Gene flow acts directly against the goals of conservationists. While the latter attempt to increase biodiversity, gene flow can act to wipe it out with devastating efficiency. Without the presence of geographical separation (allopatric speciation), the process of gene flow seems to doom each species to merging of one identical gene pool per species. Of course in practice this will never happen, there is simply too much geographical separation among plants and most animals, and too much cultural separation in humans (in the modern world geographical separation is no longer an issue for most humans), however, theoretically, as a result of gene flow, it COULD happen.
We will now go on to look at some real life examples of issues caused by gene flow, and how they lead to concerns from conservationists.
Our first example is taken from a study by Clements & Martin1 on blue tits living in Southern France. The importance of this example is that it highlights the fact that concerns for conservationists regarding gene flow can come from naturally occurring phenomenon in everyday existence.
The blue tits live among two types of oak tree – downy and holm oak. As the birds cannot carry food very far, they have to rely on food from oak trees very close to their nest. In holm oak, caterpillar levels peak at a later time than in the downy oak (June rather than May). It would seem common sense that birds nesting in the holm oak would breed slightly later than those living in the downy oak, in order to ensure greater survival for their offspring. Indeed, on the island of Corsica, this is the case – there is only holm oak here, and blue tits breed later than on mainland. However, on the mainland, all the blue tits breed at the same time. This comes from the fact that ‘gene flow is strong, for adult birds move freely between downy and holm oak’ (Stearns & Hoekstra 2005: 45)2. Without gene flow, blue tits living in holm oak would slowly become adapted to these different food patterns – however with gene flow between these blue tits and those from downy oak, it is ensured that this can never happen, and the populations will continue to display identical behavioural characteristics, even when it is clearly maladaptive.
While the problems of such a situation are obvious, the implications for conservationist concern are less so – however they are indeed existent. It can be said that, technically, there has been no loss in biodiversity overall – after all; two differing behavioural populations still exist: those on the mainland and those on the island. This is a narrow minded view to take however – gene flow has reduced the potential for biodiversity on the mainland, where the blue tits are doomed to display just one behavioural pattern. It is also possible that any future adaption’s that may be favoured by natural selection will be swamped out by gene flow, preventing what could lead, potentially, to fascinating new behavioural characteristics and even eventually new species.
The simple fact that biodiversity is lost as a result of this process is not the only problem here for conservationists. The implications of such losses can be related to the survival of the species as a whole. It seems elementary that a species that has greater variance within itself, that is a greater diversity, will have a better chance of survival in the long run. Such an example of this would be the great amount of climate change being experienced in the modern world, one specific issue being that Holm oak is better adapted to dry climates that other oak varieties. Therefore, in a situation where the climate became much drier, Holm oak would survive while other oak species would perish. As gene flow has acted to prevent most blue tit populations becoming adapted to this variety of oak, it could indirectly threaten the existence of such populations. This emphasises the importance of sub-species.
Such an issue, while undoubtedly a concern, is not the result of any human action, therefore is seemingly something that conservationists can do little about.
There are, though, some concerns regarding gene flow that are a direct result of human activity - the most widely discussed, and criticized, being the use of Genetically Modified (GM) crops by farmers. The specific importance of this example is that it emphasises that gene flow related problems can be a man made issue, as well as just what seem ‘defects’ in nature.
Genetically Modified Organisms (GMO’s) are produced by inserting genes from one organism into another using a vector. According to the Food and Agricultural Organisation of the United Nations, use of such crops climbed from an area of 1.7m to a massive 52.6m hectors between 1996 and 2001.3 It is hard to imagine the size of such a figure in 2008.
Such crops, in many ways, seem excellent – they produce much greater yields of food and have the potential to reduce world hunger. However, as put bluntly in the book ‘Genetically Engineered Food – Changing the nature of nature’4 – “they are designed not to stay put”. (Teitel & Wilson 2001: 40). In other words, they spread – and very efficiently. It isn’t hard to imagine why this is the case. GM crops are designed to be bigger, better, stronger, more resistant to disease and more able breeders than their naturally occurring cousins. “The minimum requirements for GM gene flow to occur [is] the presence of a sexually-compatible non-GM population in close proximity to the GM population” 3 - and if this is the case, it will not take long for the GM crop to spread beyond the boundaries of the field it has been planted in.
The concern for conservationists is thus an obvious one. The combination of genetic interference by mankind and gene flow has given the potential for certain organisms to completely take over areas of land, wiping out many natural species in the process. While the use of monoculture by farmers on their land is one thing, this potential spread to vast areas of surrounding land is an even greater problem.
Such problems spread beyond the inevitable loss of biodiversity for conservationists, as “crop genetic diversity is critical to the continuing development of varieties resistant to new pests, diseases, and changing climatic and environmental conditions”5, therefore, with this increased use of GM crops; we risk a situation where a single disease can act to wipe out devastating levels of crop produce. The implications here are that while the intention of using GMO’s in crop production is to produce greater food supplies; it actually risks potentially huge losses in food.
In contrast to these examples where gene flow has prevented/reduced biodiversity, we will briefly look at an example of an opposite case: where a lack of gene flow has resulted in great variance, (as explained in Douglas Futuyma 2005: 241.)6 The pocket gopher Thomomys bottae is famous for its localised variation and lives in the absence of gene flow. There have been over 150 sub-species named, with presumably much more still undiscovered. This is indeed a conservationists dream, and shows how without gene flow, populations can develop unimaginable levels of biodiversity.
Such a situation exemplifies the fantastic amount of biodiversity that can occur in the absence of gene flow. This begs the question: what can be done about gene flow? Conservationists face a dilemma in dealing with situations such as the former mentioned in this text (that of blue tits), as any attempt to stop the negative effects of gene flow would clearly be interfering with nature. There are probably some things that could be done, but it is probably seen as best to leave nature to its own accord.
There is, however, clearly action that can be taken against the case of GM crops. There are many groups working to reduce the use of such crops (including Greenpeace, sighted previously in this essay). Actions such as ensuring any GMO is grown in isolation from any of its natural relatives, and the production of strands of GMO’s that are less susceptible to breeding with natural relatives could both act to reduce the negative effects of gene flow, however the ideal situation for conservationists would be the cessation of such crops being used altogether.
In conclusion, it is clear that gene flow is an issue that causes great headaches for conservationists, whether its actions are the result of what seem to be failures of the natural world or the meddling of the human species; or whether its effects are direct and obvious to all, or more implicit in the potential problems a loss of diversity could cause.
While it could be said that in certain situations, gene flow has advantages of improving genetic variation in very small populations, it is clear that such situations are in the minority, and as a whole, gene flow will always be viewed in a negative light by conservationists, due to its potential to massively reduce variance in species as well as its potential to put the survival of whole species’ at risk.
Reference List
Clements, Alex & Martin, Jean Louis. 1991. Laying date in Mediterranean Blue Tits: effect of habitat type and geographic isolation. [Online] Available from: www.jstor.org/stable/3676515?&Search=yes&term=Clamens&term=Alex&list=hide&searchUri=%2Faction%2FdoBasicSearch%3FQuery%3DAlex%2BClamens%26gw%3Djtx%26prq%3DLaying%2Bdate%2Bin%2BMediterranean%2BBlue%2BTits%26hp%3D25%26wc%3Don&item=1&ttl=3&returnArticleService=showArticle [09/11/2008].
Food and Agricultural Organisation of the United Nations. 2002. Gene flow from GM to non-GM populations in the crop, forestry, animal and fishery sectors. [Online] Available from: www.fao.org/biotech/C7doc.htm [11/11/2008].
Futuyma, Douglas. 2005. Evolution. Sunderland: Sinauer Associates.
Greenpeace. GE agriculture and genetic pollution. [Online] Available from: www.greenpeace.org/international/campaigns/genetic-engineering/ge-agriculture-and-genetic-pol [11/11/2008].
Stearns, Stephen & Hoekstra, Rolf. 2005. Evolution – An Introduction. New York: Oxford University Press.
Teitel, Martin & Wilson, Kimberly. 2001. Genetically Engineered Food: Changing the Nature of Nature. Park Street Press.
