10/12 10: Evolution by Natural Selection

Evolution as Population Change

Evolution describes both a historical pattern—life changing through time—and the mechanisms that produce that change. The central biological distinction is that individuals develop or acclimate during their lifetimes, whereas populations evolve.

In population genetics, evolution is commonly defined as a heritable change in allele frequencies across generations. Natural selection is one important mechanism, but mutation, genetic drift, gene flow, and nonrandom mating also influence population genetic composition.

A connected explanation

A population begins with heritable variation. Environmental conditions affect which phenotypes survive and reproduce most successfully. If reproductive differences are associated with inherited genetic differences, the frequencies of alleles can change in later generations. Repeated changes can produce adaptations, divergence among populations, and eventually new species.

The essential scale of analysis is the population across generations, not the intentional improvement of an individual organism.

Takeaway: Evolution is descent with modification, measured as heritable population change across generations.

Evidence for Common Ancestry

The evidence for evolution comes from several independent lines of inquiry that converge on common ancestry and descent with modification. A strong explanation also makes testable predictions. For example, evolutionary reasoning can predict that fossils with combinations of fishlike and limbed-vertebrate traits should occur in rocks of an appropriate age and environment.

Major lines of evidence

• Fossil evidence: Fossils occur in an ordered geological sequence and document appearances, transformations, radiations, and extinctions. Transitional fossils combine traits associated with different major groups; they are not incomplete or half-finished organisms.

• Comparative anatomy: Homologous structures have a shared developmental or anatomical origin because of common ancestry. Analogous structures perform similar functions but evolved independently. Vestigial structures are reduced remnants inherited from ancestors and may still have secondary functions.

• Developmental evidence: Related organisms may share developmental pathways or structures that are later modified, reduced, or repurposed.

• Biogeography: Geographic distributions can reflect colonization, isolation, local adaptation, and diversification. Island radiations illustrate how one ancestral lineage can produce several specialized descendants.

• Molecular evidence: DNA and protein sequences, conserved genes, shared mutations, pseudogenes, chromosomal arrangements, and genetic codes can reveal degrees of relatedness and deep relationships among lineages.

• Direct observation: Changes in antibiotic resistance, antiviral resistance, pesticide resistance, and traits in natural populations demonstrate evolutionary change when heritable variants change in frequency.

No single fossil, structure, sequence, or observation is required to carry the entire explanation. The strength comes from agreement among different kinds of evidence.

Takeaway: Evolution is supported by mutually reinforcing evidence from geology, anatomy, development, geography, molecules, and observed population change.

Natural Selection and Fitness

Natural selection occurs when individuals with different heritable phenotypes leave different numbers of surviving, fertile offspring in a particular environment. It does not create traits because organisms need them, and it does not guarantee perfection. Instead, it modifies existing variation under specific conditions.

Four conditions for natural selection

1. Variation: Individuals in a population differ in phenotype.

2. Heritability: Some differences are influenced by genetic variation that can be passed to offspring.

3. Competition or limited resources: Populations generally produce more offspring than the environment can support.

4. Differential reproductive success: Some phenotypes result in greater survival or reproduction than others under the prevailing conditions.

When these conditions persist, alleles associated with successful phenotypes may become more common. The environment does not directly select an allele; it affects survival and reproduction through phenotypic differences.

Fitness and selection patterns

Fitness means relative reproductive success, not physical strength or everyday health. It depends on environmental circumstances. A trait that is advantageous during drought may be less advantageous when food, temperature, or predators change.

• Directional selection favors one extreme phenotype and shifts the population mean.

• Stabilizing selection favors intermediate phenotypes and reduces variation around the average.

• Disruptive selection favors both extremes over the intermediate phenotype and may promote population subdivision.

• Sexual selection favors traits that improve mating success, even when those traits impose survival costs.

For example, if a drought leaves mostly large, hard seeds, birds with deeper beaks may obtain more food and produce more offspring. The population changes only if beak differences are heritable or genetically associated with heritable variation.

Takeaway: Selection changes populations through unequal reproductive success acting on heritable variation.

Adaptation, Trade-Offs, and Acclimation

An adaptation is a population-level result of selection, not an intentional improvement made by an individual. If a heritable trait repeatedly increases reproductive success under particular conditions, alleles associated with that trait may become more common.

White winter coloration in an Arctic fox can reduce visibility against snow. The fox does not consciously become white because it needs camouflage. Instead, if coloration has a heritable basis and less-visible foxes leave more offspring, winter-color alleles may increase in frequency. Similar camouflage in distantly related organisms can arise through convergent evolution when similar selection pressures produce similar phenotypes.

Adaptations involve trade-offs. A feeding structure may increase food acquisition while also increasing exposure to predators. An immune response may help eliminate pathogens while damaging host tissues. Investment in reproduction may reduce long-term survival. An adaptation can also lose its advantage when the environment changes.

Adaptation versus acclimation

Acclimation is a reversible adjustment within an individual's lifetime, such as increased red-blood-cell production at high altitude. Adaptation is a heritable population-level change accumulated across generations.

Takeaway: Adaptations are inherited population characteristics shaped by environmental conditions, whereas acclimations are reversible individual responses.

Mechanisms of Population Evolution

The gene pool is the total collection of alleles in a population. Evolution occurs when processes alter the frequencies of those alleles.

Mechanisms that change genetic composition

• Mutation creates new DNA variants and is the ultimate source of new alleles. Most mutations are neutral with respect to fitness, some are harmful, and a smaller proportion may be beneficial in a particular environment.

• Sexual reproduction reshuffles existing alleles through meiosis, crossing over, independent assortment, and fertilization.

• Gene flow moves alleles among populations when individuals or gametes migrate and reproduce; it can reduce differences between populations.

• Genetic drift changes allele frequencies by chance and is especially influential in small populations.

• A founder effect occurs when a small group establishes a new population with only a sample of the original gene pool.

• A bottleneck occurs when a population is sharply reduced, leaving a chance-based sample of its former diversity.

• Natural selection changes allele frequencies when heritable phenotypes differ in reproductive success.

Hardy–Weinberg reasoning

The Hardy–Weinberg model supplies a null expectation for a population that is not changing genetically. It assumes a very large population, random mating, no mutation, no migration, no natural selection, and no genetic drift.

For two alleles, $A$ and $a$, with allele frequencies $p$ and $q$:

The expected genotype frequencies are:

Here, $p^2$ represents $AA$, $2pq$ represents $Aa$, and $q^2$ represents $aa$. A departure from these expectations indicates that one or more assumptions may not hold. It does not, by itself, prove that natural selection caused the departure; researchers must also consider sampling error, nonrandom mating, population size, migration, mutation, and drift.

Takeaway: Population genetics connects evolutionary mechanisms to measurable changes in allele and genotype frequencies.

Speciation and Reproductive Isolation

Speciation occurs when populations diverge and develop reproductive isolation, reducing or eliminating gene flow. Under the biological species concept, a species is generally a group of natural populations whose members can interbreed and produce viable, fertile offspring. This definition is less straightforward for asexual organisms, fossils, and populations that hybridize frequently.

Geographic patterns of speciation

• Allopatric speciation begins when a physical barrier such as a mountain range, river, glacier, island, habitat fragment, or long distance separates populations. The isolated populations then evolve independently through mutation, selection, drift, and different mating patterns.

• Peripatric speciation is a form of geographic speciation in which a small peripheral population becomes isolated from a much larger parent population. Founder effects and genetic drift can be especially important.

• Sympatric speciation occurs in the same geographic region. Polyploidy in plants, disruptive selection, host or habitat specialization, and assortative mating can reduce gene flow without complete geographic separation.

• Parapatric speciation occurs when neighboring populations experience different selection pressures and limited gene flow across a geographic boundary or environmental gradient.

Reproductive barriers

Prezygotic barriers act before fertilization and include habitat isolation, temporal isolation, behavioral isolation, mechanical incompatibility, and gametic incompatibility. Postzygotic barriers act after a zygote forms and include reduced hybrid viability, reduced hybrid fertility, and hybrid breakdown in later generations.

Speciation is often gradual. Populations may pass through stages of partial isolation, including local varieties, ecotypes, hybrid zones, or subspecies, before becoming fully independent species.

Takeaway: New species form when divergence is reinforced by barriers that reduce successful gene exchange.

Phylogenetic Reasoning

A phylogenetic tree is an evidence-based hypothesis about evolutionary relationships. It is not necessarily a literal record of every ancestor, and it may be revised when new fossil, anatomical, developmental, molecular, or biogeographic evidence becomes available.

Reading a rooted tree

• The root represents the oldest common ancestry shown.

• A branch represents a lineage through time.

• A node represents a common ancestor and a lineage-divergence event.

• Sister taxa share an immediate common ancestor.

• A clade includes an ancestor and all of its descendants.

• A polytomy is a node with more than two branches and may indicate unresolved relationships or rapid diversification.

The most closely related taxa are those that share the most recent common ancestor, not necessarily those that look most alike. Branches can rotate around a node without changing relationships. Unless a time scale is explicitly provided, branch length does not necessarily indicate elapsed time or amount of evolutionary change.

Building and evaluating trees

Scientists compare shared derived characters—traits that evolved in a common ancestor and were inherited by its descendants. A trait shared by a broad group may be ancestral for a smaller group, whereas a trait restricted to a particular clade may be a shared derived character.

Evidence for phylogenies can include morphology, development, DNA, RNA, protein sequences, fossil ages, anatomical transitions, and geographic distributions. Cladistics groups organisms by shared derived characters. Maximum parsimony favors the tree requiring the fewest evolutionary changes, while modern analyses may also use probabilistic or model-based methods.

Similarities must be evaluated carefully. A similarity may be homologous because of common ancestry or analogous because of independent evolution under similar environmental pressures. Hybridization and horizontal gene transfer can also make a branching tree incomplete; in some cases, a network or web better represents evolutionary history.

Takeaway: Interpret trees through common ancestry and branching order, and evaluate the evidence and assumptions behind each proposed relationship.

Evidence-Based Evolutionary Investigations

Evolutionary investigations turn concepts into questions, predictions, data, and evidence-based claims. A sound investigation defines variables, identifies limitations, and considers alternative explanations.

Modeling natural selection

A colored-paper model can represent organisms exposed to a contrasting or matching background. After each simulated generation, record phenotype counts, assign capture probabilities, and allow survivors to reproduce in proportion to survival. The prediction is that camouflage increases survival only when the background is consistent with the phenotype. The population changes because individuals with different phenotypes contribute different numbers of offspring, not because individuals intentionally change color.

The model may exaggerate selection, omit mutation and migration, and simplify reproduction. These assumptions should be distinguished from conditions in natural populations.

Finches, food, and beak variation

A strong claim about finch beaks identifies which phenotype had higher fitness under stated conditions. Evidence may include quantitative differences in survival, feeding success, or offspring production. The reasoning must connect the phenotype to heritable variation so that differential reproduction could alter the population in later generations.

Hardy–Weinberg analysis

Given genotype counts, calculate allele frequencies, expected genotype frequencies, and deviations from the null expectation. A numerical mismatch does not automatically prove natural selection. Sampling error, nonrandom mating, small population size, migration, mutation, or drift may also explain the pattern.

Constructing a phylogenetic tree

Use morphological or DNA-character data to identify shared derived characters, construct alternative trees, and defend the preferred tree. Ask whether a similarity is homologous or analogous, whether a character is ancestral or derived, and whether the proposed tree requires unnecessary evolutionary changes. Differences between trees may result from character choice, incomplete data, convergent evolution, hybridization, or horizontal gene transfer.

Takeaway: Strong evolutionary explanations link a clear claim to quantitative evidence and reasoning while acknowledging model limitations and alternative causes.

• Allele frequency