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As cells divide, they accumulate mutations that can sometimes cause cancer. Gametes have to divide like any other cell, and thus generation to generation mutations should accumulate in people's genomes. However, it doesn't appear to be the case that cancer rates increase as a species ages. Why isn't this the case? Is the process of development itself a filter that ensures that most of the cells in the body are "far from cancer"?

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  • $\begingroup$ If an organism grew from a mutated gamete, they will carry that mutation. And if the mutation is sufficiently debilitating, they won't properly develop or survive as well as other non-mutated members of the species. So the mutation would, in theory, be filtered out by natural selection. $\endgroup$ – Jam Dec 31 '18 at 21:44
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    $\begingroup$ This Wikipedia page is relevant. Individuals can inherit germline mutated forms of the TP53 gene and have a higher cancer risk. But organisms with a high cancer risk are less viable than those with a low cancer risk so their form of the gene is filtered out. $\endgroup$ – Jam Dec 31 '18 at 22:00
  • $\begingroup$ @Jam presumably this selection pressure will decrease as medical treatments become better - so we would see cancer rates increasing for younger patients in the future? $\endgroup$ – Mike Flynn Jan 7 at 19:24
  • $\begingroup$ I see what you're suggesting but I don't really know. In theory, you might be right but I think the incidence rate of the mutation would need to stay constant, and the cancer would need to affect people early enough to not affect whether they can have children. $\endgroup$ – Jam Jan 8 at 13:50
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Species age

However, it doesn't appear to be the case that cancer rates increase as a species ages

Species don't have an age. All lineages on earth are about 3.5 billions years old. No, lineage is younger or older than that. A lineage does not get its genetics magically reset when the lineage gets a new species name. For more information about the standard misunderstandings about the concept of species, please have a look at the post How could humans have interbred with Neanderthals if we're a different species?

In all cases, there is some equilibrium between the selection pressures brought about mutations that are linked to cancer (whether mutations on oncogenes or on tumor suppressor genes).

Individual age

Individual, do have an age though. Cells, within multicellular individual replicate in order to ensure the multicellular individual growth and maintenance. Those cell lineage can accumulate mutations that could be linked to cancer development. Within the individual, these mutations are not deleterious and are therefore not selected against. Actually, in case of a cancer, those mutations are, by definition, beneficial as cancerous cells proliferate (have a large fitness). As a result one should expect that old organisms are at greater risk of developing cancer than younger organisms. In humans, this correlation exists, however, the statistics are slightly more complex than that. Typically, different types of cancer seem to show different age associations. See this article from the NIH as a start for more information.

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  • $\begingroup$ I understand that species don't age, my question is "why don't they age? why haven't all species gone extinct from cancer?" $\endgroup$ – Mike Flynn Dec 31 '18 at 21:33
  • $\begingroup$ @Mike Flynn: Basically, because the individuals carrying the cancer-causing mutation die before they can reproduce, and thus pass the mutation to their offspring. A current example is the Tasmanian Devil, which seems likely to be rendered extinct by a particular cancer to which they're subject. $\endgroup$ – jamesqf Jan 1 at 4:34
  • $\begingroup$ @MikeFlynn Is your question something like "How does selection gets rid of deleterious mutations?", then? $\endgroup$ – Remi.b Jan 1 at 21:46

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