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    • #17975

      Another quick topic here I’m a bit confused about.

      Why would a species have evolved to the extreme of r-selection(=semelparity)?
      i.e. only one reproductive episode before death, like the Mayfly.

      Same thing with facilitation, with one species doing all the work, despite no direct benefit (beavers and dams).
      Even taking relative fitness into account, I still can’t see the mechanism behind it?

      Would anyone be able to enlighten me?

    • #115605

      Meant ‘K-Selected’ !

    • #115612

      You meant r selected. This given semelparous species we shall call A. Now A, being an annual species, has a very short life history. Its entire life cycle will be completed in a year or less, and thus we might presume that during this time, it has to go about its business of development, maturation and reproduction. Typically such a species will be growing throughout its life and reproducing only near the end of its life cycle, producing a large number of offspring in one bout. Naturally due to the short life cycle there is no overlap of generations, and the the only investment in each offspring is in the form of food reserves.
      If this sounds familiar to you, they are the characteristics of an r-selected species: Large number of offspring, low amount of parental investment. This is the most optimal strategy for an annual species as due to the life cycle there is a) little opportunity for repeated reproduction and b) Little opportunity for parental care. All of this deduced from just the fact that A is semelparous.
      If the environment is highly variable, investment in a small number of offspring as in iteroparity will not be optimal, due to the risk of bad years costing the life of the parent (and thus losing any offspring she might have produced in the future if she had not died). Thus the fitness of the parent is lowered as a result. (Remember if she had managed to produce these offspring she will have been more successful in terms of fitness. )
      Thus semelparity is better as the parent can now reproduce when conditions are temporarily favourable, producing a large brood and exploiting this environment, allowing the best chances of offspring survival, hence maximizing fitness.
      Fecundity(future reproduction) vs offspring (current reproduction)
      There are costs to both iteroparity and semelparity. This is illustrated above. For a semelaprous species, the cost is fecundity. If one dies while reproducing, then one can never reproduce again in the future. Simple. For an iteroparous animal with repeated reproduction, the cost is in terms of current reproduction. If you wish to be able to survive to reproduce again in the future, you must necessarily not risk all of your fitness for the current brood. For example, if you were say a cat rearing a litter, you must decide how much milk you are willing to give to your kittens. Giving too much could endanger your life by weakening you physically. At some point you have to compromise on investing too much in this litter to save for the next (or risk not being able to have a next litter).
      For more information read up on Life history theory.
      here this cost will be much smaller for our species if it was semelparous as although its investment in each offspring is small, the cost of reproduction is large because of the large number of offspring, however, whereas this will hurt the ability of a K selected iteroparous animal to reproduce in the future, it will not be as damaging to a r selected semelparous species because the species will have no need for future reproduction.
      Evolutionarily, it will have made sense for an iteroparous animal to evolve semelparity. If some genetic change caused an individual to invest more in each brood for example, by laying more eggs for the case of the mayfly, then in a highly variable environment where the parent is uncertain whether she will survive, it will be beneficial to invest more in the current offspring rather than the future offspring. Thus overtime mayflies invest more and more in their current offspring to the point where they die after reproducing.
      I will answer 2 in another post.

    • #115614

      Of course! Thanks very much. For some reason I took one reproductive event as single progeny. Thank you very much for clarifying for me.

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