PATTERNS OF EVOLUTION 3

PATTERNS OF EVOLUTION 3

1. INDIVIDUALISATION

   • acquisition of distinct identities by initially similar repeated modular units in organism eg vertebrate, teeth & anthropod appendages
   • important evolutionary phenomenon - important basis for mosaic evolution
     • eg teeth mostly uniform but differentiated into incisors, canines etc

2. HETEROCHRONY

   • evolutionary change in timing or rate of developmental events. 2 major categories
     • paedomorphosis - pattern where descendant shows underdevelopment of 1 or more characters relative to ancestor
       • 2 processes - neoteny & progenesis
         • neoteny - paedomorphosis achieved through relative slowing in rate of growth prior to reproductive maturation, resulting in sexually mature individuals with ancestrally juvenile features
         • progenesis - paedomorphosis achieved through relatively earlier reproductive maturation & cessation of growth
   • peramorphosis - any pattern of heterochrony where descendent shows overdevelopment of 1 or more characters relative to ancestor
   • useful metaphor for understanding importance of distinction between timing of development of somatic features & timing of maturation of gonads
     • beneficial in some ecological circumstances to mature & reproduce rapidly with minimum investment in somatic growth eg rapid proliferation of plant-sucking aphids each spring
       • other circumstances may pay to elaborate somatic features that confer competitive superiority in raising offspring when resources limited eg birds of prey relying on large territories to hunt
     • can be seen as consequence of adjustments to somatic growth that optimise reproductive success

3. SOMATIC

   • body features other than gonads & related reproductive structures

4. large size of human brain ascribed to extended period of growth

5. HETEROTOPY

   • evolutionary change in position within organism where phenotypic character expressed eg growth of roots from stems of woody vines
   • heteropic difference in species very common in plants eg major photosynthetic organs of most plants are leaves but photosynthesis carried out in stem of cacti
   • many bones of vertebrates arisen as sesamoids eg pandas 'thumb'
     • SESAMOID
       • bones that develop in tendons or other connective tissues subject to stress