Chordate Evolution

Chordate Evolution

Main Stages of Chordate Evolution

• Closest ancestors of chordates are unknown; theory suggests common origin with echinoderms.
• Similarity between Balanoglossus larva (hemichordate) and starfish larva (echinoderm) supports this.
• Chordates include:
  • Inferior chordates: Tunicata (Ascidia), Acrania (Cephalochordata)
  • Higher chordates: Vertebrata
• Vertebrates originated from ancient jawless organisms in the pre-Silurian period (Ordovician, ~510 million years ago).
• Vertebrate subtypes: Anamnia (Cyclostomata, Pisces, Amphibia) and Amniota (Reptilia, Aves, Mammalia).

Methods for Studying Phylogenesis

• Classic triple method:
  • Comparative Anatomy: Identifies similarities and differences in structure; uses homology, analogy, rudiments, atavisms, and transitional forms for kinship.
    • Homologous organs: Same structure/origin, different functions (e.g., tetrapod limbs).
    • Analogous organs: Similar function, different structure/origin (e.g., gills of crayfish and fish).
    • Rudiments: Lost former meaning (e.g., human appendix).
    • Atavisms: Return to ancestral traits (e.g., human tail).
  • Comparative Embryology: Studies embryo development; includes recapitulation study and identification of embryonic similarities.
    • Basic biogenetic law: $Ontogenesis, in an abbreviated form, repeats some phases of phylogenesis$
    • Law of embryonic similarity (Baer): Early developmental stages are more similar across organisms.
  • Paleontological Methods: Uses fossil transitional forms and construction of paleontological series.

Doctrines of A.N. Severtsov

• Focuses on how organism fitness increases during phylogenesis.
• Biological Progress: Increase in number, expansion of area, emergence of new systematic groups.
• Biological Regression: Decrease in number, narrowing of area, reduction of varieties.
• Main directions of evolution:
  • Arogenesis (morphophysiological progress): General increase in organization, access to new adaptive zone (via aromorphoses).
  • Allogenesis (morphophysiological adaptations): Development within one adaptive zone with similar forms (idioadaptations).
  • Morphophysiological Degeneration: Regressive changes that can lead to a new adaptive zone (e.g., parasitic lifestyle).

Regularities Reflecting Relationship Between Ontogeny and Phylogeny

• Phylembryogenesis Theory (A.N. Severtsov): Evolution occurs by changing the course of ontogenesis.
• Phylembryogenesis: Deviations from ancestral ontogeny with adaptive significance in adult forms.
• Types of phylembryogenesis:
  • Archallaxis: Change in early ontogenesis.
  • Deviation: Evasion of intermediate ontogenetic stages.
  • Anabolia: Change in final ontogenesis.
• Neoteny: Reduction of ontogenesis, early sexual maturity.
• Heterochrony: Changes in the timing of organ development.
• Heterotopia: Change in the location of organ development.
• Ontogenetic Correlations: Relative development of organs/traits; genomic, morphogenetic, ergontic.
• Phylogenetic Correlations: Stable interdependencies between organs; biological, topographic, dynamic.
• Function Change Principle: Strengthening/weakening of functions, expansion/decrease in function number, intensification of functions.
• Substitution: Organ replacement; homotopic, heterotopic, divaxia.
• Malformations:
  • Recapitulation can lead to atavistic malformations.
  • Incomplete reduction of recapitulating organs.
  • Violation of substitution processes.