Natural Selection

What you'll learn

Natural selection is the central mechanism of evolution, explaining how populations change over time in response to environmental pressures. This revision guide covers the core principles of natural selection as specified in the AP Biology curriculum, including variation, differential survival, adaptation, and the evidence supporting evolutionary theory. You'll develop the skills to analyze data, interpret evolutionary scenarios, and answer exam questions with precision.

Key terms and definitions

Natural selection — the process by which organisms with favorable inherited traits are more likely to survive and reproduce, passing those traits to offspring, causing populations to change over time

Adaptation — an inherited characteristic that increases an organism's fitness in a particular environment, having evolved through natural selection

Fitness — the ability of an organism to survive and reproduce in its environment, measured by the number of fertile offspring produced

Selection pressure — an environmental factor (biotic or abiotic) that influences which individuals survive and reproduce, thereby driving natural selection

Directional selection — natural selection favoring one extreme phenotype over others, shifting the population mean in one direction

Stabilizing selection — natural selection favoring intermediate phenotypes and selecting against extreme variants, reducing population variation

Disruptive selection — natural selection favoring both extreme phenotypes over intermediate forms, potentially leading to population splitting

Genetic variation — differences in DNA sequences among individuals in a population, providing the raw material upon which natural selection acts

Core concepts

Conditions necessary for natural selection

Natural selection operates when specific conditions exist within a population. Understanding these prerequisites is essential for analyzing evolutionary scenarios.

Variation in traits:

• Individuals within a population must exhibit phenotypic differences
• Variation arises from genetic differences (alleles), mutations, and genetic recombination during sexual reproduction
• Not all variation is heritable; only inherited variation can be acted upon by natural selection
• Examples include beak size in finches, coat color in mice, or antibiotic resistance in bacteria

Heritability:

• Traits must be passed from parents to offspring through genes
• Non-heritable variations (acquired characteristics) cannot evolve through natural selection
• The proportion of phenotypic variation due to genetic factors determines how rapidly selection can change a population

Differential reproductive success:

• Individuals with certain phenotypes survive and reproduce at higher rates than others
• Fitness differences must exist between phenotypes
• Over generations, advantageous alleles increase in frequency while disadvantageous alleles decrease
• This process is non-random, distinguishing natural selection from genetic drift

Overproduction of offspring:

• Populations produce more offspring than the environment can support
• Limited resources create competition for survival
• Not all individuals reach reproductive maturity
• This excess creates the opportunity for selection to occur

Types of natural selection

Selection operates in different patterns depending on which phenotypes confer the highest fitness in a given environment.

Directional selection:

• Favors individuals at one extreme of the phenotypic distribution
• The population mean shifts toward the favored extreme over time
• Common when environmental conditions change or populations colonize new habitats
• Example: Industrial melanism in peppered moths (Biston betularia) in polluted areas of England, where darker moths had higher survival on soot-covered trees
• Example: Antibiotic resistance evolution in bacteria, where increasing antibiotic concentration selects for increasingly resistant strains

Stabilizing selection:

• Favors intermediate phenotypes while selecting against extremes
• Reduces phenotypic variance but does not change the population mean
• Common in stable environments where the existing average phenotype is well-adapted
• Example: Human birth weight, where very small infants have higher mortality due to underdevelopment and very large infants increase maternal complications
• Example: Clutch size in birds, where too few eggs reduce reproductive output and too many eggs result in malnourished offspring

Disruptive selection:

• Favors both extreme phenotypes simultaneously while selecting against intermediates
• Increases phenotypic variance and can lead to bimodal distributions
• May result in speciation if reproductive isolation develops between the two favored forms
• Example: Beak size in African seedcracker finches, where small beaks efficiently process soft seeds and large beaks crack hard seeds, but intermediate sizes handle neither efficiently
• Less common than directional or stabilizing selection in nature

Evidence for evolution through natural selection

Multiple independent lines of evidence support the theory of evolution by natural selection. AP Biology requires familiarity with these evidence categories.

Fossil record:

• Documents the existence of species that no longer exist
• Shows progression from simple to complex forms over geological time
• Transitional fossils demonstrate intermediate forms between major groups (e.g., Archaeopteryx linking reptiles and birds, Tiktaalik showing fish-to-tetrapod transition)
• Provides direct evidence of change over time

Comparative anatomy:

• Homologous structures share common ancestry despite different functions (mammalian forelimbs in humans, bats, whales)
• Vestigial structures are remnants of features that served functions in ancestral species (human appendix, whale pelvic bones)
• Analogous structures have similar functions but different evolutionary origins, demonstrating convergent evolution (bird and insect wings)

Biogeography:

• Geographic distribution of species reflects evolutionary history
• Island species resemble nearest mainland species more than distant islands
• Unique species evolve in isolated environments (Darwin's finches in Galápagos, marsupials in Australia)
• Plate tectonics explains distribution of related species across now-separated continents

Molecular evidence:

• DNA and protein sequence similarities reflect evolutionary relationships
• Universal genetic code supports common ancestry of all life
• Molecular clocks estimate divergence times based on mutation rates
• Pseudogenes and shared "mistakes" in DNA indicate common descent

Direct observation:

• Evolution occurs in real-time in rapidly reproducing organisms
• Documented examples include pesticide resistance in insects, heavy metal tolerance in plants, and antibiotic resistance in bacteria
• Laboratory experiments demonstrate selection acting on heritable variation (e.g., E. coli long-term evolution experiment)

Fitness and adaptation

Fitness and adaptation are central to understanding how natural selection shapes populations.

Measuring fitness:

• Absolute fitness: total number of fertile offspring an individual produces
• Relative fitness: reproductive success compared to other individuals in the population
• The phenotype with highest relative fitness is assigned a value of 1.0
• Fitness is environment-specific; a trait advantageous in one habitat may be detrimental in another
• Fitness encompasses both survival to reproductive age (viability) and reproductive output (fecundity)

Adaptations arise gradually:

• Complex adaptations evolve through accumulation of small beneficial changes
• Each intermediate stage must confer some fitness advantage
• Imperfect adaptations exist because evolution works with existing variation, not toward a predetermined goal
• Constraints include developmental limitations, historical contingency, and trade-offs between competing functions

Examples of adaptation:

• Camouflage in prey species reduces predation (leafy appearance of stick insects)
• Mimicry in butterflies deters predators (monarch and viceroy butterflies)
• Antifreeze proteins in Antarctic fish prevent ice crystal formation in blood
• C4 and CAM photosynthesis pathways in plants adapted to hot, dry environments minimize water loss while maintaining carbon fixation

Common misconceptions about natural selection

Avoid these frequent errors when explaining natural selection mechanisms.

Individuals do not evolve; populations evolve:

• An individual's genotype remains constant throughout its lifetime
• Natural selection acts on individuals, but evolutionary change occurs at the population level
• Evolution is measured as changes in allele frequencies across generations

Natural selection is not goal-directed:

• Evolution does not work "toward" a predetermined outcome
• Organisms do not develop advantageous traits "because they need them"
• Selection only acts on existing variation; it cannot create variation in response to need
• The giraffe's long neck evolved because individuals with longer necks had higher fitness, not because giraffes "tried" to reach higher leaves

Acquired characteristics are not inherited:

• Changes to an organism during its lifetime due to use or disuse are not passed to offspring
• Only changes to DNA in gametes can be inherited
• Lamarckian evolution has been disproven (though epigenetic modifications can sometimes be inherited, this is beyond AP scope)

Evolution is not random:

• While mutations occur randomly, natural selection is a non-random process
• The environment determines which traits are advantageous
• Differential survival and reproduction based on fitness is predictable and directional

Worked examples

Example 1: Analyzing antibiotic resistance data

Question: A bacterial population was exposed to increasing concentrations of an antibiotic over 20 generations. The graph shows the percentage of resistant bacteria over time.

Data: Generation 0: 2% resistant; Generation 5: 15% resistant; Generation 10: 45% resistant; Generation 15: 78% resistant; Generation 20: 95% resistant

(a) Explain how natural selection has caused this change in the bacterial population. [4 marks]

(b) State two sources of genetic variation that could have produced antibiotic-resistant bacteria. [2 marks]

Model answer:

(a)

• Initially, a small proportion (2%) of bacteria possessed alleles conferring antibiotic resistance due to pre-existing genetic variation [1 mark]
• When antibiotic was present, resistant bacteria had higher fitness/survival rates than non-resistant bacteria [1 mark]
• Resistant bacteria survived and reproduced more successfully, passing resistance alleles to offspring [1 mark]
• Over successive generations, the frequency of resistance alleles increased in the population, resulting in 95% resistant bacteria by generation 20 [1 mark]

(b)

• Random mutations in bacterial DNA [1 mark]
• Horizontal gene transfer/conjugation (receiving resistance genes from other bacteria) [1 mark]

Mark scheme notes: Part (a) requires explanation of the mechanism, not just description of the trend. Must mention variation, differential survival, inheritance, and change in allele frequency. Part (b) accepts other valid sources like transformation or transduction.

Example 2: Identifying selection types

Question: In a population of marine snails, shell thickness varies from thin (2mm) to thick (8mm). Researchers measured shell thickness and survival rates under different conditions over multiple generations.

Scenario A: A predatory crab that crushes shells colonizes the area. After 10 generations, average shell thickness increased from 4.5mm to 6.2mm.

Scenario B: Snails live in an environment with moderate wave action. Very thin shells crack easily; very thick shells are too heavy for snails to move efficiently to feed. After 10 generations, average thickness remains 4.5mm but variance decreases.

Identify and explain the type of selection operating in each scenario. [6 marks]

Model answer:

Scenario A: Directional selection [1 mark]

• The selection pressure (crab predation) favors one extreme phenotype (thick shells) [1 mark]
• Thicker-shelled individuals have higher fitness and survive to reproduce more successfully [1 mark]
• The population mean shifts toward the favored extreme (from 4.5mm to 6.2mm) [1 mark]

Scenario B: Stabilizing selection [1 mark]

• Intermediate shell thickness (around 4.5mm) has the highest fitness [1 mark]
• Both extremes (very thin and very thick) are selected against, reducing population variance while maintaining the same mean [1 mark]

Mark scheme notes: Must correctly identify selection type AND explain why that type applies. Reference to specific phenotypes and fitness differences is essential for full marks.

Example 3: Evaluating evolutionary evidence

Question: Scientists discovered that certain cave-dwelling fish species lack functional eyes, while their surface-dwelling relatives have normal vision. DNA analysis shows that cave fish possess the same eye development genes as surface fish, but these genes are not expressed during development.

Explain how this evidence supports evolution by natural selection. [4 marks]

Model answer:

• The presence of the same eye development genes in both populations demonstrates common ancestry [1 mark]
• In the cave environment, vision provides no selective advantage due to absence of light [1 mark]
• Individuals with mutations that reduced energy investment in eye development had higher fitness/more energy for other functions [1 mark]
• Over many generations, natural selection favored non-functional eyes in the cave environment, while maintaining functional eyes in surface populations where vision enhances fitness [1 mark]

Mark scheme notes: Must link molecular evidence to natural selection mechanism. Should explain both why the change occurred (fitness advantage) and why genes are retained (common ancestry).

Common mistakes and how to avoid them

• Confusing individual change with population evolution: Remember that evolution is defined as change in allele frequencies in populations over time, not changes within an individual's lifetime. An individual organism cannot evolve; it can only develop according to its existing genotype.

• Teleological language: Avoid saying organisms evolve traits "in order to" or "so that they can" survive. This implies intentionality. Instead, explain that individuals with certain pre-existing traits had higher fitness and passed those traits to more offspring.

• Ignoring the requirement for heritability: Always specify that variation must be genetic and heritable for natural selection to cause evolutionary change. Environmentally-induced variation that is not inherited cannot contribute to evolution.

• Conflating correlation with causation in data analysis: When interpreting graphs or data tables, ensure you explain the mechanism of natural selection connecting environmental change to population change, not just describe the pattern observed.

• Forgetting that fitness is relative and context-dependent: A trait's effect on fitness depends entirely on the environment. Always specify the environmental context when discussing whether a trait is advantageous or disadvantageous.

• Oversimplifying selection pressures: Many traits are influenced by multiple selection pressures simultaneously. Consider trade-offs, pleiotropy, and competing selective forces when analyzing complex scenarios.

Exam technique for "Natural Selection"

• Command word precision: "Explain" requires you to make a point and develop the reasoning (often cause and effect). "Describe" requires factual recall without explanation. For natural selection questions, explanations should include: variation exists, differential survival occurs, traits are inherited, and allele frequencies change.

• Use the data provided: Questions presenting graphs, tables, or experimental data require you to cite specific values or trends from the data in your answer. Generic responses without reference to the given information will not earn full marks.

• Structure mechanism explanations systematically: Follow a logical sequence: (1) variation in the population, (2) selection pressure acts, (3) differential survival/reproduction, (4) inheritance of advantageous traits, (5) change in allele frequency over generations. This framework works for most "explain natural selection" questions.

• Allocate time according to marks: Each mark typically requires one distinct, correct point. A 4-mark question needs four separate creditworthy statements, not one point repeated four ways. Check mark allocations and ensure your answer depth matches.

Quick revision summary

Natural selection is the non-random process by which organisms with advantageous inherited traits survive and reproduce more successfully than others, causing allele frequencies to change in populations over time. Selection can be directional, stabilizing, or disruptive depending on which phenotypes confer highest fitness. Evidence for evolution includes fossils, comparative anatomy, biogeography, molecular data, and direct observation. Fitness is environment-specific and relative. Evolution occurs in populations, not individuals, and is not goal-directed. Variation must be heritable for selection to cause evolutionary change.