GEOS 26400/36400/BIOS 23255/EVOL 32300 Principles of Paleontology

Diversity, Origination, and Extinction III (Morphological and taxonomic diversity)

I. Background
      -diversity: number of taxa
      -disparity (morphological diversity): magnitude of differences among taxa
            -Some morphological distinction is implicit in erection of different taxa, but
                  magnitude of difference is unspecified.
            -Erection of species need not imply morphological difference (e.g., sometimes
      species distinctions based on fact that populations are found in different places).
      -taxonomic proxies
            -Collection of morphological data is laborious.
            -Common operational assumption: origin and number of higher taxa provides
                  index of morphological diversification and disparity.
                  -E.g., Erwin et al. (1987) study of Paleoz. and Mesoz. diversifications
                  *Assumption needs to be tested (q.v.)

II. Measuring disparity
      -Basic requirement: morphological data, preferably multivariate, on large sample of species.
            -data sources
                  -measurement of museum specimens
                  -coding of characters from specimens or from illustrated descriptions
      -Morphospace: multidimensional space whose bases are morphological variables
            (continuous or discrete)  [example: coiling parameters]
      -Disparity measures
            -range (area, volume, hypervolume...) in morphospace
                  -intuitively reasonable, but biased by sample size
            -average dissimilarity (distance in morphospace) among species
                  -variance = average squared distance of all species from mean
                        -similar measure = mean squ. dist. of all species from each other

III. Accuracy of taxonomic proxies
      -Compare diversity to disparity in a few cases where we have data on both.
            -Trilobites: measure shape of outline of cranidium (part of head)
            -Blastozoan echinoderms: discrete characters covering entire form
            -Crinoid echinoderms:      "                                   "
      -Lower taxa (genera and species): poor proxies for disparity.
      -Higher taxa (orders and suborders): reasonable proxies in some cases, not all.
      *Methodological conclusion: Higher taxa are sometimes fair proxies, but it's still
            worth using explicitly morphological data.
      *Remaining evolutionary question: Are discordances between diversity and disparity
            just noise, or is there some evolutionary information?

IV. Some simple evolutionary "rules" during diversification (if taxonomic rates constant)
      **Not an exhaustive list of possibilities!
      1. No temporal change in size of morph. differences from ancestor to descendant)
            -Expectation: disparity (variance) increases linearly with time
                  *{Note similarity to molecular diffusion and random walk, where probable
extent of occupied space increases as square root of time.  If we measured maximum or
minimum morphology in clade evolving by rule IV.1, we'd expect them to increase as square
root of time.  Since variance is a squared measure, it goes up linearly with time.}
      2. Morphological steps become smaller over time
            -Possible reasons: increasing ecological "crowding"; increasing rigidity of development
            -Expectation: disparity keeps increasing, but at ever slower rate
      3. Morphological steps become larger over time
            -Perhaps reflects ecological opportunities following "key innovation"
            -Expectation: disparity increases at an increasing rate
      4. Morphological boundaries (constraints, limits) reached
            -Expectation: disparity and range in morphospace increase only up to a point.
      **Direct testing requires detailed evolutionary tree with ancestor-descendant pairs.
      **Pattern of disparity provides indirect test when evolutionary tree not known.
      **Consistency between observation and model ("rules") need not imply that those
            rules were actually in operation.
V. Some simple evolutionary rules during decline in diversity
      1. Origination and extinction essentially "random" with respect to morphology (on suitably large scale).
            -Expectation: Disparity stays high during decline in diversity.
      2. Large-scale selectivity against whole regions in morphospace
            -This can be origination bias and/or extinction bias.
            -Expectation: Disparity and diversity decline together.
      *Again, disparity-diversity comparisons provide indirect tests.

VI. Examples
      -Trilobites: consistent with rules IV.3 and V.1
      -Blastozoa: consistent with rules IV.2, V.1 (early) and V.2 (later)
      -Blastoidea: consistent with rules IV.1 and V.1
      -Crinoidea: consistent with rule IV.4

VII. Caveats
      -Foregoing models stated in terms of morphological rules.
            -But changes in taxonomic rates can produce similar patterns.
      -E.g., common observation: decline in taxonomic rates during diversification
            -This can yield decline in rate of increase of disparity (mimicking rule IV.2)
      -Inference of rules from large scale patterns is indirect.
            -Some studies have used estimated evolutionary trees to test directly for
changes in morphological step size (e.g. Wagner on blastozoans)
      -Morphological disparity may not indicate ecological or functional disparity