Charles Darwin is widely regarded as the father of the modern science of evolution. Indeed, his ideas—particularly those published in his 1859 book The Origin of Species and his 1871 book The Descent of Man—are still celebrated for how far-seeing and essentially correct we now recognize them to be.
And Darwin’s theory of evolution has had an enormous impact. In fact, since the middle of the 20th century when Darwin’s ideas were combined with an emerging understanding of genetics, evolution has become the central framework that unites all other subfields of biology.
But despite the great many things about which Darwin was correct, he was wrong about others.
Perhaps the single most important thing that Darwin got wrong was the nature of heredity. Understanding how traits are passed from generation to generation was the missing link in Darwin’s theory. And when that mystery was eventually solved, the plausibility of his theory suddenly snapped into focus, laying the foundation for all of modern biology.
The first studies that unlocked the mystery of heredity were done around the same time as Darwin was developing and publishing his theory by a little known Augustian monk named Gregor Mendel. Mendel’s approach to understanding heredity involved meticulously planting and cross-breeding different varieties of pea plants. He carried out these experiments in a small garden greenhouse at the monastery where he lived and worked.
The results of Mendel’s experiments showed that traits of pea plants, like the color of their flowers and whether the seed coats were smooth or wrinkly, acted in a predictable manner. Importantly, he also noted that the traits did not blend together as previously thought: crossing purple-flowered pea plants with white-flowered pea plants does not give you light purple or pink-colored pea plants.
Learn more about genome mutations.
Mendel realized that his studies had implications for more than just pea plants– they were clues about the nature of heredity. Unfortunately Mendel’s work would not become widely known to the scientific community for several decades, until they were rediscovered in 1900.
In the meantime, Darwin developed his own theory of heredity, called pangenesis. In a book called The Variation of Animals and Plants Under Domestication, published in 1868, Darwin suggested cells all over the body produce small particles he called gemmules that travel to the sex cells, where they become incorporated into sperm or eggs. Darwin suggested that when a sperm fertilizes an egg, the gemmules inside them combine to give offspring a full mixture of all bodily traits present at the time of conception on the bodies of both the mother and father.
We now recognize three ways in which Darwin’s pangenesis idea was flawed.
Learn more about what Darwin knew and why it matters.
First, gemmules don’t exist. Instead, we now know that traits are encoded by the sequence of the DNA bases adenine (A), guanine (G), thymine (T), and cytosine (C) that make up our genomes.
Second, Darwin assumed that the traits of each parent would always blend together, like different colors of paint. As Mendel discovered, traits do not actually blend together. A pea plant with wrinkly seeds and a pea plant with smooth seeds will produce hybrid offspring that have smooth seeds, NOT semi-wrinkly seeds or some other intermediate trait.
Third, Darwin imagined that gemmules bringing information from around the body at the time of reproduction would include traits acquired during an individual’s lifetime. According to Darwin’s pangenesis theory, a gymnast that developed strong, flexible muscles through years of training would be expected to have children that inherit their mother’s strength and flexibility because these would be passed along through gemmules. In this regard, Darwin’s theory of pangenesis echoed the ideas of an earlier evolutionary thinker, French biologist Jean Baptiste Lamarck, who suggested that giraffes evolved their long necks because each generation giraffes stretched their heads a little closer to to the treetops, and passed their slightly longer necks to their offspring.
Darwin imagined that that gemmules bringing information from around the body at the time of reproduction would include traits acquired during an individual’s lifetime.
The great German biologist August Weismann would later prove that Darwin and Lamarck were wrong about the heritability of acquired traits through a simple experiment using mice. Weismann amputated the tails of mice and allowed them to breed, repeating the procedure over five generations of mice. Out of 901 amputees, not a single tailless mouse was born, showing that acquired traits are not heritable.
On the other hand, discoveries in the 21st century have caused biologists to reconsider whether acquired traits can ever be heritable. Researchers studying the biological side effects of a fungicide called vinclozolin found that rats exposed to the chemical when they were pregnant gave birth to male pups that developed abnormal sperm as adults. The real surprise came when the descendants of the rats who were exposed as fetuses also developed deformed sperm. It wasn’t just the children of the rats exposed as fetuses, but also their grandchildren and great-grandchildren!
We now know this happens when certain molecules, called methyl groups, are attached to the parts of the genomes that code for proteins, called genes. Methyl groups being added or deleted can affect whether — and when — those genes are turned on or off. A gene that is turned on is using the information encoded in its sequence of nucleotide bases to make proteins. Genes that are turned off are not making proteins.
Offspring inherit not only their parents’ genes but also the methyl groups attached to their genes. This is known as epigenetic inheritance. While most epigenetic effects resulting from methlyation seem to disappear after a few generations, another important part of inheritance is neither genetic, nor epigenetic: we call it learning.
Learn more about gene flow vs. natural selection.
Birds, whales, and primates, all use teaching and learning to pass information between generations. Humpback whales and white-crowned sparrows learn to sing by listening to the songs of their parents as well as those of other individuals. For species capable of cultural learning, evolution goes beyond genetic adaptation, making it possible to survive a larger variety of challenges.
So, while Darwin may have been wrong about the specifics of how heredity works, we are still discovering new ways in which his ideas were ahead of their time.
Professor Scott Solomon, Ph.D. is an Associate Teaching Professor at Rice University, where he teaches ecology, evolutionary biology, and scientific communication. You can stream his course What Darwin Didn’t Know: The Modern Science of Evolution Wondrium.
