On a pedigree the squares represent the males and the circles represent the females. If the shape is colored then that person has whatever trait is being shown. Since the genotypes of males are "XY" they would inherit the sex-linked recessive trait, because they would only inherit a Y from their father. In a girl it would be different because she would have two X's, one from mom and one from dad, the genotype of a girl is "XX". She would have to inherit a bad x from mom and dad to have the trait. If the father has the sex-linked recessive trait then his son will also have it because he only has one Y to give to the offspring. With his daughter it would be different, because he would pass down an X, assuming the X from the mother is a good X, then the child wouldn't inherit the trait, since the trait is recessive.
* Not all organisms are the same and the differences can be passed down.
*Not all offspring can survive and sometimes the ones that do survive can't produce offspring.
*Since not all offspring can survive, the offspring fight for the limited things.
*Each individual offspring has its own good traits and bad traits. All organisms are different. The individuals that are best fit have more success in reproduction. The offspring are more inevitable to survive or have what it needs to survive. Other organisms have less offspring or they die. The species change over time.
*Species alive now, are descended from ancestral species.