Most of us know that as living things evolve, they acquire traits that help them thrive in their home environments. But how are certain traits “chosen” for future generations and how are others discarded? It all comes down to natural selection, or the process by which species pass on the characteristics that allow them to survive.
What does 'natural selection' mean?
Natural selection is a mechanism of evolution—Not evolution itself. “Natural” refers to how traits are passed on without intentional outside interference. (In contrast, “artificial selection” is used for what we normally call breeding or breeding, such as with dogs or commercial fruit varieties.) Despite the use of the word “selection,” natural selection does not refer to a conscious decision about which traits can remain and which should disappear. Instead, it invokes the passive process by which certain traits manage to endure, which can best be understood through the well-known phrase “survival of the fittest.”
Simply put, living beings with the skills necessary to survive are those that can pass their genes on to their offspring. This is true for any living being, from animals and plants to fungi and single-celled organisms such as algae. A mammal that dies before it can reproduce cannot pass on its genetic material; an herb that dries up before it can become seed (or used to pollinate another plant) loses the opportunity to contribute to the next generation. If the longevity of a living being (compared to the rest of its species) determines its ability to continue its genetic lineage, it is inevitable that the traits that lead to survival will be transmitted over many generations. Meanwhile, traits that are not conducive to survival gradually disappear.
The original peppered moth (top) and the dark peppered moth (bottom).
Credit: Gail Hampshire/Wikimedia Commons
A famous example of this process can be found in the peppered moth, an insect native to Europe that once had light gray wings with dark spots. The peppered moth frequently camouflaged itself behind the bark of trees, which, until the Second Industrial Revolution, was a suitably light shade of gray or brown. Thanks to the sharp increase in manufacturing after 1870, the air in England constantly contained dark smoke, which covered nearby trees and lichens. The light-colored peppered moths suddenly became visible to predators, while those with darker spots could more easily hide among the trees. Between 1848 and 1900 (enough time for more than 50 generations of moths), the once rare dark peppered moth became common. Only in rural regions that were not affected by soot could the light-colored moth still thrive.
How do we discover natural selection?
The modern study of natural selection began with Charles Darwin, an English naturalist best known for his contributions to the theory of evolution. During a five-year mapping trip along the coast of South America, Darwin collected a variety of samples of plants, animals, rocks and fossils that guided his research. It was while he was collecting these samples from the remote Galapagos archipelago that Darwin fell in love with the native finches, which seemed to have beaks of different sizes and shapes. Spanish settlers living on the islands reported to Darwin that local finches tended to look different and eat varied diets depending on which island they called home. This gave Darwin the idea that a species could branch into new species to adapt to particular environments.
An original illustration showing the differences between the head and beak shapes of four finch species.
Credit: Charles Darwin
Darwin later noticed similar discrepancies among mockingbirds, which appeared very different from island to island. He began to wonder if species could adapt to their environments. It wasn't until Darwin read about the economist Thomas Robert Malthus. population theorywhich promoted the idea of ”survival of the fittest” among humans, that Darwin realized that the rest of the living world probably functioned the same way.
Alfred Russell Wallace, a younger English naturalist, came to a similar conclusion. Wallace had conducted extensive field studies in the Amazon, but had lost his notes and samples in a shipwreck on the return voyage. He returned with vigor to his work in the Malay Archipelago (now Malaysia and Indonesia). While he was there, he noticed what Darwin had observed: that living things seemed to pass on traits to their offspring, allowing species to adapt and differentiate.
Credit: Fernando Venzano/Unsplash
Wallace described his thoughts in a letter to Darwin. The men proceeded to simultaneously publish their evolutionary research, which was then presented at a meeting of the Linnean Society of London in 1858. Although both naturalists' papers were highly respected, Darwin published his evolutionary bomb On the origin of species a year later, largely expelling Wallace from the public consciousness. (Rumor has it that Wallace didn't care; he had still been promoted to high academic status and reportedly loved being Darwin's biggest fan.)
Darwin defined evolution as “descent with modification,” meaning that species can change over many generations. In the mid-19th century, this was a revolutionary concept: most people believed that species were static and had nothing to do with their suspiciously similar predecessors. Today we know species not only evolve but do so by filtering out unfavorable traits in favor of more useful ones. This often results in a differentiation of species, also known as speciation.
What is speciation?
Sometimes certain members of a species will adapt to fit a particular niche while the rest of the species maintains its genetic line. This can happen when environmental changes occur or a population's geographic area expands.
A 1918 illustration of the Abert's squirrel (top) and the spiced Kaibab squirrel (bottom).
Credit: Louis Agassiz Fuertes/Gilbert Hovey Grosvenor/Edward William Nelson/U.S. National Geographic Society/Wikimedia Commons
Speciation occurred with Darwin's finches, which found homes on several islands in the Galapagos archipelago. While some finches evolved broad, blunt beaks that allowed them to crack open nuts and large seeds, others evolved to possess slender beaks, which could extract cactus fruits and flower nectar while keeping the finch a safe distance from the plant's thorns. The vast fissure that forms the Grand Canyon has also acted as a catalyst for forced geographic speciation, leading to the evolution of the Kaibab Squirreldescendant of the extant Abert's squirrel.
How do scientists study natural selection today?
Unlike Darwin and Wallace, we now have decades of genetic research at our disposal as we delve deeper into the science behind natural selection. DNA sequenceo determine the order of nucleotides in DNA, allows geneticists to track genetic variation within and between species. Genotyping, or tracking which small genetic variations are responsible for certain traits, helps researchers see how characteristics are passed between a parent organism and its offspring. Scientists can then use CRISPR either Fanzor-ωARN modify these genetic chains and see how the changes affect the phenotype or observable characteristics of an organism.
A laboratory full of DNBSEQ sequencing machines.
Credit: RPSkokie/Wikimedia Commons
Fossils It also helps archaeologists, paleontologists and zoologists document speciation or phenotypic changes. when unearthing bones and preserved soft tissues, researchers can look back in time to see what a living thing was like tens of thousands of years ago. Radiocarbon dating or use coalThe radioactive isotope of to determine the age of an organic material, meanwhile, helps researchers determine when that organism lived. This helps create a timeline along which that organism evolved.
Although we now know that natural selection exists, we still have a lot to learn about how it works and how it transforms. Landthe diverse ecosystems over time. Researchers like Hopi Hoekstra, an evolutionary biologist who studies Adaptive melanism in pocket mice.and Sarah Otto, a theoretical biologist who focuses on mathematical and statistical modeling of evolutionary processes, continue to analyze the nuances of processes such as natural selection. And while they do so, nature will continue to filter out the countless characteristics of living things, maintaining those that are conducive to survival and eliminating those that are not.
