Genetic Drift and Population Evolution: Key Concepts and Equations

Genetic drift

Change in allele frequencies from generation to generation due to random chance.

Random and unbiased

Drift affects alleles equally; frequencies can go up or down at random.

Effect of population size

Drift is stronger in small populations and weaker in large ones.

Loss of genetic variation

Drift removes alleles over time, reducing diversity.

Population bottleneck

Drastic population reduction causes random loss of alleles and reduced diversity.

Founder effect

Small group starts a new population with different allele frequencies than the original.

Allele fixation

Random process where one allele becomes fixed and others are lost.

Population differences

Even identical starting populations can diverge by drift alone.

Effective population size (Ne)

Number of breeding individuals contributing genes to the next generation.

Effective population size equation

Ne = 4NmNf / (Nm + Nf); small Ne = strong drift.

Coalescence time

Average generations to a common ancestor ≈ 2Ne.

Neutral mutation

Evolves by drift alone; no effect on fitness.

Beneficial mutation

Selection acts, but drift adds randomness that can speed or slow fixation.

Deleterious mutation

Can fix in small populations, increasing inbreeding load and mutational meltdown.

Adaptive valley

Drift can raise an allele's frequency until selection can act on it.

Drift vs selection

Outcome depends on population size and selection strength.

Nes parameter

Nes = Ne × s; compares strength of selection to drift.

Nes >> 1

Selection dominates over drift.

Nes << 1

Drift dominates over selection.

Nes ≈ 1

Both drift and selection influence allele fate.

Neutral theory of molecular evolution

Most mutations are neutral and evolve by drift.

Nearly neutral theory

Mutations vary slightly in effect; small populations behave more neutrally.

Mutation-drift balance

Mutation adds new alleles, drift removes them; balance maintains polymorphism.

Goal of detecting selection

Identify genes or regions under selection using DNA sequence comparisons.

dN/dS ratio

Compares nonsynonymous (dN) to synonymous (dS) substitutions to detect selection.

dN/dS = 1

Gene evolves neutrally.

dN/dS < 1

Gene under purifying selection.

dN/dS > 1

Gene under positive selection.

Synonymous mutation

Does not change amino acid; usually neutral.

Nonsynonymous mutation

Changes amino acid; can affect fitness.

McDonald-Kreitman test

Compares NS/S ratios within and between species to detect selection.

Positive selection (MK test)

NS/S within < NS/S between.

Purifying selection (MK test)

NS/S within > NS/S between.

Fixed mutation

All individuals share same base difference from another species.

Polymorphic site

Variation among individuals within a species.

FST

Measure of allele frequency difference between populations.

High FST

Indicates local adaptation.

Local adaptation

Populations evolve traits suited to different environments.

Signatures of selection

Positive selection in immunity, sensory, gamete genes; purifying in cytoskeletal genes.

Take-home message

Selection alters amino-acid-changing vs silent mutations and leaves a molecular signature.

Gene flow

Mixing of alleles between populations through migration.

Effect of gene flow

Makes populations more genetically similar over time.

Role in evolution

Introduces new alleles and counteracts drift and selection.

Dispersal

Movement of individuals or gametes; can be passive or active.

Passive dispersal

Movement by wind, water, or pollinators.

Active dispersal

Movement by animals like birds or insects.

Discrete populations formula

Δp = m(pm - p); change in allele frequency by migration rate.

Continuous populations formula

Depends on migration distance variance (σ²m²).

Hardy-Weinberg departures

Used to estimate gene flow when populations differ in allele frequency.

Linkage disequilibrium

Non-random association of alleles after limited recombination.

Selection-migration balance

Opposing forces of local adaptation (selection) and homogenization (migration).

Balance equation

p² = 1 - (m/s).

Selection > migration

Local adaptation dominates; alleles fixed locally.

Migration > selection

Gene swamping; local adaptation lost.

Cline

Gradual change in allele frequencies across geography.

Short cline

Strong selection or limited gene flow.

Long cline

Weak selection or high gene flow.

Speciation

Formation of new species from ancestral populations.

Phenetic species concept

Defined by physical traits and appearance.

Phylogenetic species concept

Smallest group sharing a common ancestor on a phylogenetic tree.

Biological species concept

Interbreeding populations reproductively isolated from others.

Reproductive isolation

Reduction of gene flow through biological differences.

Premating barrier

Potential mates do not meet or recognize each other.

Prezygotic barrier

Mating occurs but fertilization fails.

Postzygotic barrier

Hybrids inviable or sterile.

Intrinsic postzygotic barrier

Genetic incompatibility reduces hybrid fitness.

Extrinsic postzygotic barrier

Hybrids less fit in environment or behaviorally disadvantaged.

Dobzhansky-Muller incompatibility

Incompatible alleles from different populations cause hybrid problems.

Haldane's Rule

Hybrid issues occur in heterogametic sex (XY or ZW).

Allopatric speciation

Occurs when populations are geographically separated.

Sympatric speciation

Occurs without geographic separation.

Diversification rate

Change in number of species over time (speciation minus extinction).

Monophyletic group

Common ancestor and all descendants.

Paraphyletic group

Common ancestor and some descendants.

Polyphyletic group

Multiple ancestors; not including common ancestor.

Sister taxa

Two groups sharing an immediate common ancestor.

Parsimony method

Tree requiring fewest evolutionary changes is most likely.

Tree thinking

Understanding evolutionary relationships through phylogenetic trees.

Microevolution

Changes in allele frequency within populations.

Macroevolution

Large-scale evolutionary change, including speciation.

Hierarchical classification

Domain → Kingdom → Phylum → Class → Order → Family → Genus → Species.

Phylogeny

Evolutionary history of a species

Tree parts

Root, node, branch, tip, sister taxa.

Sister species or clades

Two taxa sharing a recent common ancestor.

Parsimony principle

Fewest evolutionary changes = most likely explanation.

Ancestral state (plesiomorphy)

Original trait condition.

Derived state (apomorphy)

Newly evolved trait condition.

Synapomorphy

Shared derived trait unique to a clade.

Autapomorphy

Trait unique to one lineage.

Homology

Trait inherited from a common ancestor.

Analogy (homoplasy)

Similar trait evolved independently.

Evolutionary reversal

Derived trait returns to ancestral form.

Polytomy

Node with more than two branches; uncertain relationships.

3 species concepts

biological, phylogenetic, phenetic

3 reproductive isolating barriers

premating, prezygotic, postzygotic