AP Biology · Unit 7 · Topic 7.1
Introduction to Natural Selection
Suppose a drought ends and the next generation of a plant population contains more individuals with water-conserving leaves. Did the plants change because they needed to survive, or did plants that already differed leave different numbers of descendants? Those explanations predict different observations. Natural selection connects differences among individuals to changes across generations.
Follow the trait into the next generation
A population contains individuals of the same species in a shared area. Individuals can differ in traits such as leaf structure or flowering time. Some differences have a heritable genetic basis; others reflect nutrition, age, or growing conditions. A visible difference alone does not establish inheritance.
For evolutionary change by natural selection, connect three observations: individuals vary, some of that variation is inherited, and variants differ consistently in their contribution to future generations. If water-conserving plants produce more offspring that reach the next reproductive stage, and those offspring inherit the relevant variation, the population can become richer in that variation. The original plants do not have to transform into a new type.
Selection acts through phenotypes—the traits organisms express in their environment. Its evolutionary consequence is a change in the population's inherited composition. Merely counting survivors documents one part of this chain, not necessarily its completion.
Why limited resources enter the explanation
Darwin and Wallace independently developed natural selection as an explanation for evolutionary change. Producing many offspring does not mean all can mature and reproduce: space, food, water, or suitable nesting sites may be limited. A trait affecting access to such resources can therefore affect reproductive success. Competition need not involve a fight; seedlings shading one another can experience unequal access to light.
A selective pressure is an environmental factor associated with differences in survival or reproduction. Predators and competitors are biotic factors; temperature and water availability are abiotic factors. The environment does not evaluate organisms or plan a better species. “Favored” means a variant has a reproductive advantage under specified conditions, not that it is universally superior.
Surviving longer is not the same as leaving more descendants
Evolutionary fitness concerns reproductive contribution, not strength, health, or lifespan alone. To compare two variants, use the same life-cycle interval. Counting eggs for one variant and breeding adults for another would not be a meaningful comparison.
Consider an invented one-generation model of an asexual organism with faithfully inherited types A and B. Each type starts with 100 individuals. Assume no immigration or mutation, nonoverlapping generations, and that the table counts every offspring reaching the next breeding census.
| Type | Starting individuals | Survive to reproduce | Recruited offspring per survivor | Next-generation individuals |
|---|---|---|---|---|
| A | 100 | 80 | 2 | 160 |
| B | 100 | 50 | 4 | 200 |
A has higher survival: 80% versus 50%. But each starting A contributes, on average, \(0.80(2)=1.6\) recruited offspring, while each starting B contributes \(0.50(4)=2.0\). B's greater output per survivor more than offsets its lower survival. Of the 360 individuals in the new generation, B accounts for:
B rises from 50% to about 55.6%. Relative fitness can be expressed by dividing each contribution by the largest contribution in this comparison: \(w_A=1.6/2.0=0.8\) and \(w_B=1\). This normalization compares contributions; a relative fitness of 0.8 does not mean an 80% probability of survival.
An advantage can reverse when conditions change
Now use a separate hypothetical asexual population with inherited types D and W. Initially there are 40 D and 60 W. During a dry interval, each starting D contributes 3 individuals to the next generation, while each W contributes 1. The new counts are \(40(3)=120\) D and \(60(1)=60\) W. D's frequency becomes \(120/180=2/3\), even though no W individual turns into D.
Suppose the next interval is wet and reverses the contributions: D contributes 1 per parent and W contributes 3. Starting from those 120 D and 60 W, the following generation contains \(120(1)=120\) D and \(60(3)=180\) W. D's share falls to \(120/300=0.40\). The dry-condition advantage is not a permanent ranking.
Notice that D's count stays at 120 during the wet interval while its frequency falls. More W individuals enter the denominator. A population-level comparison therefore needs both the number of a type and the total population size. Returning to 40% also does not erase the changes that occurred between the two censuses.
This deliberately simple model assumes faithful inheritance, fixed contributions within each interval, and no mutation, migration, or random sampling. It is not a forecast of unlimited population growth. In a real habitat, changing density can alter resources and reproductive success.
What the counts are calculating
You do not need a genotype-equilibrium equation to explain the preceding change. The underlying calculation is a weighted share. Let \(p\) be the initial fraction of D, \(N\) the starting population size, and \(r_D\) and \(r_W\) the mean recruited offspring per starting individual. Then D starts with \(Np\) individuals and W with \(N(1-p)\). Multiplying each count by its contribution gives the next-generation counts. Divide D's count by their sum:
The common factor \(N\) cancels. For the dry interval, substituting \(p=0.40\), \(r_D=3\), and \(r_W=1\) gives \(1.2/(1.2+0.6)=2/3\). If the two nonzero contributions are equal, the fraction remains \(p\). This is optional mathematical reasoning for the stated asexual model, not a general shortcut for allele frequencies in a sexually reproducing diploid population.
A plausible story still needs evidence
Imagine observing narrow leaves more often after several dry years. That pattern alone does not identify natural selection. Existing plants might produce narrower leaves in response to water shortage, narrow-leaved plants might arrive from elsewhere, or a small sample might shift by chance.
For a hypothetical study, compare inherited variants under shared growing conditions, measure reproductive contribution in replicated dry and wet environments, and follow the next generation. Common growing conditions help separate some environmental effects from inherited differences, but parental environmental effects can persist; one shared-environment comparison does not prove a DNA mechanism. Record immigration and use repeated populations to evaluate chance variation.
A supported explanation should identify the variant, the environmental condition, the measured reproductive difference, and evidence that relevant differences are transmitted. Avoid replacing those links with “the population adapted because it had to.”
Keep the mechanism separate from its alternatives
Mutation produces new alleles; recombination can assemble existing alleles into new combinations. Selection does not generate a particular useful mutation on demand. It changes representation through differences in reproductive success. A variant that was already present can become more common after conditions change.
A short-term physiological response within an individual is not, by itself, evolutionary change. Likewise, evolution is not synonymous with natural selection: genetic drift changes frequencies through chance, and gene flow moves inherited variation between populations. Selection can produce adaptations, but it does not guarantee perfection or anticipate a future environment. A trait's benefit may come with a cost elsewhere.
Test the explanation, not just the vocabulary
1. Type A survived better in the table. Why did type B increase?
The relevant comparison includes reproduction after survival. A contributed 160 recruited offspring and B contributed 200 from equal starting counts. B therefore occupies \(200/360\), or about 55.6%, of the new generation. Survival alone omitted the second factor.
2. Both types now contribute two recruited offspring per starting individual. D initially represents 40%. What happens in the ideal model?
D contributes \(0.4N(2)=0.8N\) individuals and W contributes \(0.6N(2)=1.2N\). D's share is \(0.8N/(2N)=0.4\). Population growth alone does not establish a selective advantage; finite real populations can still fluctuate through chance.
3. D fell from 66.7% to 40%. Must D individuals have died during that interval?
No. In the supplied model the next-generation D count equals the preceding D count, 120, while W increases from 60 to 180. The percentages concern replacement generations and their totals, not a direct tally of deaths among the same individuals.
4. All plants develop smaller leaves during drought. Has natural selection been demonstrated?
No. This observation could reflect an environmental response within individuals. Check heritable differences and whether those differences predict contributions to the next generation. A shared response is not enough to establish either link.
5. A storm removes a random half of a small population. Is any resulting frequency change necessarily selection?
No. If removal is unrelated to inherited traits, the frequency shift is consistent with genetic drift. If a heritable anchoring trait systematically changes survival and later reproductive contribution, selection is a plausible additional mechanism. “Many died” does not distinguish them.
6. Repair this claim: “The dry environment made every organism acquire type D so the species could survive.”
In our model D and W existed before the dry interval. D individuals contributed more descendants under dry conditions, and faithful inheritance increased D's share of the next generation. No individual conversion, planned goal, or guaranteed species survival is part of the explanation.
Concept references: OpenStax Biology 2e: Understanding Evolution; OpenStax Biology 2e: Adaptive Evolution; UC Berkeley Understanding Evolution: Natural Selection. Scope checked against College Board AP Biology Course and Exam Description, Topic 7.1. These are external references. The explanations, hypothetical datasets, calculations, graph, and questions were independently composed for this lesson; no source illustrations or worksheets are reproduced.
Official curriculum reference: College Board AP Biology course page. The explanation and worked example are independently written for this study site.