Recently, the question of how prevalent in nature are truly non-overlapping generations has piqued my interest. There are many methodologies which make the assumption that generations are non-overlapping. Or in other cases, it is a simplification we may make to more easily understand underlying processes or to be able to obtain an answer in a first go at a research question.
I think there has been much progression from this assumption across the field, clarifying how results may differ for organisms with overlapping generations (e.g. see 1-5). My question may therefore be less relevant than I originally anticipated. Yet, I have not been able to brush it aside, since I naively thought the answer must already exist. So I’ll pose it to a more general audience here at the Molecular Ecologist: “What percent of described species exhibit truly non-overlapping generations?”
Cicada molts – photo by Flickr user PG Palmer.
Well, after much searching, emailing, twittering, and asking around; the answer has decidedly been that we really don’t know enough about many organisms’ life histories to have a good answer for this question.
There are many annual plants out there, but lots have seed banks that last greater than one season. Some anadromous fish might have fallen into the category, but they often are out at sea for multiple years while an intervening generation returns to spawn, and some mixing among the generations can occur. Some frogs get close, with very low annual survival, but overlap can still occur. The main group that seems our best bet are insects. Even though you have many common insects that reproduce continually, such as ants and termites, you also have things like cicadas that, at least in one geographic region, come out every n years, reproduce, and die. As long as there is no complicating diapause in adults, many insects seem to fit a similar life history (only on a shorter time span), but again, from my preliminary searches, not enough is known to be able say x% of insect species on the planet have non-overlapping generations.
I think the general thought on the topic is that non-overlapping generations are not the norm, but only a small fraction of extant organisms. However, insects are so speciose, that I would wonder if that pushes the percent of non-overlapping generation species higher than one might think. I don’t have an answer, but I’m curious to know, so anyone out there with insights, do please chime in! If your study species has non-overlapping generations (a new adult generation only exists once the entire previous adult generation is dead), let me know in the comments or feel free to email me. If you know a whole clade that fits the bill, even better.
I’ll end on this note. I was incredibly surprised to learn that there was no clear answer out there. This may have been obvious to any naturalist or taxonomist, but I found it quite exciting that something I thought could be so simply answered is not at all.
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1. Felsenstein J (1971) Inbreeding and variance effective numbers in populations with overlapping generations. Genetics, 68:581-597.
2. Nunney L (1993) The influence of mating system and overlapping generations on effective population size. Evolution, 47:1329-1341.
3. Ellner S, Hairston NG (1994) Role of overlapping generations in maintaining genetic variation in a fluctuating environment. American Naturalist, 14:403–17.
4. Eldon B, Wakeley J (2006) Coalescent processes when the distribution of offspring number among individuals is highly skewed. Genetics, 172:2621-2633.
5. Balloux F, Lehmann L (2012) Substitution rates at neutral genes depend on population size under fluctuating demography and overlapping generations. Evolution, 66:605-611.