When researchers at Emory University in Atlanta trained mice to fear the smell of almonds (by pairing it with electric shocks), they found, to their consternation, that both the children and grandchildren of these mice were spontaneously afraid of the same smell. That is not supposed to happen. Generations of schoolchildren have been taught that the inheritance of acquired characteristics is impossible. A mouse should not be born with something its parents have learned during their lifetimes, any more than a mouse that loses its tail in an accident should give birth to tailless mice.
If you are not a biologist, you’d be forgiven for being confused about the state of evolutionary science. Modern evolutionary biology dates back to a synthesis that emerged around the 1940s-60s, which married Charles Darwin’s mechanism of natural selection with Gregor Mendel’s discoveries of how genes are inherited. The traditional, and still dominant, view is that adaptations – from the human brain to the peacock’s tail – are fully and satisfactorily explained by natural selection (and subsequent inheritance). Yet as novel ideas flood in from genomics, epigenetics and developmental biology, most evolutionists agree that their field is in flux. Much of the data implies that evolution is more complex than we once assumed.
Some evolutionary biologists, myself included, are calling for a broader characterisation of evolutionary theory, known as the extended evolutionary synthesis (EES). A central issue is whether what happens to organisms during their lifetime – their development – can play important and previously unanticipated roles in evolution. The orthodox view has been that developmental processes are largely irrelevant to evolution, but the EES views them as pivotal. Protagonists with authoritative credentials square up on both sides of this debate, with big-shot professors at Ivy League universities and members of national academies going head-to-head over the mechanisms of evolution. Some people are even starting to wonder if a revolution is on the cards.
In his book On Human Nature (1978), the evolutionary biologist Edward O Wilson claimed that human culture is held on a genetic leash. The metaphor was contentious for two reasons. First, as we’ll see, it’s no less true that culture holds genes on a leash. Second, while there must be a genetic propensity for cultural learning, few cultural differences can be explained by underlying genetic differences.
Nonetheless, the phrase has explanatory potential. Imagine a dog-walker (the genes) struggling to retain control of a brawny mastiff (human culture). The pair’s trajectory (the pathway of evolution) reflects the outcome of the struggle. Now imagine the same dog-walker struggling with multiple dogs, on leashes of varied lengths, with each dog tugging in different directions. All these tugs represent the influence of developmental factors, including epigenetics, antibodies and hormones passed on by parents, as well as the ecological legacies and culture they bequeath.
Image courtesy the author.
The struggling dog-walker is a good metaphor for how EES views the adaptive process. Does this require a revolution in evolution? Before we can answer this question, we need to examine how science works. The best authorities here are not biologists but philosophers and historians of science. Thomas Kuhn’s book The Structure of Scientific Revolutions (1962) popularised the idea that sciences change through revolutions in understanding. These ‘paradigm shifts’ were thought to follow a crisis of confidence in the old theory that arose through the accumulation of conflicting data.
Then there’s Karl Popper, and his conjecture that scientific theories can’t be proven but can be falsified. Consider the hypothesis: ‘All sheep are white.’ Popper maintained that no amount of positive findings consistent with this hypothesis could prove it to be correct, since one could never rule out the chance that a conflicting data-point might arise in the future; conversely, the observation of a single black sheep would decisively prove the hypothesis to be false. He maintained that scientists should strive to carry out critical experiments that could potentially falsify their theories.
Everything from diet to air pollution to parental behaviour can influence gene expression
While Kuhn and Popper’s ideas are well-known, they remain disputed and contentious in the eyes of philosophers and historians. Contemporary thinking in these fields is better captured by the Hungarian philosopher Imre Lakatos in The Methodology of Scientific Research Programmes (1978):
... continue reading