Why is more than one reproductive isolation barrier needed to completely isolate a species?

Which of the following is a period in the paleozoic era? devonian cambrian permian all of the above

In this picture, groups of pigment that capture light energy are located on the

Which statements explains what the fossil records indicates? check all that apply. a) earth is 4.5 billion years old b) many organisms have become extinct c) mammals have always existed on earth d) mass extinctions have occurred several times on earth e) the amount of water on earth has decreased over time?