Reproductive Success (RS) is defined in many ways in different places by different people, but one of the most common definitions is simply the number of offspring an individual produces. This definition is further modified in most cases to mean only those individuals that will be fertile, i.e., capable of producing further offspring. RS is important in understanding Natural Selection (NS). In the simplest model, a heritable feature that increases RS will be selected for over time in a population because individuals with higher RS will contribute more offspring to future generations and this, in turn, causes the frequency of the RS-enhancing allele (gene variant) to become more common over generational time in that population.

Fitness is a property of an allele that refers to its relative likelihood of representation in future generations in a population. An allele with higher fitness will be more likely to be represented in future generations within a population than an allele for the same gene with lower fitness. The important thing here is that the likelihood of future representation has to be due to a feature of that allele, and not random effects.

At first glance RS and Fitness are the same, or similar, but one might immediately notice that RS is a feature of an individual (that has offspring) while fitness is a feature of an allele. So, it is possible that a given individual will have a relatively high RS but contain a particular allele with low fitness. Presumably the higher RS of that individual is due to high-fitness alleles of other genes. In this way, fitness and RS are different, but when considering large scales of time and large populations, the two can be (perhaps) safely conflated because things average out over time and the different alleles are being independently assorted over generational time, so each allele gets to have its day, sometimes, independently of other lower-fitness alleles. By this way of thinking, RS and fitness can be safely considered as measures of roughly the same thing, but with caveats.

RS is usually measured, in actual experimental work or field observations, as the number of offspring observed for an individual, but to make sure that RS is correlated with fitness, one might measure grand-offspring in order to factor out infertile offspring and other factors that may affect one generation but that do not apply over the long term. Again, RS and fitness are then, it would seem, equatable but with caveats.

RS is the number of offspring or grand-offspring but kin selection may apply as well. This is where an individual foregoes some of its own reproduction for the benefit of a relative, causing indirect fitness, a measure of this contribution devalued by the probability of the two individuals sharing the same allele by common descent. One can state that a measure of RS is still a measure of fitness because over the long term, again, things average out, but equating fitness and RS is done, again, with caveats.

There may be an optimal number of offspring an individual may have, above which longer term reproduction is reduced. A litter that is too large may result in a set of adults that are smaller than ideal and will thus have fewer offspring, or in the case of serial reproduction, if parental investment is spread out over several offspring, having too many offspring in a row may cause a deficit for all of the offspring, or for the later offspring that get less care because less energy is available, or earlier offspring may get short changed by being left on their own sooner. Putting this another way, the ultimate long-germ fitness strategy may be to have X offspring, where having more or fewer than X results in a suboptimal outcome. In this way, increasing RS from zero towards X increases fitness, but increasing RS beyond X decreases fitness.

So, RS equals fitness except:

  • RS is a measure applied to an individual while fitness is ideally applied to alleles for a gene or some other genetic construct;
  • The offspring-fertility link can be misleading. A queen bee with an allele that allows her to produces more sterile offspring may also produce more fertile offspring;
  • RS is fitness plus or minus random effects;
  • RS usually does not consider indirect fitness;
  • RS is selected to be optimized while fitness is selected to be maximized.

Equating RS and fitness is therefore only a rough approximation. When initially learning about Natural Selection students are often led to believe that RS and fitness are the same, which is only true with these (and possibly other) caveats. Equating RS and fitness in pedagogy risks skipping past and perhaps never understanding the caveats, and these caveats are very far from trivial. They are, in many cases, the point of specific evolutionary research projects.


  1. #1 JohnR
    July 19, 2013

    Excellent point, and worth making; as is usually the case, shorthand approximations are generally helpful, but may be a bit misleading. For those of us who muddle along as naturalists and strive for the [airy wave of hand] Big Picture, it’s usually enough to link the gene-influenced factor of some kind to an increased generational output to an increased relative genetic presence over time in a population. However, as you note, for those doing the real digging, a more nuanced view is important. That also helps to spawn beer-enhanced arguments over definitions, which, I still believe, is where the real spade-work of science gets done.

  2. #2 Greg Laden
    July 19, 2013

    “That also helps to spawn beer-enhanced arguments over definitions, which, I still believe, is where the real spade-work of science gets done.”

    And frequently there must be a napkin.

  3. #3 Jim Thomerson
    July 19, 2013

    I understand ‘Simple Darwinian Fitness’ to be a relative measure of how many offspring an individual successfully raises to sexual maturity, compared to the rest of the population. Anyone know where this definition comes from, as I do not recall? This is not the same as crude reproductive success; total number of offspring produced.

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  5. #5 Jim K
    July 19, 2013

    Wonder what effect a “greedy” allele might have = perhaps continuing to eat despite supposedly being sated; or (if one can have it) the sort of allele affecting behaviour whose accumulation in society led to (the production of) Easter Island, if you get my drift. Potential links to “my king/Party/Country right or wrong” ?

  6. #6 Jim K
    July 19, 2013

    P.S. Beer & napkins all round…

  7. #7 daedalus2u
    July 20, 2013

    This is one of the things that I am working on right now. There are “greedy alleles”, they code for an endonuclease that causes a double-strand break and inserts itself into the genome. I suspect that preventing lethal overwhelming of the genome by such things is why eukaryotes have limited DNA replication fidelity. It isn’t a bug, it is a feature that prevents selfish DNA from selfishly replicating itself.

  8. [...] Reproductive Success and Fitness are not the same thing � Greg … [...]

  9. [...] the time, so it’s interesting to see how they definitions are much more nuanced than people Reproductive success and fitness are not the same thing and can be found over at Greg Laden’s blog (imaginative [...]

  10. #10 john b.
    January 7, 2014

    you have a good point here. overall, fitness has to be present in order to increase chances of RS and RS can also be a probable result of good fitness.

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