Let us clarify the principles of evolution by addressing a few misconceptions.
• Does the use or disuse of some structure or be-havior cause an evolutionary increase or decrease in 16 Chapter 1 The Major Issues
try it yourself O N L I N E
that feature? You may have heard people say some-thing like, “Because we hardly ever use our little toes, they will get smaller and smaller in each succeeding generation.” This idea is a carryover of the biologist
Jean Lamarck’s theory of evolution through the inher-itance of acquired characteristics, known as Lamarck-ian evolution.According to this idea, if giraffes stretch their necks as far out as possible, their offspring will
1.2 The Genetics of Behavior 17 Figure 1.10 Evolutionary trees
(a) Evolutionary relationships among mammals, birds, and several kinds of reptiles.
(b) Evolutionary relationships among various species of mammals.
(a)
Dinosaurs
Turtles Mammals Lizards Snakes Crocodiles Birds
Platypus (b)
Millions of years ago
0 2.5 7 25 38 54 65 135 195
Cattle and sheep
Insectivores Rodents Marsupials
Bats
Humans
Carnivores Rabbits
Apes Whales
and dolphins
Early mammal-like reptiles
Elephants
Monkeys Horses and
rhinoceroses
be born with longer necks. Similarly, if you exercise your arm muscles, your children will be born with big-ger arm muscles, and if you fail to use your little toes, your children’s little toes will be smaller than yours.
However, biologists have found no mechanism for La-marckian evolution to occur and no evidence that it does. Using or failing to use some body structure does not change the genes.
(It is possible that people’s little toes might shrink in future evolution if people with even smaller little toes have some advantage over other people. But we would have to wait for a mutation that decreases little toe size—without causing some other problem—and then we would have to wait for people with this mu-tation to outreproduce people with other genes.)
• Have humans stopped evolving? Because mod-ern medicine can keep almost anyone alive, and be-cause welfare programs in prosperous countries pro-vide the necessities of life for almost everyone, some people assert that humans are no longer subject to the principle of “survival of the fittest.” Therefore, the ar-gument goes, human evolution has slowed or stopped.
The flaw in this argument is that the key to evolu-tion is not survival but reproducevolu-tion. For you to spread your genes, of course you have to survive long enough to reproduce, but what counts is how many healthy children (and nieces and nephews) you have. Thus, keeping everyone alive doesn’t stop human evolution.
If some people have more children than others do, their genes will spread in the population.
• Does “evolution” mean “improvement”? It de-pends on what you mean by “improvement.” By defini-tion, evolution improves the average fitness of the
pop-ulation, which is operationally defined as the number of copies of one’s genes that endure in later genera-tions.For example, if you have more children than av-erage, you are by definition evolutionarily fit, regardless of whether you are successful in any other sense. You also increase your fitness by supporting your brother, sister, nieces and nephews, or anyone else with the same genes you have. Any gene that spreads is by def-inition fit. However, genes that increase fitness at one time and place might be disadvantageous after a change in the environment. For example, the colorful tail feath-ers of the male peacock enable it to attract females but might become disadvantageous in the presence of a new predator that responds to bright colors. In other words, the genes of the current generation evolved be-cause they were fit for previous generations; they may or may not be adaptive in the future.
• Does evolution act to benefit the individual or the species? Neither: It benefits the genes! In a sense, you don’t use your genes to reproduce yourself; rather, your genes use you to reproduce themselves (Dawkins, 1989). A gene spreads through a population if and only if the individuals bearing that gene reproduce more than other individuals do. For example, imagine a gene that causes you to risk your life to protect your chil-dren. That gene will spread through the population, even though it endangers you personally, provided It is possible to slow the rate of evolution but not just
by keeping everyone alive. China has enacted a policy that attempts to limit each family to one child. Successful enforcement of this policy would certainly limit the possibility of genetic changes between generations.
© Alain Le Garsmeur/CORBIS
Sometimes a sexual display, such as a peacock’s spread of its tail feathers, leads to great reproductive success and therefore to the spread of the associated genes. In a slightly changed environment, this gene could become maladaptive. For example, if an aggressive predator with good color vision enters the range of the peacock, the bird’s slow movement and colorful feathers could seal its doom.
© F. J. Hierschel/Okapia/Photo Researchers
18 Chapter 1 The Major Issues
that it enables you to leave behind more surviving children than you would have otherwise. A gene that causes you to attack other members of your species to benefit your children could also spread, even though it harms the species in general, presuming that the be-havior really does benefit your children, and that oth-ers of your species do not attack you or your children in retaliation.
S T O P & C H E C K
5. Many people believe the human appendix is useless.
Should we therefore expect that it will grow smaller from one generation to the next?
Check your answer on page 21.