The fossil record shows that the average size of black bears was large during cold glacial periods and got smaller during warmer time periods. what is this an example of?
directional selection
disruptive selection
sexual selection
stabilizing selection

The correct answer is -

directional selection.

Directional selection is one of the three modes of natural selection where an extreme trait of the population is selected over other traits, which causes allele frequency to change over time in one particular direction (that is with respect to the phenotype).

As per the information in the question, in the fossil records, average size of black bears became large and small in their respective cold glacial and warmer periods.

Thus, it indicates directional selection as extreme phenotypes (large and small) are selected during the climatic periods.

you and your brother should be dizygotic twins (fraternal) twins since identical twins can only be one gender: boy and boy   or   girl and girl.

the answer is "a.) well–developed vascular system."

explanation: