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For much of the twentieth century it was assumed that genes alone mediate the transmission of biological information across generations and provide the raw material for natural selection. In Extended Heredity, leading evolutionary biologists Russell Bonduriansky and Troy Day challenge this premise. Drawing on the latest research, they demonstrate that what happens during our lifetimes—and even our grandparents’ and great-grandparents’ lifetimes—can influence the features of our descendants. On the basis of these discoveries, Bonduriansky and Day develop an extended concept of heredity that upends ideas about how traits can and cannot be transmitted across generations. Extended Heredity reappraises long-held ideas and opens the door to a new understanding of inheritance and evolution.
Why does heredity need to be extended?
We are at an interesting moment in the history of biology. Classical genetics, molecular biology, and genomics have greatly enriched our understanding of how organisms function, why individuals vary, and how biological variation is transmitted from parents to their offspring. But, along the way, biologists have made many discoveries that can’t be shoehorned into the conventional picture. For example, every first-year biology student learns that “acquired traits” can’t be passed on to descendants, but a great deal of evidence now contradicts this conventional wisdom. Taken together, these discoveries strongly suggest that genes are not the whole story, and that heredity needs to be extended to encompass a variety of non-genetic factors that operate alongside genes.
What does extended heredity have to do with evolution?
Heredity is one of the essential ingredients required for evolution to occur. If some individuals have particular features that enable them to produce more surviving offspring, and if those features are heritable, then those features will be represented in a greater proportion of individuals in the next generation. This simple formula is the essence of Darwin’s theory, and it was developed long before the discovery of genes. But, in the 20th century, evolution came to be defined in purely genetic terms because biologists assumed that only genes could be passed on to descendants. So what happens if there’s more to heredity than genes, and if nongenetic hereditary factors operate by very different rules? As we show, extended heredity broadens our understanding of how evolution works and leads to some surprising conclusions.
Weren’t such ideas—so-called “Lamarckian” or “soft” inheritance—refuted long ago?
The history of heredity—in particular, how heredity came to be defined in exclusively genetic terms—is a fascinating story in its own right. A commonly held view is that, after a lengthy scientific debate involving numerous experiments, the evidence ultimately showed that Mendelian genes (which were later recognized as DNA sequences) are the sole bearers of heredity. The actual history is far messier. In fact, the rejection of nongenetic forms of hereditary was never well-justified by evidence or logic, and current efforts to dismiss nongenetic inheritance as irrelevant to evolution don’t fare much better. On the other hand, some of the arguments made by proponents of an “extended evolutionary synthesis” are problematic as well, and so we search for a firm middle-ground.
What would you say to a skeptic?
Many biologists are wary of such unorthodox ideas, and some simply wonder what the fuss is about. After all, evolutionary biology has been wonderfully successful without extended heredity, so why open this can of worms? But science progresses by constantly updating knowledge and reassessing ideas. Not everyone will agree with our concept of extended heredity, but we hope to at least convince skeptics that non-genetic inheritance is real and should no longer be neglected in evolutionary thinking. There is a wealth of intriguing evidence out there that challenges conventional ideas, and we should confront this evidence and see where it leads us.
Is all this just an academic debate or are there practical implications?
Heredity is extremely relevant to health and many other practical concerns. Although our primary focus is on evolution, we also consider some of the practical implications of extended heredity. For example, we are exposed in our daily lives to many substances, such as the BPA found in certain plastic products, that have been shown to affect embryonic development in other animals. Many people are now aware that maternal smoking or obesity can harm a developing foetus, but few know that paternal lifestyle and health can also affect the foetus by reprograming the genes carried in sperm. There’s a disturbing historical dimension to this as well. It’s hard to believe today, but doctors and scientists used to believe so strongly in the exclusive role of genes in heredity that they denied the possibility that toxins ingested by pregnant women—most notably alcohol—could cause congenital abnormalities. We’ve obviously come a long way since then, but the idea that our children’s health and features could be shaped, not only by the genes that we pass to them, but also by our own lifestyle choices is still not widely appreciated.
Russell Bonduriansky is professor of evolutionary biology at the University of New South Wales in Australia. Troy Day is a professor in the Department of Mathematics and Statistics and the Department of Biology at Queen’s University in Canada. His books include Biocalculus and A Biologist’s Guide to Mathematical Modeling in Ecology and Evolution (Princeton).