GATE Ecology and Evolution Evolution Syllabus

Name the three classic modes of natural selection acting on a quantitative trait.

Stabilizing selection, directional selection, and disruptive (diversifying) selection.

Describe stabilizing selection and its effect on phenotypic variance.

Stabilizing selection favors intermediate phenotypes and selects against both extremes, keeping the population mean constant while reducing phenotypic variance. Classic example: human birth weight.

Describe directional selection and its effect on the trait distribution.

Directional selection favors one extreme phenotype, shifting the population mean toward that extreme over generations and reducing variation at the disfavored tail. Example: peppered moth melanism; antibiotic/pesticide resistance.

Describe disruptive (diversifying) selection and its effect.

Disruptive selection favors both extreme phenotypes and selects against intermediates, increasing variance and producing a bimodal distribution; it can promote sympatric speciation. Example: bill sizes in seed-eating finches.

Which mode of selection increases phenotypic variance, and which decreases it?

Disruptive selection increases phenotypic variance (bimodal); stabilizing selection decreases it. Directional selection shifts the mean while typically reducing variance at one tail.

What is frequency-dependent selection, and how do its two forms differ?

Selection in which a genotype's fitness depends on its frequency. Negative frequency-dependent selection favors rare phenotypes (maintains polymorphism); positive frequency-dependent selection favors common phenotypes (reduces diversity).

What is sexual selection, and what are its two main mechanisms?

Selection arising from differential mating success. Mechanisms: intrasexual selection (competition among one sex, usually males, e.g., antlers) and intersexual selection (mate choice, usually female choice of male ornaments, e.g., peacock tail).

Approximately how old is the Earth and when did life first appear?

Earth is about $4.6\times10^{9}$ years (4.6 billion years) old; the earliest evidence of life (prokaryotic cells/stromatolites) dates to roughly $3.5$–$3.8\times10^{9}$ years ago.

What is the Oparin–Haldane hypothesis on the origin of life?

Life arose through chemical evolution: simple inorganic molecules formed organic monomers in a reducing primitive atmosphere, accumulating in a "primordial soup," then polymerized into macromolecules and eventually self-replicating systems (coacervates/protocells).

What did the Miller–Urey experiment (1953) demonstrate?

Passing electric sparks through a simulated reducing primitive atmosphere ($\ce{CH4}$, $\ce{NH3}$, $\ce{H2}$, $\ce{H2O}$) abiotically produced amino acids and other organic molecules, supporting chemical evolution and the Oparin–Haldane hypothesis.

What is the "RNA world" hypothesis?

The proposal that early life used RNA as both genetic material and catalyst (ribozymes), before DNA and proteins took over those roles, because RNA can store information and catalyze reactions (e.g., self-splicing introns, the ribosome's catalytic RNA).

What is the endosymbiotic theory and who is most associated with it?

Lynn Margulis's theory that eukaryotic mitochondria and chloroplasts originated from free-living prokaryotes (alpha-proteobacteria and cyanobacteria) engulfed by a host cell; supported by their own circular DNA, double membranes, and 70S ribosomes.

List the geological eons/eras in order from oldest to youngest.

Hadean → Archean → Proterozoic (these three within the Precambrian) → Phanerozoic, which is divided into the Paleozoic → Mesozoic → Cenozoic eras.

What was the Cambrian explosion?

A relatively rapid diversification about $540$ million years ago at the start of the Cambrian, during which most major animal phyla (body plans) first appear in the fossil record.

What is biodiversity, and what are its three commonly recognized levels?

Biodiversity is the variety of life. Levels: genetic diversity (within species), species diversity (number/abundance of species in a community), and ecosystem (community/habitat) diversity.

What is taxonomy versus systematics?

Taxonomy is the science of identifying, naming, and classifying organisms (nomenclature and hierarchy); systematics is the broader study of the diversity of organisms and their evolutionary relationships (phylogeny), of which taxonomy is a part.

List the Linnaean taxonomic hierarchy from broadest to most specific.

Domain → Kingdom → Phylum (Division in plants) → Class → Order → Family → Genus → Species.

What is binomial nomenclature and who introduced it?

Carolus Linnaeus's two-name system for species: a capitalized Genus name followed by a lowercase specific epithet, italicized (e.g., $Homo\ sapiens$); governed by international codes of nomenclature.

Name the three domains of life and the basis for the system (Carl Woese).

Bacteria, Archaea, and Eukarya. Based on Carl Woese's comparison of small-subunit (16S/18S) ribosomal RNA sequences, revealing Archaea as distinct from Bacteria.

Name Whittaker's five kingdoms and the key criteria used.

Monera, Protista, Fungi, Plantae, and Animalia. Criteria: cell structure (prokaryotic vs eukaryotic), level of organization, mode of nutrition (autotrophic vs heterotrophic), and phylogenetic relationships.

Contrast artificial, natural, and phylogenetic systems of classification.

Artificial systems use one or few arbitrary characters (e.g., Linnaeus's sexual system); natural systems use many overall morphological/anatomical similarities; phylogenetic systems group organisms by evolutionary (ancestor–descendant) relationships.

What is cladistics, and what is its grouping criterion?

Cladistics (phylogenetic systematics, founded by Willi Hennig) classifies organisms strictly by shared derived characters (synapomorphies), grouping taxa into clades that include a common ancestor and all its descendants (monophyletic groups).

Define a clade and the three types of grouping in cladistics.

A clade is a monophyletic group (an ancestor plus all its descendants). Monophyletic = ancestor + all descendants; paraphyletic = ancestor + some descendants (e.g., reptiles excluding birds); polyphyletic = members from different ancestors without their common ancestor.

Distinguish a plesiomorphy, an apomorphy, and a synapomorphy.

A plesiomorphy is an ancestral (primitive) character state; an apomorphy is a derived (novel) state; a synapomorphy is a shared derived character used to define a clade; a symplesiomorphy (shared ancestral trait) cannot define a clade.

Distinguish homology from homoplasy (analogy/convergence).

Homology is similarity due to shared common ancestry (e.g., tetrapod forelimbs); homoplasy is similarity due to convergent or parallel evolution, not common descent (e.g., bird and insect wings). Only homologies inform phylogeny.

What is phenetics (numerical taxonomy), and how does it differ from cladistics?

Phenetics classifies organisms by overall similarity across many equally weighted characters (without regard to ancestral vs derived states or evolutionary history), often producing phenograms; cladistics uses only shared derived characters to infer evolutionary relationships.

What principle is used to choose among competing cladograms (trees)?

Maximum parsimony — the preferred tree is the one requiring the fewest evolutionary changes (character state transformations), i.e., the simplest hypothesis explaining the data.

What is the principle of resource allocation in life-history theory?

An organism has a finite energy/resource budget that must be partitioned among competing functions—growth, maintenance/survival, and reproduction—so investment in one function reduces what is available for others.

What is a trade-off in life-history evolution? Give an example.

A trade-off occurs when an increase in fitness via one trait causes a decrease via another because resources are limited. Example: the trade-off between number (quantity) and size (quality) of offspring, or between current reproduction and future survival (cost of reproduction).

Compare $r$-selected and $K$-selected life-history strategies.

$r$-strategists: high reproductive rate, many small offspring, little parental care, early maturity, short lifespan, unstable environments. $K$-strategists: low reproductive rate, few large offspring, much parental care, late maturity, long lifespan, stable environments near carrying capacity $K$.

What is the principle of allocation as it relates to the cost of reproduction?

Because resources allocated to reproduction cannot also be used for survival and maintenance, increased current reproductive effort typically reduces future survival and fecundity—this trade-off shapes the evolution of optimal clutch size, age at maturity, and semelparity vs iteroparity.