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I recently saw a few sources stating that Crocodiles do not biologically age, implying negligible senescence. Basically, a 70 year old crocodile is in the same physical/health condition as a 7 year old crocodile. However, crocodiles are K-selected, meaning that populations are generally close to their carrying capacity. If they do not exhibit natural senescence, isn't that evolutionarily disadvantageous, because the younger crocodiles with more slightly more fit genes will not necessarily replace the older generations, meaning that crocodile populations would adapt to environmental changes significantly slowly. Can someone please explain to me how this lack of senescence can apply for this type of species?

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    $\begingroup$ Your premise is almost certainly wrong. As a rule of thumb, virtually every popular account that claims certain animals don't age is bullshit. Unless you have a peer-reviewed paper that explicitly states the animals doesn't undergo senescence, assume that it does. In this case this paper offers an unconfirmed claim that old crocodiles are senescent. $\endgroup$ – iayork May 31 '16 at 13:59
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You seem to use the term evolutionary advantageous vs disadvantageous for the species rather than for individuals. It is a very common misconception from the general public. You might want to follow an introductory course to evolutionary biology such as Understanding Evolution (UC Berkeley) for example.

younger crocodiles with more slightly more fit genes

"fit genes" does not mean much. Again an introductory course to evolutionary biology should help you there.

Answer

You'd right though that absence of senescence causes longer generation time and therefore lower response to selection pressure (such as those caused by an environmental change). But populations don't evolve in order to maximize the probability of the survival of this population (related to first comment above).

Those that live longer, leave more offspring and therefore long-lived individuals are expected to dominate the population in the long run. The evolution of senescence is not exactly that easy though otherwise no species would show any sign of senescence (see Why does evolution not make our life longer?).

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  • $\begingroup$ Sorry, I'm a bit confused on two things. Firstly, for the younger generation being fitter, doesn't that generally apply in times of selective pressure? In this case, the slightly more fit individuals will reproduce a bit more and thus, more offspring will carry on those genes. And also, during selective pressure, more fit individuals will reproduce, so generally the alleles for the more advantageous genes will appear in higher frequencies, causing the population to adapt to environmental stresses. $\endgroup$ – Ham Radio May 30 '16 at 17:52
  • $\begingroup$ Yes, you are right, the expected the change in population mean fitness is increasing by the value of the additive genetic variance in the previous generation. Of course, it is only the expectation from a distribution. I removed this small piece of sentence. $\endgroup$ – Remi.b May 30 '16 at 17:56
  • $\begingroup$ The main point is that you seem to think that what is increases the probability of a population to survive should be selected for while this is not what natural selection does. $\endgroup$ – Remi.b May 30 '16 at 17:58
  • $\begingroup$ So as of crocodiles, they have been able to maintain the negligible senescence due to a lack of selective pressures? Otherwise, crocodilian populations would have lower responses to environmental stresses. $\endgroup$ – Ham Radio May 30 '16 at 18:02
  • $\begingroup$ It is possible that an eventual quasi-absence of external causes of mortality (such as predation and parasitism) would remove the interaction between age and reproductive success resulting in more efficient elimination of old-age deleterious mutations. I realize I might use concepts you could be unaware of here. I would strongly recommend that you read Why does evolution not make our life longer?, you will like this post! $\endgroup$ – Remi.b May 30 '16 at 18:22

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