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Systematic variation in extinction probability with biological or ecological traits. Associations can change between events and taxonomic levels: geographic range, body size and sexual dimorphism have each been linked to survival differences in particular datasets, but none is a universal predictor across all mass extinctions.

Four documented examples, each tied to its actual context, are:

These examples demonstrate event- and group-dependent selectivity. They do not define one universal vulnerability ranking: the bivalve study, for instance, did not find body-size selectivity at the same end-Cretaceous genus-level event.

The biological species concept groups natural populations that actually or potentially interbreed and are reproductively isolated from other such groups. Its central criterion is reproductive isolation and gene flow, rather than visible similarity. It is hard to apply directly to fossils, organisms reproducing strictly by asexual reproduction, and geographically separated populations whose potential interbreeding cannot be tested.

The phenetic species concept groups organisms by overall similarity in measured phenotypic traits. A practical morphological species concept uses anatomical form as its main evidence; strict phenetic classification can combine many kinds of measured traits. This makes such approaches usable for preserved specimens and much of the fossil record, but the decision depends on which traits are measured and how similarities are weighted.

At least four difficulties arise. Cryptic species may be reproductively isolated while looking nearly identical, so similarity can lump separate lineages. Conversely, sexual dimorphism, juvenile/adult differences and phenotypic plasticity can make one interbreeding species look like several, causing over-splitting. Convergent evolution can make distantly related organisms resemble one another in ecologically selected traits. Finally, continuous geographic or temporal variation supplies no unique gap at which to place a boundary: character choice, sample size, preservation and similarity thresholds can alter the inferred species delimitation. These are distinct from the biological concept's practical difficulty of testing actual interbreeding.

Extinction selectivity means that the probability of extinction depends systematically on biological or ecological traits, rather than being equal across all species. An empirical association needs a specified event, group and taxonomic level; a trait protective in ordinary background turnover may not protect during every mass extinction.

Geographic range: at the end-Cretaceous crisis, geographically widespread marine bivalve genera had lower extinction intensity than narrowly distributed genera. This makes range breadth a demonstrated selective trait for that dataset. The same analysis found no clear selectivity by body size, so that result should not be generalized to every trait or event.^[ref]

Body size: large-bodied mammals suffered disproportionately during late-Pleistocene and later Quaternary losses, including the loss of many giant herbivores and carnivores. A global analysis over the last $125000$ years found strong size-selective losses across continents and trophic modes. Large size can correlate with slow reproduction, high resource demand and exposure to human hunting, but the observed size pattern and its possible mechanisms should be distinguished.^[ref]

Sexual dimorphism and reproductive investment: a study of fossil cytheroid ostracods found much higher estimated extinction rates in species with stronger male–female carapace dimorphism, interpreted as greater male investment in reproduction. This supplies a third empirical trait association; the observed relationship is not a universal claim that sexual selection is always harmful.^[ref]

The examples concern different time intervals and levels, including genera in the first example and species in the third. They demonstrate extinction selectivity without assuming one set of trait effects is universal across all crises.

A species concept based on overall measured similarity among organisms, with explicit or implicit choices of characters, weights and grouping thresholds. A morphological species concept emphasizes anatomical similarity, while phenetic comparisons can use a wider range of phenotypic traits. Cryptic species, phenotypic plasticity, sexual dimorphism and convergent evolution explain why similarity need not coincide with reproductive isolation or a unique evolutionary lineage.