rather fuzzy concept that may include multiple processes, from plasticity to natural selection. A comprehensive model of the role of different components of adaptability in population persistence, and how some tipping points can lead to extinction in the presence of environmental stressors, is provided by Botero et al. (2015).

See also the talk by Richard Frankham at this conference, Tuesday 20 August 10:00

Acknowledgements

Funding for this work and travelling support to I. Scotti were provided by the FORESTERRA-INFORMED research programme and by the ANR-MRSEI "NETTREE" (ANR-18-MRS1-0019) fund.

Useful references

Angeloni, F., Ouborg, N. J., & Leimu, R. (2011). Meta-analysis on the association of population size and life history with inbreeding depression in plants. Biological Conservation, 144(1), 35–43. 

Aravanopoulos, F. A. (2011). Genetic monitoring in natural perennial plant populations. Botany, 89(2), 75–81. https://doi.org/10.1139/b10-087

Aravanopoulos, F. A., Tollefsrud, M., Graudal, L., Koskela, J., Kätzel, R., Soto, A., … Bozzano, M. (2015). Genetic monitoring methods for genetic conservation units of forest trees in Europe. European Forest Genetic Resources Programme (EUFORGEN). Biodiversity International, Rome , Italy.

Botero, C. A., Weissing, F. J., Wright, J., & Rubenstein, D. R. (2015). Evolutionary tipping points in the capacity to adapt to environmental change. Proceedings of the National Academy of Sciences, 112(1), 184–189.

Charlesworth, D., & Charlesworth, B. (1987). INBREEDING DEPRESSION AND ITS EVOLUTIONARY CONSEQUENCES. In Ann. Rev. Ecol. Syst (Vol. 18). Retrieved from www.annualreviews.org

Clements, C. F., & Ozgul, A. (2016). Including trait-based early warning signals helps predict population collapse. Nature Communications, 7, 10984.

Dobzhansky, T., & Ayala, F. J. (1973). Temporal Frequency Changes of Enzyme and Chromosomal Polymorphisms in Natural Populations of Drosophila. Proceedings of the National Academy of Sciences, 70(3), 680–683.

Frankham, R. (1995). Inbreeding and extinction: a threshold effect. Conservation Biology, 9(4), 792–799.

Gunderson, L. H. (2000). Ecological Resilience—In Theory and Application. Annual Review of Ecology and Systematics, 31(1), 425–439. https://doi.org/10.1146/annurev.ecolsys.31.1.425

Hoelzel, A. R., Bruford, M. W., & Fleischer, R. C. (2019). Conservation of adaptive potential and functional diversity. Conservation Genetics, 1–5. https://doi.org/10.1007/s10592-019-01151-x

Holling, C. S. (1973). Resilience and Stability of Ecological Systems. Annual Review of Ecology and Systematics, 4(1), 1–23. https://doi.org/10.1146/annurev.es.04.110173.000245

Luque, G. M., Vayssade, C., Facon, B., Guillemaud, T., Courchamp, F., & Fauvergue, X. (2016). The genetic Allee effect: A unified framework for the genetics and demography of small populations. Ecosphere, 7(7), e01413.

Ouborg, N. J., & Van Treuren, R. (1994). The significance of genetic erosion in the process of extinction. IV Inbreeding load and heterosis in relation to population size in the mint Salvia pratensis. Evolution, 48(4), 996–1008.

Saccheri, I., Kuussaari, M., Kankare, M., Vikman, P., Fortelius, W., & Hanski, I. (1998). Inbreeding and extinction in a butterfly metapopulation. Nature, 392(6675), 491–494.

Starrfelt, J., & Kokko, H. (2012). Bet-hedging-a triple trade-off between means, variances and correlations. Biological Reviews, Vol. 87, pp. 742–755.

Stephens, P. A., Sutherland, W. J., & Freckleton, R. P. (1999). What Is the Allee Effect? Oikos, 87(1), 185–190.

BONUS: The oak and the reed (J. de la Fontaine, 1668)

The oak one day address'd the reed:--
'To you ungenerous indeed
Has nature been, my humble friend,
With weakness aye obliged to bend.
The smallest bird that flits in air
Is quite too much for you to bear;
The slightest wind that wreathes the lake
Your ever-trembling head doth shake.
The while, my towering form
Dares with the mountain top
The solar blaze to stop,
And wrestle with the storm.
What seems to you the blast of death,
To me is but a zephyr's breath.
Beneath my branches had you grown,
That spread far round their friendly bower,
Less suffering would your life have known,
Defended from the tempest's power.
Unhappily you oftenest show
In open air your slender form,
Along the marshes wet and low,
That fringe the kingdom of the storm.
To you, declare I must,
Dame Nature seems unjust.'
Then modestly replied the reed:
'Your pity, sir, is kind indeed,
But wholly needless for my sake.
The wildest wind that ever blew
Is safe to me compared with you.
I bend, indeed, but never break.
Thus far, I own, the hurricane
Has beat your sturdy back in vain;
But wait the end.' Just at the word,
The tempest's hollow voice was heard.
The North sent forth her fiercest child,
Dark, jagged, pitiless, and wild.
The oak, erect, endured the blow;
The reed bow'd gracefully and low.
But, gathering up its strength once more,
In greater fury than before,
The savage blast
O'erthrew, at last,
That proud, old, sky-encircled head,
Whose feet entwined the empire of the dead!