Back To CourseCollege Biology: Help and Review
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Angela has taught college Microbiology and has a doctoral degree in Microbiology.
Think about the following scenario: a child decides that she wants to be a bodybuilding champion and earn the Ms. Intergalactic title. She spends six hours every day for ten years in the gym and earns the coveted crown as Ms. Intergalactic! Having accomplished this goal, Ms. Intergalactic decides that since she has worked so hard, she should have children so she can pass on her strength to her children. The question is, will the children born to Ms. Intergalactic inherit her strength?
You may have already correctly assumed that, no, the children would not inherit her strength. However, if you posed that same question in the early 1800s, the theory of evolution that was prevalent at the time would have said that Ms. Intergalactic's children could, in fact, inherit her strength.
Jean-Baptiste Lamarck (1744-1829) was a French naturalist and biologist who laid the groundwork for the current theories of evolution. Scholars before Lamarck had attempted to explain evolution as a natural, organic changing of species, but what made Lamarck stand apart was that he was the first person to develop and present a plausible mechanism for this change.
Lamarck's theory can be boiled down to two broad claims. First, new species evolved from previous species. Today, we know that each organism carries DNA that encodes the characteristics or traits that we associate with its species. This DNA is inherited from parent to offspring and small changes in the DNA over millions of years result in the diversity of species that we see today.
As you can imagine, in the 1700s, DNA had not even been discovered yet, and genetics experiments were not available to Lamarck. He based this tenet on observation and his idea that all species could be ordered from simplest to most complex. By examining this order, you could trace the development of new traits and species up the complexity ladder.
Now, with our contemporary understanding of Charles Darwin's theory of evolution, we know this tenet at its most basic is correct: New species result from small, incremental changes inherited from parent to offspring. Experimental evidence has continued to support the basics of Lamarck's claim. However, the details of his theory, including the ordering of species based on complexity, are not accurate. Organisms found in the fossil record show the same complexity as organisms found today.
Lamarck's second broad claim was, acquired traits are inherited by offspring. Lamarck's second tenet is that this evolution of species occurs by the inheritance of acquired characteristics (like Ms. Intergalactic's strength). We now know that this is incorrect, but let's examine what Lamarck meant by 'acquired characteristics.'
Lamarck's theory stated that evolution worked on individuals, like Ms. Intergalactic. He postulated that any change to an individual during its lifetime would be passed directly to its offspring. So if mom has big muscles, then baby has big muscles. His classic example involved the necks of giraffes.
Lamarck observed that giraffes have disproportionately long necks. He also observed that giraffes seemed to be built perfectly for eating the leaves from the tops of trees. He hypothesized that the original giraffe had a normal sized neck but that by constantly stretching its neck up to reach leaves on higher branches, the neck would become stronger and longer. Any offspring of that giraffe would then inherit a longer and stronger neck. By each individual increasing its neck length a small amount, then passing that on to its descendants, it becomes clear how the neck was able to become so long in the modern giraffe.
While this theory is wrong, the question of why giraffes have such long necks remains, but it can be explained using what we know about DNA and Darwin's theory of evolution. In this theory, giraffes with longer necks are able to eat more leaves, which makes them healthier and able to produce more offspring. The genes, or DNA, that encode this long neck are then inherited by their offspring. Over many years, the number of offspring with the 'long neck' genes outnumbers the offspring with the 'normal neck' genes. Eventually, the 'normal neck' genes are lost, and we have what we see today, long-necked giraffes.
Okay, now back to Lamarck. Besides hypothesizing that excessive use of a structure causes it to become stronger and be passed on to the next generation, he also suggested that the opposite was true. Failure to use an organ would lead to a weakening and eventual loss of that trait in offspring. Lamarck cited blindness in moles that live underground as an example of loss by disuse.
Lamarck's work didn't go unchallenged, however. One infamous challenge came from the German biologist August Weismann. Weismann supported the idea that traits are passed only through sperm and egg cells, and changes to the body had no impact on what offspring would inherit. Weismann's experiment to prove Lamarck wrong involved cutting the tails off mice, mating them, and checking the offspring for tails. Weismann hypothesized that if a mouse without a tail produced offspring without tails it would be the ultimate example of Lamarck's definition of disuse. If Lamarck was correct, the disuse of a tail due to removal would be passed on to successive generations. All told, Weismann cut the tails off 901 mice over five generations. His results? Every mouse, in every generation, was born with a normal tail. This was enough to convince Weismann that Lamarck's theory of inheritance of acquired characteristics did not correctly explain how traits were inherited.
Despite being ultimately wrong about how traits are inherited, Lamarck is important because he was the first person to use observations of the natural world to develop a complete theory of evolution. Without the benefit of technology, he attempted to explain exactly how different species come to exist. By laying this foundation, Lamarck attracted attention to evolution and helped inspire others, including Charles Darwin, to develop their own theories.
Jean-Baptiste Lamarck (1744-1829) was a French naturalist and biologist who laid the groundwork for the current theories of evolution. Lamarck's theory can be boiled down to two broad claims. First, new species evolved from previous species. Second, acquired traits are inherited by offspring.
At its core, the first theory is right, but his second theory ended up being wrong. One infamous challenge came from the German biologist August Weismann. Weismann supported the idea that traits are passed only through sperm and egg cells, and changes to the body had no impact on what offspring would inherit.
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Back To CourseCollege Biology: Help and Review
24 chapters | 433 lessons