What is the selective pressure in this example of natural selection?

When organisms are classified ,they are given a two part name

Error analysis: what might be the reason that some of your percentages didn't exactly match your predicted results? gametes aren't responsible for inheritance. mice don't have large litters, so the sample size was not large enough. the wrong type of mice were used.

If a human has 23 pairs of chromosomes in every muscle cell of its body how many chromosomes will be in a human egg or sperm