The meaning of the adaptation to tolerate lactose.

The question that is most often asked about lactose tolerance is that if drinking milk is an evolutionary adaptation shouldn't we continue to reap its fitness benefits? Although this question may be frowned upon by individuals who study evolutionary biology it is an important question to answer as it addresses the issues that  are important to understanding of evolution; adaptation and natural selection. 

I will explore this question in two parts first I will explain the adaptation of lactose tolerance and then I will explore how natural selection effects life history evolution. 

Is Lactose tolerance an adaptation? 

Lactose tolerance occurs due to a genetic mutation(s) that confers an ability to digest lactose from milk. Lactose tolerance appeared as a fortuitous mutation, yet it got selected as a result of the fitness benefits it conferred; it had beneficial effects on the adaptedness (absolute fitness) of individual humans within an environmental context. Natural selection then occurred as a result of differences in adaptedness of individuals within a specific population; i.e. some individuals could drink milk into adulthood whereas others couldn’t. Over time, those individuals that could drink milk into adulthood survived and reproduced at relatively higher rate than those who could not. Over many generations, through the process of adaptation the trait for digesting lactose became adaptive within a spatio-temporal context.

Even though the trait arose as a fortuitous mutation it became adaptive as it created differences in absolute fitness and set stage for natural selection to occur. Lactose tolerance gene was definitely an adaptation to milk drinking in the spatio-temporal context it evolved in; must this mean that this trait is as adaptive today as it was in our past? Does a trait’s noble legacy justify its credentials in the present? This may not always be true and the answer depends on the differences between past and present and the nature of life history evolution.

It is important at this point to understand that all adaptations have three aspects.

a) Historical; an adaptive trait is present today in an organism because it provided fitness benefits to its bearers and there was a selection for the adaptive trait.

b) Spatial: An adaptive trait is a specific solution to a problem posed by a feature of a given environment.

c) Eventual: Adaptive traits have the ability to confer (present and future) fitness benefits to its bearers.

Whether or not an adaptive trait persists through time depends on a number of factors; the environment being the most critical of them. If there are minimal changes in the environment (over evolutionary time) such that the plasticity allows the trait to persist, then the trait remains a legible solution to the problems posed by the environment. On the contrary, if the environment has changed substantially enough so that the plasticity of the trait does not buffer the changes, then the trait finds itself in uncharted waters. Sober pointed out in “The Nature of Selection” that “the benefits that account for a traits fixation need not persist into the present. “The benefits that account for a traits fixation” are governed by the environment (Sober, 1993).

If one considers just the historical aspect of an adaptive trait then “a trait can be an adaptation for performing some task, even though performing that task in the present environment confers no benefit” (Sober, 1993).

This statement can be misunderstood as the reader might comprehend that a traits noble legacy justifies it present credentials. An adaptive trait remains an adaptation no matter what happens. I would like to clarify what I think Sober meant for the reader to read between the lines; an adaptive trait remains an adaptive trait, retrospectively. A trait is an adaptation just because it was an adaptation in the spatial context it evolved. The trait conferred fitness benefits to its eventual bearers. The trait will still be called adaptive; the process which shaped the trait will still be called adaptation, but retrospectively. Whether an adaptive trait is adaptive today depends on the environmental context of the trait. If the environment is substantially different from the environment in which the trait evolved and became adaptive then the trait may not confer any fitness benefits (Remember that the trait is still adaptive but…you guessed it…retrospectively). If the environment changes, the trait remains adaptive looking at the past; whether the trait still persists as an adaptation in the future is another issue altogether.

There are three scenarios a trait may experience if the environment changes; a) the trait may not confer any fitness benefits, i.e. remain neutral and adaptive retrospectively; b) the trait may happen to confer fitness benefits, persist being adaptive; and c) the trait confers negative fitness benefits and become maladaptive.

Going back to the adaptive trait of Lactose tolerance, we can either say that Lactose tolerance was an adaptation in the spatio-temporal context that favored its prevalence or lactose tolerance is an adaptation retrospectively. Both statements are correct as long as you understand the definition of an adaptation. The question therefore is not whether lactose tolerance is an adaptation. It most definitely is! The question is whether lactose tolerance is an adaptation in today’s environmental context? 

In the arid climates of Middle and Near east dairy has been suggested to be the major source of clean uncontaminated water (Cook & Al-Torki, 1975). The individuals that could drink milk into adulthood happened to have higher relative fitness than those who could not. In northern Europe dairy was the source of calcium, this must have been crucial in place where there is very little sunlight (Flatz & Rotthauwe, 1973).

Thus the trait was selected and as the populations of individuals travelled across the world the trait travelled with them. In certain places the trait might have been still adaptive; yet, in other places the adaptive trait might have lost its relevant environmental context. Rehydration through consumption of dairy is not a necessity when you live next to a river, for example.

Thus we can say that in certain populations in the past (and today) the adaptive trait of lactose tolerance lost its relevance, as it could not confer fitness benefits. It is lingering in our gene pool as a consequence of fitness benefits it conferred to its bearers in their environmental context. Those fitness benefits are not relevant in those populations whose environmental context has changed. Thus lactose tolerance does not remain an adaptation in those populations; however, that does not take away the adaptive legacy of the trait. The trait has become a not-so-benign ghost of Christmas past.

Lactose Tolerance is a mal-adaptation

Now that I have established that a trait can lose its adaptive significance in a changed environmental context I will try to make the case that this trait is not just a neutral adaptation but has the potential to be transformed into a mal-adaptation in todays environmental context. Lactose tolerance and the resultant consumption of milk have been linked to many chronic diseases of the western civilization. Chronic consumption of milk has been associated primarily with auto-immune diseases (Type 1 Diabetes, Rheumatoid Arthritis, and Multiple Sclerosis) (Elliott, Harris, Hill, Bibby, & Wasmuth, 1999; Malosse, Perron, Sasco, & Seigneurin, 1992), consumption of dairy has also been correlated to prostate cancer and insulin resistance (Pereira et al., 2002; Qin et al., 2004). The ghost-adaptation of lactose tolerance is making us realize the necessity of changing our ways (I couldn’t help myself there ). Lactose tolerance today is a mal-adaptation as the trait was previously selected for by natural selection, lost its relevance, and now is becoming a crucial factor in the development of chronic diseases.

Natural Selection and Life History Evolution

There is another important aspect to the “adaptation” of consuming dairy into adulthood. It is centered on the workings of natural selection.

Natural selection operates most quickly on traits and behaviors that are expressed during the reproductive years of the life cycle. A trait that hampers reproductive ability in the young will be quickly eliminated from the population. A trait that is expressed in the in the older individuals will be removed much more slowly. As lactose tolerance was beneficial for individuals within a certain environmental context it got selected irrespective of the long-term effect it caused.

The following table may help to explain which traits get naturally selected. Traits and adaptations may have in all four aspects: a) short term benefit, b) short term cost, c) long term benefit, and d) long term cost

Traits that have short (during reproductive years) and long term benefits (longevity) will get naturally selected so will traits that have short term benefit and a long term cost. Traits with short term cost will get weeded out if they negatively affect an organisms’ fit with the environment. Lactose tolerance, in todays changed environmental contexts falls into the category of traits that have short term benefit and long term cost.

Summary

Thus, the question, “if lactose tolerance is an adaptation shouldn’t we continue to drink milk?” has two sub-questions. First, is lactose tolerance an adaptation in todays environmental context? And second aspect deals with nature of life history evolution and natural selection. The answer to the first question tell us that lactose tolerance is does not remain an adaptation in a changed environmental context, moreover it has become a mal-adaptation due to enabling of chronic consumption of milk. Dairy consumption has been correlated locally as well as globally with many diseases of civilization. The answer to the second question addresses the issue that a trait that confers fitness benefits in reproductive life will tend to get selected even though it has long term costs.

Summary Statement: Lactose tolerance is a retrospective adaptation that has the potential to become mal-adaptive in a changed environmental context. 

References

Cook, G. C., & Al-Torki, M. T. (1975). High intestinal lactase concentrations in adult Arbs in Saudi Arabia. British Medical Journal, 3(5976), 135–136.

Elliott, R. B., Harris, D. P., Hill, J. P., Bibby, N. J., & Wasmuth, H. E. (1999). Type I (insulin-dependent) diabetes mellitus and cow milk: casein variant consumption. Diabetologia, 42(3), 292–296.

Flatz, G., & Rotthauwe, H. W. (1973). Lactose nutrition and natural selection. The Lancet, 302(7820), 76–77.

Malosse, D., Perron, H., Sasco, A., & Seigneurin, J. M. (1992). Correlation between milk and dairy product consumption and multiple sclerosis prevalence: a worldwide study. Neuroepidemiology, 11(4-6), 304–312.

Pereira, M. A., Jacobs Jr, D. R., Van Horn, L., Slattery, M. L., Kartashov, A. I., & Ludwig, D. S. (2002). 

Dairy consumption, obesity, and the insulin resistance syndrome in young adults. JAMA: the journal of the American Medical Association, 287(16), 2081–2089.

Qin, L.-Q., Xu, J.-Y., Wang, P.-Y., Kaneko, T., Hoshi, K., & Sato, A. (2004). Milk Consumption Is a Risk Factor for Prostate Cancer: Meta-Analysis of Case-Control Studies. Nutrition and Cancer, 48(1), 22–27. doi:10.1207/s15327914nc4801_4

Sober, E. (1993). The nature of selection: evolutionary theory in philosophical focus. University of Chicago Press. Retrieved from http://books.google.com/books?hl=en&lr=&id=6bLZQzDd0f4C&oi=fnd&pg=PR9&dq=nature+of+selection&ots=NqnNZ0V_xs&sig=yt2t6n05ZurIb8JigyFXDenPRVE