New sequence variants arise.
Mutation is not directed toward what the organism needs.
Side 106
Evolution
A study of how populations change through time. Evolution is not one mechanism: mutation, recombination, selection, drift, migration and developmental constraints jointly shape lineages and adaptation.
variation→inheritance→sorting→population→lineage
06study lenses
36working concepts
V0working model
106Side
Evolution requires heritable differences among individuals.
Variation enters through mutation, recombination and other processes before selection can sort it.
Existing variants are reshuffled.
Sexual reproduction creates new combinations without creating every allele from scratch.
Genes act through development and environment.
Selection sees expressed traits, not DNA in isolation.
Populations already contain alternatives.
Rapid adaptation can use variants that predate the new pressure.
Not every imaginable form is reachable.
Developmental history and architecture limit possible variation.
Traits can improve one function while harming another.
Fitness effects are multidimensional and context-dependent.
Natural selection changes variant frequencies when traits affect reproductive success.
Selection is statistical sorting across generations, not foresight.
Relative reproductive contribution.
Fitness belongs to a trait or genotype only within a specified environment.
One end of a trait distribution is favored.
Persistent pressure can move the population mean.
Intermediate forms are favored.
Selection can reduce variance around a local optimum.
Extremes outperform intermediates.
This can maintain diversity or contribute to divergence.
Value changes with rarity.
A strategy may succeed precisely because few others use it.
Mating success changes evolution.
Traits can spread despite survival costs if they improve reproductive access.
Not all evolutionary change is adaptive.
Random sampling and migration can move allele frequencies independently of fitness.
Chance changes frequencies.
Drift is strongest in small populations.
A new population begins from a small sample.
Its genetic composition can differ sharply from the source population.
Population collapse removes variation.
Recovery in numbers does not immediately restore lost diversity.
Migration moves variants among populations.
Flow can spread adaptation or prevent local divergence.
Genetic population size differs from headcount.
Unequal reproduction and demographic structure alter the strength of drift.
Some variants have little fitness effect.
Neutral evolution provides a baseline against which selection can be inferred.
Lineages split when gene flow falls and divergence accumulates.
Species boundaries are biological patterns summarized by several competing concepts.
Barriers reduce interbreeding.
Geography, behavior, timing and ecology can all restrict gene flow.
Populations accumulate differences.
Selection and drift can push separated populations along different paths.
Hybrids may not form or persist.
Prezygotic and postzygotic barriers stabilize lineage separation.
Boundaries can remain porous.
Genes sometimes cross species lines long after divergence.
Different definitions answer different questions.
Reproductive, phylogenetic and ecological concepts need not classify every case identically.
One lineage can diversify rapidly.
New ecological opportunities can generate many descendant forms.
Evolution is reconstructed from multiple independent records.
Confidence grows when fossils, genetics, anatomy, geography and observed change converge.
Time-ordered morphology.
The fossil record is incomplete but still constrains sequence and timing.
Shared characters infer common descent.
Trees are hypotheses updated as new data change relationships.
Sequences preserve ancestry and change.
Shared errors, conserved regions and divergence patterns reveal lineage history.
Location records historical separation and dispersal.
Island and continental patterns often make little sense without common descent.
Change can be measured directly.
Resistance, experimental populations and field studies show evolution on human timescales.
Independent methods can agree.
A strong evolutionary inference survives different evidence types with different failure modes.
Adaptation is a historical claim, not a label for anything useful.
A trait can be functional today without having evolved for its current use.
What effect increases fitness?
Current contribution must be distinguished from origin.
How did the trait arise?
Selection can modify structures that originally served another role.
Old traits acquire new functions.
Feathers and other structures can be co-opted.
Best available is not perfect.
Evolution works with inherited architecture rather than designing from zero.
Adaptive value is conditional.
A trait can switch from beneficial to harmful when surroundings change.
Past adaptation can become present liability.
Rapid environmental change can outpace biological adjustment.