Are all individuals of a species the same?

The definition of a species states that all individuals within a species will have the same main characteristics (Collins Dictionary, 2020). These Burchell’s zebras all share the same characteristic striped appearance for example.


A group of Burchell’s zebras. Maasai Mara National Reserve, Kenya

So each individual within a species must share similar characteristics. But does this mean each will respond to environmental change in the same way? Traditionally a species is assumed to be one homogenous unit, assuming that all individuals within the species have similar environmental tolerances irrespective of their origin within the species range. Accordingly all individuals should be able to exist anywhere within the species’ range.

But is it that simple? Do all individuals respond the same way or do some individuals have the ability to respond differently? And more pressingly: Will all individuals within a species be affected by climate change in the same way?

Kohlekraftwerk Lünen

Coal Powerplant; Kohlekraftwerk. Lünen, Germany

Greenhouse gas emissions caused by human activity have enhanced the greenhouse effect; resulting in additional warming of the Earth’s surface (Houghton et al., 1990). Of this extra heat, 93% has been absorbed by the ocean leading to surface water warming of 0.11°C per decade in the Indian Ocean, 0.07°C per decade in the Atlantic and 0.05°C per decade in the Pacific (Hoegh-Guldberg et al., 2014).

Increased mean temperatures are redistributing species across the globe with many cold-water species moving polewards; including brown seaweeds. In Europe the cold-water cuvie kelp is already showing range shifts toward northern regions and local extinctions in its southern range (Müller et al., 2009) and by 2200 two common rockweeds will be lost from European shores south of Bordeaux (Jueterbock et al., 2013).

Forest kelp or cuvie (Laminaria hyperborea)

Forest kelp or cuvie (Laminaria hyperborea). Shetland, UK

If all individuals of a species respond the same way to warming then those at the edge of the species’ range will be more likely to be subjected to temperatures outside of their tolerance. These edge populations will therefore be most susceptible to range shifts and local extinctions. Those individuals within the central range should have tolerance to the warming since there is a greater margin before they are exposed to intolerable temperatures. Thus the species should be maintained within the central range and lost at the edges.

But what if all individuals do not respond the same way? In that case central populations could be in just as vulnerable a state.

If individuals respond differently, than those from different locations might be able to tolerate temperature changes to different degrees. Therefore those in central populations may also be exposed to temperatures outside of their tolerance.

Consequently if all individuals do not respond to warming in the same way then the species could be vulnerable to climate change throughout its whole range.

So either all individuals are the same and the edge populations are most vulnerable or individuals can be different and then all populations could be vulnerable.

evil sargassum

Gulfweed (Sargassum), Rockweed (Fucus) and Sea lettuce (Ulva). Llangennith, Wales, United Kingdom

Well, a systematic review of literature relating to marine plants and seaweeds appears to help answer this question. A striking 90% of studies observed within species differences to temperature change, meaning that individuals do in fact respond differently to temperature (King et al., 2018). Therefore all populations could be vulnerable. We could see local extinctions from both the edge and the central range of many important seaweed species.

To add to that, many seaweeds have limited dispersal. This results in highly structured distributions with closely related individuals remaining in close proximity. These individuals of close relation will possess similar characteristics, including temperature tolerance. Accordingly most individuals within the population will respond similarly to warming.

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Sardine run. Tonga

In a highly mobile species with high dispersal, for example many fish species, migrant fish can move polewards to track the tolerable temperature and replace previously resident fish that can no longer tolerate the temperature in their previous habitat. These migrants can also contribute vital genetic material to the populations allowing the population to adapt to warming.

But seaweeds are not mobile; they cannot migrate since they are generally permanently attached to a hard surface. Consequently migration cannot readily contribute to the population. This means that struggling resident seaweeds cannot be replaced by migrant seaweeds. No migration and limited dispersal also means that no new genetic material is added to the population, therefore hindering adaptation to warming.

Figure 1 from King et al., 2018
Diagrammatic representation of how low and high dispersing species will respond to warming at the edge and centre of their range. Ovals represent the experienced warming within the edge and central range. Under warming conditions low dispersing species, such as seaweeds, with individuals that tolerate different temperatures will not be able to disperse into future suitable habitats and will become locally extinct.

It seems then that whether a population is at the edge or in the centre of the species range warming can be disastrous for seaweeds.

But individuals having different tolerances might also help protect a species, given a human little help. If individuals tolerant to warming are relocated to population susceptible to warming we may be able to boost local temperature tolerances. This concept of ‘Assisted migration’ is not new, having already been developed in terrestrial forests (McLachlan et al., 2007; Aitken & Whit­lock 2013; Williams and Dumroese, 2013) and seagrass meadows (Katwijk et al., 2016).

There are far more inherent logistical problems relocating seaweeds compared to the far simpler reseeding of terrestrial trees and seagrasses, however there is growing indication that this may become a feasible option in the face of climate change.

EUO © OCEANA Carlos Minguell 34111

Rhizomes of seagrass (Posidonia oceanica). Pecorini a Mare, Filicudi island, Eolian islands, Italy. © OCEANA

So we can conclude that just because all individuals from a species must share similar characteristics this does not mean that they are all exactly the same. We need to stop considering species as a single homogenous unit and instead see all individuals as unique in their own small way.

This post was based off the research of King et al., (2018).

If you enjoyed this post you can find the article here:

King, N.G., McKeown, N.J., Smale, D.A. and Moore, P.J., 2018. The importance of phenotypic plasticity and local adaptation in driving intraspecific variability in thermal niches of marine macrophytes. Ecography41(9) 1469-1484.


Aitken, S. N. and Whitlock, M. C. 2013. Assisted gene flow to facilitate local adaptation to climate change. Annual Review of Ecology, Evolution, and Systematics. 44: 367–388.

Collins Dictionary (2020) Definition of ’species’. HarperCollins Publishers. Date accessed 25/06/2020;

Houghton, J.T., Jenkins, G.J. and Ephraums, J.J., (1990) Climate change: the IPCC scientific assessment. Intergovernmental Panel on Climate change (IPCC), Cambridge University Press; Cambridge, United Kingdom

Hoegh-Guldberg, O., R. Cai, E.S. Poloczanska, P.G. Brewer, S. Sundby, K. Hilmi, V.J. Fabry, and S. Jung, 2014: The Ocean. In: Climate Change 2014: Impacts, Adaptation, and Vulnerability. Part B: Regional Aspects. Contribution of Working Group II to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Barros, V.R., C.B. Field, D.J. Dokken, M.D. Mastrandrea, K.J. Mach, T.E. Bilir, M. Chatterjee, K.L. Ebi, Y.O. Estrada, R.C. Genova, B. Girma, E.S. Kissel, A.N. Levy, S. MacCracken, P.R. Mastrandrea, and L.L.White (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 1655-1731.

Katwijk, M. M. et al. 2016. Global analysis of seagrass restoration: the importance of large-scale planting. Journal of Applied Ecology. 53: 567–578.


McLachlan, J. S. et al. 2007. A framework for debate of assisted migration in an era of climate change. Conservation Biology 21: 297–302.

Müller, R., Laepple, T., Bartsch, I. and Wiencke, C., 2009. Impact of oceanic warming on the distribution of seaweeds in polar and cold-temperate waters. Botanica Marina52(6): 617-638.

Jueterbock, A., Tyberghein, L., Verbruggen, H., Coyer, J.A., Olsen, J.L. and Hoarau, G., 2013. Climate change impact on seaweed meadow distribution in the North Atlantic rocky intertidal. Ecology and evolution3(5):1356-1373.

Williams, M. I. and Dumroese, R. K. 2013. Growing assisted migration: synthesis of a climate change adaptation strategy. – In: Haase, D. L. et al. (tech. coord.), National Proceedings: Forest and Conservation Nursery Associations – 2012. Proceedings RMRS-P-69. Fort Collins, CO, USA Dept of Agriculture, Forest Service, Rocky Mountain Research Station, 90–96.

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