“We happy few: using structured population models to identify the decisive events in the lives of exceptional individuals”

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Robin Snyder and Stephen Ellner

Identical phenotypes? Luck determines who produces most of the offspring. Luck counts more when you're young/small

Evidence of long-distance dispersal in mathematical biologists (Homo mathematicus). These two mathematical biologists (coincidentally the authors of the above paper) were recaptured near Salt Lake City, UT, far from both of their home ranges. The purple-banded individual on the left has a home range centered on Ithaca, NY, while the brown-banded individual on the right has a home range centered on Cleveland, OH.
(Credit: Peter Adler)

In most plant populations, the vast majority of offspring are produced by a few individuals. Even if all individuals are identical, they will encounter different luck during their lives—some living long while others die young, even if all of them have the same risk of death at a given age. In the end, only a “happy few” end up leaving many offspring. What kinds of luck contribute most to becoming one of the happy few? Is it survival when you’re young? Fast growth when you’re small? Or something else?

Robin Snyder and Stephen Ellner show how to use a popular form of demographic model (integral projection models) to calculate how strongly the probability of having many offspring is affected by luck in survival or growth rate, at different ages or sizes. Working with two empirical examples, they found that for a tropical tree, having many offspring was mostly a matter of surviving long enough, while for a dune shrub, both fast growth and high survival were necessary. In both cases, luck while young and small counts more than luck while older and larger.

Two immature Homo mathematicus individuals competing for a territory.
(Credit: Peter Adler)

To have many offspring is to have high fitness, so this study identifies which parts of the life cycle are most critical for fitness and thus under the highest selective pressure. It also highlights the large role of random chance in lifetime offspring production. Read the Article