Chapter 15, Problem 29A

Summary Introduction

To discuss:

Directional selection of clam muscle size if sea stars eat clams by pulling apart two halves of a clam’s shell.

Introduction:

Natural selection acts to select the individuals that are best adapted to survive and reproduce. It acts on an organism’s phenotype and changes allelic frequencies. Natural selection can alter an organism’s phenotype through stabilizing selection, directional selection and disruptive selection.

Answer to Problem 29A

For a population of clams to survive and reproduce they must develop strong muscles to remain closed for longer periods of time so that sea stars are not able to open them and eat. Directional selection of clam muscle size will make the clams more fit to survive and reproduce. Soon the population will consist of clams with bigger and stronger muscles.

Explanation of Solution

If an extreme version of a trait makes an organism more fit, directional selection might occur. This form of selection increases the expression of the extreme version of a trait in a population.

Sea stars eat clams by pulling apart two halves of a clam’s shell. This forces clams to resist their opening by sea stars. In doing so, they increase their muscular strength. In due course of time, only those clams that can resist opening of their shells or remain closed for long time due to their strong muscles will survive and reproduce.