How humans are changing the earth’s fauna

By Mikayla Heiss
BU News Service

BOSTON — Pictures of puppies with saucer eyes and floppy ears stare at the audience. The audience stares back. These cute creatures illustrate an astonishing truth: humans are influencing the evolutionary process.

Large eyes and droopy ears are a few results of artificial selection, said Barbara Schaal, dean of the faculty of arts and sciences at Washington University and former science adviser to President Barack Obama. Artificial selection is the generational transfer of human-desired animal or plant traits through intentional or unintentional actions like breeding or domestication.

Humans and other organisms have always had a natural relationship–as one changes, the other reacts, Schaal said. As time progressed, though, humans expanded their civilization, growing plants and taming animals. 

“[Humans] will select individuals with…favorable characteristics,” Schaal said. “This can be done consciously, or it could be done without even thinking about it.”

There are three pathways of domestication. One pathway occurs naturally as animals bond with humans. Human food scraps, for example, attract animals, and as the animals rely on humans as a food source, a relationship forms, Schaal said.

In a prey pathway, animals gain a different purpose, and once beneficial traits may be lost or altered. Prehistoric horses, for example, were first hunted. Eventually, though, herds were established as humans discovered the benefits of keeping their dinner close. Previously a food supply, the horses became a method of transportation during hunts.

Direct pathways are conscious selections, such as selectively breeding koi fish for their appearances, or corn for their soft kernels, Schaal said. 

But problems arose when humans domesticated their surroundings on a large scale.

The head shape of canines has changed due to artificial selection. The elongated great wolf skull, lined with sharpened teeth, illustrates its predatory nature, Schaal said.

Domestic dogs have a shorter snout and larger skull shape due to artificial selection. When dogs first associated with humans, they were thrown into a unique environment. While relatively safe in the confines of human society, dogs began competing among themselves.

Adaptability and high reproductive rates were valued, which may have led to changes in the developmental process. Embryonic tissues responsible for head and neck development were genetically altered, resulting in large skulls that are small at the crown.

The impacts of domestication are so great that researchers suggest certain dog breeds may reach sexual maturity before skeletal maturity. Earlier skeletal maturity may be linked to a cartilage disorder in certain breeds, like dachshunds. The disorder causes short, thickened and curved bones to develop.

The evolution of canine facial muscles has also been altered by human involvement. Researchers studying canine eyes found puppies have muscles in the eyebrows that resemble a human expression for sadness.

Dogs hijacked the human care-giving response, the researchers said. The dog’s gaze causes the release of oxytocin, a chemical involved in mother-infant attraction. By eliciting a caregiving response, dogs with expressive eyes were artificially selected by humans. 

While this trait is pointless for wolves, it makes for an effective begging technique in domestic animals, Schaal said.

The brain is also smaller as a result of domestication. Compared to their wolf ancestors, domestic dogs have a 30% reduced brain size. Humans tended to accept timid dogs, which chips away at a part of the brain responsible for behavioral responses, Schaal said. During the developmental stage, brain cells are also less active in domestic dogs, which alters the flight-or-fight response.

An onlooker doesn’t have to wait long to see humanity’s impact on the evolutionary process, Schaal said. Researchers in 1959 selectively bred silver foxes for human-preferred traits. By 1969, the silver fox population had floppy ears, shorter snouts and curly tails. 

“You end up getting a fox that looks like one of my border collies,” Schaal said. 

Ongoing research and new technologies offer fresh input on artificial selection’s effects on the evolutionary process. Genomics allowed researchers to track changes on a molecular level, Schaal said.

Recently, scientists used bones from archaeological digs to extract, analyze and compare species’ DNA. With this technique, researchers can create family trees and observe changes over time that humans have potentially caused in fauna.