Dear EarthTalk:

Are any plants and animals evolving backwards by incorporating traits that they lost long ago?

Joseph R., New Bedford, MA

Regressive evolution, in simple terms, refers to the loss of previously evolved traits. It occurs because specific characteristics are no longer beneficial to the organism’s survival due to changes in lifestyle or the environment. Gene preservation and decay depend on the level of pleiotropy (the frequency at which a gene influences more than one trait). However, regressive evolution remains somewhat speculative due to the limited research on the molecular level regarding the re-emergence of genes. According to University of Bologna researcher Giobbe Forni, further understanding will “be possible only if we dedicate more effort to observing and analyzing the amazing diversity of organisms."

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Over the past 50 million years, whales and dolphins (cetaceans) have evolved in remarkable ways as they transitioned from land-dwelling mammals back to life in the ocean. Unlike early vertebrates that first moved from water to land over 350 million years ago, cetaceans represent a secondary return to aquatic environments. During this transition, they developed specialized adaptations such as streamlined bodies, flippers, tail flukes, and in some species, echolocation. At the same time, they experienced regressive evolutionary changes, including the reduction of hind limbs and loss of most body hair. One explanation for this shift is that aquatic environments offered abundant food resources and reduced competition, so natural selection favored traits that improved survival and reproduction in marine habitats.

Another example is cave fish that undergo regressive evolution in their eyes, which are not needed in pitch-blackness. Cave fish don’t return to a primordial ancestor without eyes; biological processes to produce eyes stop, leaving the eye underdeveloped, covered by skin. Regressive evolution is like the evolutionary process that creates more complex. This is best said by William R. Jeffery, a biologist at the University of Maryland: “But the eye didn't go in reverse. It just stopped going forward.”

Lastly, tomato plants on the western islands of the Galápagos have reverted to an ancestral chemical defense, producing alkaloids that are unnatural to modern-day tomatoes but identical to those of their eggplant relatives, which date back millions of years. Researchers found that four amino acid changes in an enzyme led to regressive evolution due to harsher conditions on the western islands, specifically in areas with barren land and poor soil, which revived ancient defensive traits in tomato plants.

It may not be possible to completely revert to a past ancestor, as mimicking the extinct environment and lifestyle is nearly impossible due to the impact of anthropogenic influences. Regressive evolution is a fascinating process that is accessible to learn more about. Steps to gain a better understanding of this include reading about evolution, which is driven by the same principles; taking free online courses and receiving academic credit; and visiting local or online museums and attending public lectures.

CONTACTS

• Harvard Museum of Natural History Evolution Matters Lecture Series: https://linkly.link/2Bof5.

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