Source: BIOL 110 Learning Guide, Exploring Life, Evolution, and Earth's Changing Environment Difficulty: Introductory | Prerequisites: Read the companion notes on Properties of Life, Evolution and Phylogenetic Trees first.
This set of notes focuses on how natural selection works as a mechanism, using concrete examples from the tutorial: a beetle population under predation, the Grants' research on Galápagos finches during drought, and antibiotic resistance in bacteria. These case studies illustrate the same four-step logic, and the course will expect you to apply that logic to new scenarios on exams. You should already be comfortable with the definitions of evolution, adaptation, and natural selection from the companion notes.
Natural selection follows a consistent pattern: a population has genetic variation, some of those variants confer a survival or reproductive advantage in the current environment, and because the traits are heritable, the next generation skews toward the advantageous variant. The Galápagos finch data show this happening in real time over a single drought, with beak depth shifting measurably in one generation.
Natural selection
The process by which organisms with heritable traits better suited to their environment survive and reproduce at higher rates, gradually shifting the population's characteristics. Think of it as the environment filtering which genetic variants get passed on.
Genetic variation
Differences in DNA among individuals in a population. Without variation, natural selection has nothing to act on.
Heritability
The extent to which a trait is passed from parent to offspring through genes. A trait must be heritable for natural selection to change its frequency in the population.
Selective pressure
An environmental factor (predation, drought, disease, competition) that causes differential survival or reproduction among individuals with different traits.
Directional selection
A mode of natural selection in which one extreme of a trait is favoured, shifting the population's average in that direction. The finch beak-depth increase during drought is a textbook example.
Antibiotic resistance
The ability of bacteria to survive exposure to an antibiotic, typically conferred by a gene that was already present in the population before the antibiotic was applied. Selection amplifies the resistant variant by killing non-resistant individuals.
The tutorial illustrates natural selection with a population of beetles that vary in colour. The logic runs in four steps, and every natural-selection scenario you meet on an exam follows the same pattern.
Step 1 – Genetic variation exists in the population. The beetle population contains individuals of different colours (green and brown, for example). This variation has a genetic basis.
Step 2 – Competition for resources. There are limited hiding places, food, or mates. Not every individual can survive and reproduce.
Step 3 – Differential survival based on traits. Birds prey on the beetles. Brown beetles blend into the bark better than green beetles, so brown beetles are eaten less often. More brown beetles survive to reproductive age.
Step 4 – Heritability passes the advantageous trait to offspring. Because colour is heritable, surviving brown beetles produce offspring that are also predominantly brown. Over generations, the proportion of brown beetles in the population increases.
The result is not that individual beetles "turned brown." The population shifted because brown beetles survived and reproduced at higher rates.
Peter and Rosemary Grant studied medium ground finches (Geospiza fortis) on the Galápagos Islands. Their data from a severe drought in 1977 provide one of the clearest real-world demonstrations of natural selection.
Before the drought (1976 population): Beak depth varied across the population. The most common beak depth was approximately 8.8 mm.
During the drought: Food became scarce. Only large, hard seeds remained available. Finches with deeper (larger) beaks could crack these seeds; finches with shallower beaks could not get enough food and died at higher rates. More than half the population perished.
After the drought (survivors): The surviving population had a higher average beak depth. The most common beak depth among survivors shifted upward.
1978 offspring: Because beak depth is heritable, the offspring of the survivors also showed larger average beak depth than the original 1976 population. The most common beak depth in the 1978 offspring generation was approximately 9.8 mm.
Key takeaways from the data:
The most common beak depth of the initial population (8.8 mm) was poorly represented among survivors. (True)
More than half the initial population died during the drought. (True)
Finches with larger beaks had a survival advantage. (True)
The average beak depth of the 1978 offspring population was larger than that of the original 1976 population. (True)
The most common beak depth increased from 8.8 mm to 9.8 mm. (True)
Less than half the population died. (False, more than half died.)
A common exam question: will finch beaks keep getting larger? The answer is C: it depends on changes in the environment. If wet conditions return and small seeds become abundant again, smaller beaks may become advantageous, and the population could shift back. Natural selection has no fixed direction; it tracks the current environment.
The tutorial presents a passage about antibiotic resistance that contains a deliberate misconception for students to find. The misconception is that the resistance mutation "arose because of applying the antibiotic." That is wrong. The mutation existed in the population before the antibiotic was applied. The antibiotic did not cause the mutation; it selected for bacteria that already carried it, by killing all the non-resistant individuals. The survivors divide, and the next generation is overwhelmingly resistant.
This is the same four-step pattern: variation exists, selective pressure is applied, individuals with the advantageous trait survive, the trait is heritable.
Antibiotic resistance is one of the most pressing public-health problems worldwide, and it is a direct consequence of natural selection. Every time antibiotics are used, the selective pressure favours any bacterium that carries a resistance gene. This is why doctors stress completing a full course of antibiotics: stopping early leaves surviving (potentially resistant) bacteria to repopulate.
The finch research also illustrates why conservation biologists monitor trait shifts in wild populations. Rapid environmental change (drought, habitat loss, climate shifts) can drive measurable natural selection within a few generations, which has implications for species' long-term viability.
"The antibiotic caused the bacteria to mutate." Mutations arise randomly, not in response to need. The antibiotic selected for pre-existing resistant variants by killing everything else.
"Natural selection makes organisms perfectly adapted." Selection only works with the variation that exists at the time. It produces "good enough for now," not perfection. Trade-offs are common.
"Evolution has a goal or direction." There is no endpoint. If the environment shifts, the direction of selection shifts with it. Finch beaks got larger during drought but could get smaller again if conditions change.
"Stronger, faster, or bigger is always better." Fitness is context-dependent. During the drought, larger beaks were better. In a wet year with abundant small seeds, smaller beaks might be advantageous.
⚠️ Be able to walk through the four steps of natural selection for any scenario. The exam may give you a novel organism and environment and ask you to explain how natural selection operates.
⚠️ Know the finch data. The shift from 8.8 mm to 9.8 mm most common beak depth, the fact that larger beaks survived the drought, and the fact that the 1978 offspring showed the shift because beak depth is heritable.
⚠️ Distinguish natural selection (the mechanism) from evolution (the measurable outcome). The bird-beak drought scenario is natural selection. The ten-year population shift from 30% short tails to 45% short tails is evolution.
⚠️ The antibiotic-resistance misconception is a favourite exam question. Mutations are random and pre-existing; the antibiotic selects, it does not cause the mutation.
⚠️ What will happen to finch beaks in the future? The answer is that it depends on the environment (option C). This tests whether you understand that natural selection has no fixed direction.
True or False: The antibiotic causes bacteria to develop resistance mutations. (False, mutations are random and pre-existing; the antibiotic selects for them.)
Fill in the blank: The four requirements for natural selection are genetic variation, competition for resources, differential __________, and heritability. (survival and reproduction)
True or False: Finch beaks will continue to get larger indefinitely. (False, the direction depends on the environment.)
Fill in the blank: The most common beak depth in the 1978 finch offspring was approximately __________ mm. (9.8)
True or False: Natural selection acts on individuals, not populations. (True in one sense: selection pressures affect individual survival. But evolution, the outcome, is measured at the population level.)
Q: Using the beetle example, explain the four steps of natural selection. Include the terms genetic variation, heritability, and competition for resources.
A: (1) The beetle population shows genetic variation in body colour, with both green and brown individuals. (2) There is competition for resources such as hiding places on tree bark. (3) Brown beetles are better camouflaged and avoid predation, so they survive at higher rates (differential survival). (4) Because colour is heritable, the offspring of surviving brown beetles are also predominantly brown, so the proportion of brown beetles increases over generations.
Q: A population of birds on an island has beaks of varying sizes. During drought, only small seeds are available and birds with smaller beaks feed more successfully. Is this natural selection or evolution? Explain.
A: This is natural selection. It describes the mechanism: a selective pressure (drought reducing seed size) causes differential survival (small-beaked birds feed better and reproduce), and the trait is heritable. It describes the process acting in one generation, not a measured population-level change over time.
Q: A researcher reports that over ten years, the proportion of short-tailed birds increased from 30% to 45%. Is this natural selection or evolution?
A: This is evolution. It documents a change in the heritable characteristics of the population over time. It does not identify which specific selective pressure caused the change, only that the population shifted.
Q: Find the misconception in this statement: "The resistance mutation arose by mutation because of applying the antibiotic."
A: The misconception is that the antibiotic caused the mutation. Mutations occur randomly and independently of the selective pressure. The resistance gene was already present in some bacteria before the antibiotic was applied. The antibiotic killed non-resistant bacteria, leaving the resistant ones to survive and reproduce.
Q: What will happen to the size of beaks in medium ground finches in the future, and why?
A: It depends on changes in the environment. If conditions change (for example, if rain returns and small seeds become abundant), smaller beaks may be favoured again. Natural selection has no fixed direction; it tracks current environmental conditions.
The four-step natural selection framework reappears throughout BIOL 110, including in units on genetics (how variation arises through mutation and recombination), gene expression (how heritable traits are encoded), and human evolution (the lactose tolerance case, covered in the companion notes).
Antibiotic resistance connects to microbiology and public health topics later in the course. Understanding that selection acts on pre-existing variation is essential for those discussions.
The finch data set is a classic example of directional selection, a concept that becomes more precise when you study population genetics.
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