How can a species switch from r to K - reproductive strategy in a single generation?

Question

I ask because it seems as though humanity is in the midst of doing exactly this. Only a few decades ago, human population was growing exponentially, and it seemed destined to keep growing until hitting constraints of food supply, at which point we would devolve into resource wars and famine. But then it didn't happen -- over the last few decades, fertility rates in the developed and developing world have plummeted and the world population is expected to stabilize in the second half of this century around 10-11 billion people. Demographers list a number of reasons for this -- education and empowerment of women, urbanization, decreased child mortality and increased life expectancy, availability of safe and effective contraception, etc. The common thread is that these trends encourage people to invest more into fewer offspring. It's as though our entire species is spontaneously deciding to switch from an r-strategy of reproduction to a K-strategy. My question is whether this is unprecedented or not. I had the impression that r or K strategy was typically a function of species, but here we have a case where humans seem to be switching between the two strategies in a single generation. Are there other species which can change their entire reproductive strategy when the population approaches its carrying capacity? If so, what kinds of environmental cues or epigenetic factors trigger the switch? How do they manifest themselves in the case of humanity's current switch?

Answer 1

The idea of a species switching from r-selection to K-selection is very interesting! It reminds me of a nonfiction book called The Children of Men by P.D. James, in which humans become infertile because of our reproductive decisions. I highly recommend. However, let’s return to your question.

While our fertility rates may have decreased, we have not switched between r and K. We were previously and still are a K-categorized species. Although it was not uncommon in the past for a woman to have 12 children, she did not have them all at the same time. K species can have many offspring, just not all at once, which differs from an r species. For example, a spider would be an r organism because of its ability to have hundreds of offspring within one egg. Also, an r species generally can only reproduce once in their lifetime whereas a K species can reproduce multiple times within their life span.

Regarding if it is possible for r-species to switch to K-species, I am uncertain. If it is, both the environment and mortality of a species would need to change. This would have to take place over more than one generation. For example, an r-species would need to have a lower mortality rate in order to provide prolonged care for a K-type offspring. R-species live in unstable environments, which is why they do not reach their carrying capacity despite their large number of offspring. Therefore, it is more important that they have many offspring in order to have at least a few live. This means that the environment would also need to become more stable for an r-species to change to a K-species.

In relation to infertility linked with environmental and epigenetic factors, I believe it is both. While I have not looked much into it, typing “epigenetics infertility” into the Google search bar resulted in a display of many research papers on the topic that you may be interested in. If you have access to a database of peer-reviewed articles through your school, it would be a great resource to use for this topic as well.

I hope this answer helped! Let me know if you would like to discuss it further or have any questions related to r and K species.