Lamarck's Evolutionary Hypotheses

How did Lamarck propose that species evolve?

Darwin wasn't the first scientist to suggest that characteristics of species could change over time. Throughout the eighteenth century, a growing fossil record supported the idea that life somehow evolved. Ideas differed, however, about just how life evolved. The French naturalist Jean-Baptiste Lamarck proposed two of the first hypotheses. Lamarck suggested that organisms could change during their lifetimes by selectively using or not using various parts of their bodies. He also suggested that individuals could pass these acquired traits on to their offspring, enabling species to change over time. Lamarck published his ideas in 1809, the year Darwin was born.

Lamarck's Ideas Lamarck proposed that all organisms have an inborn urge to become more complex and perfect. As a result, organisms change and acquire features that help them live more successfully in their environments. He thought that organisms could change the size or shape of their organs by using their bodies in new ways. According to Lamarck, for example, a water bird could have acquired long legs because it began to wade in deeper water looking for food. As the bird tried to stay above the water's surface, its legs would grow a little longer. Structures of individual organisms could also change if they were not used. If a bird stopped using its wings to fly, for example, its wings would become smaller. Traits altered by an individual organism during its life are called acquired characteristics.

Lamarck also suggested that a bird that acquired a trait, like longer legs, during its lifetime could pass that trait on to its offspring, a principle referred to as inheritance of acquired characteristics. Thus, over a few generations, birds like the one in Figure 16–6 could evolve longer and longer legs.

FIGURE 16–6 Acquired Characteristics? According to Lamarck, this black-necked stilt's long legs were the result of the bird's innate tendency toward perfection. He claimed that if a water bird needs long legs to wade in deep water, it can acquire them by making an effort to stretch and use its legs in new ways. He also claimed that the bird can then pass the trait on to its offspring.

Evaluating Lamarck's Hypotheses Today, we know that Lamarck's hypotheses were incorrect in several ways. For one thing, organisms don't have an inborn drive to become more perfect. Evolution does not mean that over time a species becomes “better” somehow, and evolution does not progress in a predetermined direction. We now also know that traits acquired by individuals during their lifetime cannot be passed on to offspring. However, Lamarck was one of the first naturalists to suggest that species are not fixed. He was among the first to try to explain evolution scientifically using natural processes. He also recognized that there is a link between an organism's environment and its body structures. So, although Lamarck's explanation of evolutionary change was wrong, his work paved the way for later biologists, including Darwin.

In Your Notebook Why are Lamarck's ideas called scientific hypotheses and not scientific theories?

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