Since I’m part of a reading group on evolutionary medicine this semester (I am no expert of evolution OR medicine, but love to learn from experts!), I wanted to post some cool things I’m learning 😀
[NOTE: To learn more about the topic with in-depth explanations & fascinating examples, please see EVOLUTION AND THE ORIGINS OF DISEASE.]
One fascinating topic is why we humans are susceptible to disease. If natural selection*** shapes successful traits, then why do we catch and develop so many diseases??
Evolutionary medicine is the field that tries to figure out why natural selection has left us so susceptible to illness (physical and mental). Here are some ways to explain our vulnerability to getting sick:
Reproduction vs. Health
I feel it is a common misconception that evolution wants us to live longer healthier lives (e.g. when people say that we’ve evolved to eat a certain way that allows us to live long and be disease-free).. and it is disturbing and heartbreaking for some to find out that evolution pretty much doesn’t care about your happiness, health, or longevity. 😦 Natural selection does not shape organisms to increase all those things, it shapes them to improve our fitness (Fitness NOT meaning long healthy lives [and with a six pack, preferably], but having more healthy children). So a trait that actually harms health will still get inherited if it increases reproduction!
One example I have heard of is having attractive female proportions (waist to hip ratio and all): it increases the chances that you will have more children by making a female desirable by men, but it is associated with higher risk of some diseases in old age.Another interesting example is higher mortality of males in adulthood- natural selection can favor such traits as risk taking, (which is important in attracting females as males compete for female attention), though it can decrease the lifespan of people whose personalities allow for increased risk taking.
I’ve seen that some paleo diet followers discuss evolution as a benevolent force that has figured out a way for humans to live long and prosper, and while that’s not true on the level of the individual (it technically “cares” that the species prospers by spreading), it doesn’t mean that you can’t use evolutionary theory to personally get healthier.
Our genes don’t match the environment!
Humans have created quite amazing conditions for ourselves- sanitation, roads, safe desktop jobs, public transit, etc. Things that make life comfortable and pleasant. And technologically advanced societies see higher rates of various disorders- autoimmune disease, obesity, drug abuse and so on. Many versions of certain genes are only problematic in modern environments. Proponents of all sorts of paleo-related diets, for example, claim our evolved preference for sugar and salt is dangerous in the world where processed foods are cheap and omnipresent (though adaptive in the wild as sugary ripe fruit are nutrient & calorie-rich). Another example is nearsightedness– it’s a problem in societies where kids begin reading early and is not a problem in populations that hunt & gather.
Other explanations are:
- Pathogens simply evolve faster than their hosts (ourselves) so we will never have an immune system that is not vulnerable to some disease.
- There are also tradeoffs: a certain trait can have great benefit in one way, yet it may have negative effect in other respects (again- being a seductive mess on a motorcycle might make females go crazy over you & want to reproduce, but it also makes one susceptible to dying from unsafe choices).
Once again, here is a GREAT read on EVOLUTION AND THE ORIGINS OF DISEASE by Dr. Nesse (MY INSTRUCTOR!) and Dr. Williams with MUCH more comprehensive explanations AND more interesting examples than I have in this blog entry. 😉
- Evolution: change in genetic makeup of a population over generations; it requires genetic variation. The variation in genes arises from mutations and recombination.
- Natural selection favors traits that allow an organism to produce more offspring [that is healthy enough to produce its own]
- Fitness does not mean personal health & longevity. Fitness means how good you are at leaving a successful offspring.
- Inclusive Fitness: unlike previously thought, evolution doesn’t work on the level of groups/species but on the level of individuals (so traits that aren’t “good” for the whole species but are good for this individual having more kids are going to be selected for). E.g. genes that make one aggressive to others will still pass on if it leads to this individual reproducing successfully.
HOWEVER, nice helpful personality traits are successful & are passed on (humans are incredibly altruistic vs. other animals) as it makes one do nice things for close relatives. Since you share genetic material (50% with each parent and siblings, 25% with cousins), the individual’s reproductive success actually includes not only how many healthy kids you produce, but how many your closest relatives do also!
- There are no traits/genes that are awesome universally. The benefit of a certain trait is always in the context of the environment. E.g. sodium retention is prevalent in people that evolved around the equator since it gave them a selective advantage (salt is necessary to your body but is lost via sweat and urine.. you’d sweat way more in the hot climates).
[***Natural selection: imagine a group of people/dolphins/bugs. If this group’s members differ in some way that influences the likelihood that they’ll be part of the group in the future, this group will end up changing with time. So if some members have a genetic variation that influences how many kids they’ll have, in time this group will change and have more of the genetic trait that resulted in more kids!
A popular example is trees that once had light barks but got covered in black soot. The group of moths that used to hang out by the tree had variation in color- some white, some black.. The white ones will end up being eaten up by birds simply because they are now super visible on the dark bark and, in time, majority of moths will be black (the group has changed!) . Thus, a genetic trait is only “successful” in a context of an environment. There is nothing beneficial to being a black moth other than you’re less visible on black bark and thus will end up having more offspring than the white moths in the group.]