Modes of Selection and Human Impact
Darwin's Hypothesis and the Angraecum sesquipedale Orchid: Darwin hypothesized the existence of a moth with an exceptionally long proboscis after observing the Angraecum sesquipedale orchid in Madagascar, which has a nectar tube 11-12 inches (approximately cm) long. He predicted that such a moth would be required to pollinate this orchid. This moth, Xanthopan morganii praedicta, nicknamed "Darwin's moth," was indeed discovered, but its interaction with the orchid was not documented until , years after Darwin's prediction, when it was observed for the first time in nature. This serves as a classic example of co-evolution. The orchid has been highly selective about its pollinator, and the moth has evolved a specific adaptation to access the nectar.
Modes of Natural Selection Defined: Natural selection refers to changes in the genetics of a population over time. The "modes" of natural selection describe how a particular characteristic influences which genetic variants are favored to survive and reproduce in an environment. It dictates the direction of genetic change by determining which phenotypes are most advantageous.
Stabilizing Selection:
Definition: This mode of natural selection favors intermediate phenotypes and selects against extreme variations. The average or middle characteristic is the most successful.
Example: Human Baby Weight: Babies that are either too lightweight (underdeveloped, requiring respirators) or too large (risk of diabetes for the mother, complicated delivery) face higher health risks. The average baby weight is favored for survival and successful development.
Example: Snow Hares: Snow hares change color with the seasons (white in winter, brown in spring/summer) triggered by temperature. In recent years, abnormal weather patterns (warm December, late snow in January ) caused a delay in their color change. Brown hares on a snowy background or white hares on a green lawn become highly visible to predators, demonstrating that the intermediate, environmentally-appropriate coloration is most advantageous.
Phenotype: Stabilizing selection typically favors only one phenotype (the intermediate).
Directional Selection:
Definition: This mode of natural selection favors one extreme phenotype over others, leading to a shift in the population's genetic variance towards that extreme. The middle is the least favorable place to be.
Example: Peppered Moth (Biston betularia): Initially, light-colored peppered moths were prevalent as they blended with pale tree bark. During the Industrial Revolution, soot from factories darkened trees, making light moths conspicuous to predators (birds). Darker (melanic) peppered moths, a previously rare phenotype, became advantageous as they could camouflage better. The population shifted to primarily black moths. As pollution declined, trees lightened, and the selection pressure reversed, favoring the return of lighter-colored moths. This is an example of an environment-driven shift where one extreme ( phenotype) is favored.
Phenotype: Directional selection favors one specific phenotype, which is an extreme version of a characteristic.
Diversifying (Disruptive) Selection:
Definition: This mode of natural selection favors both extreme phenotypes over intermediate ones. It can lead to the divergence of a population into two or more distinct groups, potentially resulting in speciation.
Example: Beach Mice along the Gulf of Mexico: Along the thousands of kilometers of coastline, sand color varies in shades. Mice inhabiting these different sand colors have evolved diverse fur colors (up to different phenotypes were mentioned) to match their local environment, allowing them to hide from predators. Being an intermediate color would make them stand out on either light or dark sand.
Example: Pupfish in Death Valley: Pupfish were isolated in separate ponds due to intervening sand. Each pond developed slightly different environmental characteristics (e.g., highly saline, acidic, low oxygen levels, very hot). The pupfish in these ponds adapted to their specific extreme conditions, leading to diversification. Some pupfish even evolved a form of internal fermentation to produce energy, though this process also produces ethanol, which is toxic and can cause liver failure.
Sexual Selection:
Definition: A mode of natural selection where individuals with certain inherited traits are more likely to obtain mates than other individuals. It is often more complex in humans due to psychological and cultural factors (e.g., fashion, accessories).
In Nature: Generally simpler, driven by the female's choice or male-male competition.
Male Display: Males often develop exaggerated traits to attract females. Examples include the large antlers of elk, moose, and deer, or the vibrant and colorful plumage of male peacocks and male birds like blue jays. Female birds (like a pictured female of a pie, likely a magpie) are typically duller because they don't expend energy on showy displays; it's the male's role to attract.
Male Competition: Males compete for access to females. This can involve physical contests, such as moose locking antlers during fights. These fights can lead to injuries or even death if antlers become permanently interlocked.
Display of Size/Threat: An example is a cat fluffing its tail. When agitated or playful, cats' fur stands on end, making them appear larger and more intimidating. This can be interpreted as a form of display, where a larger-looking male might deter rivals or attract mates.
Artificial Selection:
Definition: A process where humans intentionally breed plants or animals for specific desirable traits. Humans act as the selective agent.
Example: Corn: Over approximately years, humans have transformed wild teosinte (a small, grass-like plant with few small kernels) into the large, kernel-filled corn cobs we consume today, by selectively breeding plants with more edible kernels.
Broad Applications: Artificial selection is pervasive in agriculture and animal husbandry, applied to:
Livestock: Cows are bred for increased milk production or specific meat qualities. Chickens are bred for higher egg yields or larger breast meat.
Companion Animals: Dogs, cats, and horses have been bred for thousands of years for specific appearances, temperaments, or abilities.
Advanced Reproductive Technologies in Animals:
Artificial Insemination (AI): Common practice, even involving shipping semen on dry ice across continents for insemination. This method can be referred to as the "turkey baster method."
Gamete Intrafallopian Transfer (GIFT) / Gamete Intrafallopian Perfusion (GIP): For extremely valuable animals (e.g., horses worth millions of dollars), more advanced techniques are used. Under microscopic guidance and ultrasound, sperm are directly guided to the oviduct where the egg is ovulating. This bypasses the natural journey where millions of sperm often die, significantly increasing the chance of fertilization for a successful pregnancy.
Human-Driven Natural Selection (Unintended Consequences):
Pesticides: The overuse of pesticides drives natural selection in insect populations. Insects with natural resistance to pesticides survive, reproduce, and pass on their resistance, leading to "superbugs" that are harder to control.
Antibiotics: Similar to pesticides, the improper use of antibiotics (e.g., not finishing the full course) contributes to the evolution of antibiotic-resistant bacteria. When an antibiotic course is stopped prematurely, the most resistant bacteria survive and proliferate, forming a new, harder-to-treat population. This not only affects the individual but also spreads resistant pathogens to others, creating a public health crisis concerning antibiotic resistance. The full course of antibiotics is crucial to eliminate as many bacteria as possible, minimizing the chance for resistant strains to emerge and spread.