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There aren't really any good evolutionary explanations for the decrease in sperm count. Changes in the distribution and frequency of any given genotype/phenotyp

There aren't really any good evolutionary explanations for the decrease in sperm count. Changes in the distribution and frequency of any given genotype/phenotype literally takes generations to occur, and would require some kind of selective pressure (as you say, competition and medically induced fertility are possibilities). This is happening too fast and too uniformly.

jondubois 8y ago
>> would require some kind of selective pressure Not selective pressure, but lack of it. When you take away selective pressure for a specific trait, organisms become weaker on average in respect to that trait; when a trait loses importance, populations divert their resources to different traits.
- Dylan16807 8y ago
  For a gene to decay without selective pressure takes a very long time, especially when your population is not on the edge of starvation.
  - londons_explore 8y ago
    A selective pressure going away is as strong as a selective pressure appearing. Eg. 100 years ago, 20% of children would die as infants. Imagine all of those children died of a mutation which causes half the usual sperm count, together with a lack of resistance to polio/cholora. Now that polio/cholora don't exist, all those people survive, and within 3 generations, nearly everyone has at least a grandparent with that 'low sperm count' mutation.
    - Dylan16807 8y ago
      > Eg. 100 years ago, 20% of children would die as infants. Imagine all of those children died of a mutation which causes half the usual sperm count, together with a lack of resistance to polio/cholora That wouldn't be a steady state. A gene that powerfully negative, with no upsides, would decrease in prevalence by a significant amount every generation. > Now that polio/cholora don't exist, all those people survive, and within 3 generations, nearly everyone has at least a grandparent with that 'low sperm count' mutation. Everyone has an ancestor with the gene. But they only have a 1/8, 1/16, 1/32 chance of inheriting it from that specific ancestor. Generation to generation, the percentage of people with the gene will be almost identical. Just like eye and hair color.

jondubois 8y ago

>> would require some kind of selective pressure Not selective pressure, but lack of it. When you take away selective pressure for a specific trait, organisms become weaker on average in respect to that trait; when a trait loses importance, populations divert their resources to different traits.

Dylan16807 8y ago
For a gene to decay without selective pressure takes a very long time, especially when your population is not on the edge of starvation.
- londons_explore 8y ago
  A selective pressure going away is as strong as a selective pressure appearing. Eg. 100 years ago, 20% of children would die as infants. Imagine all of those children died of a mutation which causes half the usual sperm count, together with a lack of resistance to polio/cholora. Now that polio/cholora don't exist, all those people survive, and within 3 generations, nearly everyone has at least a grandparent with that 'low sperm count' mutation.
  - Dylan16807 8y ago
    > Eg. 100 years ago, 20% of children would die as infants. Imagine all of those children died of a mutation which causes half the usual sperm count, together with a lack of resistance to polio/cholora That wouldn't be a steady state. A gene that powerfully negative, with no upsides, would decrease in prevalence by a significant amount every generation. > Now that polio/cholora don't exist, all those people survive, and within 3 generations, nearly everyone has at least a grandparent with that 'low sperm count' mutation. Everyone has an ancestor with the gene. But they only have a 1/8, 1/16, 1/32 chance of inheriting it from that specific ancestor. Generation to generation, the percentage of people with the gene will be almost identical. Just like eye and hair color.