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Examining how environmental pressures shift the distribution of traits in a population.
Imagine a population of light-colored moths living in a forest. If a nearby factory covers the trees in black soot, what happens to the moths that can't hide? Nature doesn't just pick winners randomly; it follows predictable patterns that reshape entire species.
In any population, most traits—like height, beak size, or skin color—follow a Normal Distribution. If you graph these traits, they form a 'bell curve.' The highest point of the curve represents the mean (average) phenotype, where most individuals are found. The 'tails' of the curve represent the extremes. For example, in a forest of medium-sized squirrels, the peak of the curve is the average size, while the far left represents tiny squirrels and the far right represents giant squirrels. Natural selection acts like a sculptor, 'pushing' on this curve based on which squirrels survive to reproduce.
Quick Check
In a normal distribution graph, what does the highest point of the curve represent?
Answer
The highest point represents the mean, or the most common phenotype in the population.
Directional Selection occurs when environmental changes favor individuals at one extreme of the trait range. This causes the entire bell curve to shift left or right. A classic example is the Peppered Moth. Originally, light-colored moths were common. However, during the Industrial Revolution, soot darkened the trees. Suddenly, dark moths had a survival advantage. Over time, the 'peak' of the population shifted from light to dark. If we let represent the frequency of the dark trait, we see increase steadily over generations as the environment changes.
1. A population of bacteria is exposed to an antibiotic. 2. Most bacteria die, but a few with a rare 'resistance' gene survive. 3. These survivors reproduce, passing on the resistance. 4. The graph of 'Resistance Level' shifts entirely toward the high-resistance extreme.
Quick Check
If a climate becomes much colder and only the thickest-furred animals survive, which pattern of selection is occurring?
Answer
Directional Selection, because the population is shifting toward one extreme (thicker fur).
Sometimes, it pays to be average. Stabilizing Selection occurs when the environment favors the intermediate variants and acts against the extreme phenotypes. This doesn't shift the curve left or right; instead, it 'squeezes' the curve. The peak gets taller and the tails disappear. A perfect example is human birth weight. Babies that are very small may have health complications, while babies that are very large may have trouble during birth. Thus, the 'average' weight is favored by nature, keeping the population's birth weight within a narrow range.
1. Robins typically lay 4 eggs. 2. If they lay more (e.g., 7-8), the parents can't provide enough food, and the chicks starve. 3. If they lay fewer (e.g., 1-2), there is a high risk that a predator eats them all, leaving no survivors. 4. Result: The population 'stabilizes' at 4 eggs, and the extremes are eliminated.
Disruptive Selection is the rarest and most dramatic pattern. It occurs when environmental conditions favor individuals at both extremes of the phenotypic range over those with intermediate traits. This 'pushes down' on the center of the bell curve, eventually creating two distinct peaks (a bimodal distribution). This often happens when a habitat has two distinct food sources or types of shelter, and being 'in the middle' makes you poorly adapted for either one. This pattern is a key driver of speciation, or the formation of new species.
1. In a specific environment, only two types of seeds are available: very hard seeds and very soft seeds. 2. Birds with large, powerful beaks can crack the hard seeds. 3. Birds with small, nimble beaks can handle the soft seeds. 4. Birds with medium-sized beaks are not strong enough for hard seeds and not nimble enough for soft seeds. 5. The population splits into two groups: Large-beaked and Small-beaked.
Which type of selection would likely occur if a population of lizards lived in an environment with only very large and very small rocks, but no medium-sized rocks for cover?
In a graph of Stabilizing Selection, what happens to the width of the bell curve?
Directional selection is the most likely pattern to occur when a species migrates to a new environment with different resources.
Review Tomorrow
In 24 hours, try to sketch the three types of selection graphs from memory and label which one leads to 'speciation'.
Practice Activity
Find a news article about 'superbugs' (antibiotic-resistant bacteria) and identify which of the three patterns of selection is being described.