External And Internal Environments

Competition and the external environment In direct contact Most often biotic

Not in direct contact More often biotic

(often allopatric) Constraint and the internal environment

Limits on runaway change by directional evolution of parts Structural brakes upon change

Variational brakes

Lamarckian inheritance

(does not occur) Design limits of Bauplan

Rarity of new mutation allayed by recomination in sexual organisms; serious in asexual organisms


Organism, deme Clade

Speciation Extinction

Source of stasis in punctuated equilibrium 'Asexual' by budding from one indiviudal Isolation; drift and selection

Generally absent (except hybridization between species in some clades) Inherent in 'birth' process and ever present

Anagenesis (gradualism within species); rare by punctuated equilibrium Directional speciation


Species selection

Differential speciation Differential extinction

Species drift Founder drift

More likely to produce and effect by differential elimination More likely to produce and effect by differential birth

Punctuated equilibrium suppresses anagenesis by statis Positive correlation of frequency of speciation and extinction apparently unbreakable Sufficient change per new clade, but low number of species in clades continued

Table 6.1 Continued.

Feature Organism level

Species level

Developmental brakes Von Baer's law of complex

Hold of homology


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