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Evolutionary Biologists Can Learn from Catholics

May 30, 2024

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As Catholics, we follow Pope St. John Paul II in believing that “science can purify religion from error and superstition.” The most prominent modern example of such purification is the contribution of evolutionary biologists to a Catholic understanding of the origin and development of life on earth. Currently, however, evolutionary biologists may have more to learn from Catholics than vice versa.

This may surprise those outside the world of science who often assume that evolution is a settled dogma in biology. In reality, evolution remains very controversial, and in recent years, its theorists have run into several massive stumbling blocks. These include the apparent impossibility of defining the concept of species, the incoherence of attributing goal-oriented adaptations to chance, and the incapability of mere matter to account for “emergent” phenomena like self-consciousness.

These stumbling blocks present no challenge to the observed facts of evolution, and yet, for many serious thinkers, they still appear insurmountable. And perhaps they are insurmountable if biologists remain constrained by the modern secular worldview. With the aid of the Catholic intellectual tradition, however, the philosophy of Aristotle and St. Thomas Aquinas could provide a ready path to their overcoming.

Consider the definition of “species.” Evolution requires all life to be divisible into distinct types. Otherwise, one cannot speak meaningfully about one life form changing into another, but only about an innumerable series of unique individuals. But based on observations of external traits and genetic analyses alone, it is surprisingly difficult to pin down what constitutes a type. Is there such a thing as a lion, for example, if no two “lions” are physically identical?

The language of nature and form has been sustained through the ages primarily not by science, but by the Church . . .

Some of us learned in school that species are defined by their individuals’ ability to produce fertile offspring together. This definition makes little sense, however, when applied to populations that are sexually compatible but live separately and appear and behave differently from each other. And it makes no sense at all when applied to organisms that reproduce asexually, like bacteria. Consequently, most biologists consider the reproductive species definition to be insufficient.

But the need for an adequate definition of species remains. To quote the evolutionary biologist Ernst Mayr in Animal Species and Evolution, “Whoever . . . denies that species are non-arbitrarily defined units of nature . . . evades the issue.” This makes the scientific community’s inability to settle on a species definition all the more maddening.

Enter Aristotle and St. Thomas. These thinkers, both keen students of the natural world, define species as sets of diverse individuals that nevertheless partake in a common metaphysical nature or “substantial form” that is manifested in their bodies. This definition is coherent because it is not circular. It does not attempt to make sense of the physical with the physical, but grounds our understanding of the world in metaphysical realities. It is also perfectly compatible with evolution, as neither Aristotle nor St. Thomas denies the ability of a lineage of organisms’ “primary matter” to receive a new nature or form.

Of course, the language of nature and form has been sustained through the ages primarily not by science, but by the Church, which uses it to explain articles of faith like grace and the Eucharist. But the truth is the truth, and if Aristotle and St. Thomas have contributed to a Catholic knowledge of religion, why may not Catholics, having benefited from that knowledge, contribute to secular scientists’ knowledge of biology?

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Motivated by this question, Fr. Mariusz Tabaczek, a Dominican professor at the Angelicum in Rome, has spent the last ten years integrating Aristotelianism, Thomism, and the observations of Charles Darwin. This year, he published a four-part “hylomorphically-grounded essentialist definition of species.” It is probably the most accurate, and maybe the only adequate, biological definition of species to date.

Fr. Tabaczek has also brought Aristotle and St. Thomas to bear on the issue of goal-oriented adaptations. The biologist Francisco J. Ayala raised the problem of “creativity” when he noted that natural selection produces organisms positively directed toward ends, which should be impossible for a purely chance-based mechanism. On Fr. Tabaczek’s account, the Aristotelian-Thomist notion of plural causality solves this riddle by regarding random mutation as just an accidental cause of natural selection while elevating “living beings that strive to survive and produce offspring” to be selection’s proper cause.

Fr. Tabaczek believes the notion of plural causality accounts for “emergence” in evolution too. Scientists have long observed that complex systems can generate phenomena beyond the sum of their underlying parts. The human brain, for instance, generates self-consciousness despite its composition of un-self-conscious matter. This is a stumbling block to physicalists, but it makes sense, per Fr. Tabaczek, when we recognize that metaphysical forms, like a human’s rational soul, possess causal power.

It will take astute minds to evaluate the nitty-gritty details of Fr. Tabaczek’s arguments, and it remains to be seen whether those arguments, regardless of their validity, will have any salient impact on mainstream biology. Nevertheless, they show that Catholic thought continues to be relevant in the modern age, even to the cutting edge of science.

This is not to suggest that the faith per se gives us special insight into the workings of the natural world. To quote Pope St. John Paul II again, “Religion is not founded on science nor is science an extension of religion.” But if Catholics can learn from secular scientists, it stands to reason that secular scientists can learn from Catholics. There is no need for us to be shy about or ashamed of our intellectual tradition. To the contrary, we may be surprised by how many earthly dilemmas it clarifies and resolves.