Comparative Anatomy reviewer Part 2

Homologous Structures

The bones are color-coded to demonstrate that all of the organisms in the picture must have evolved from a common ancestor. Homology (shared characteristics among different species) is presented as solid evidence for biological evolution.



  • anatomical features that have the same form or function in different species that have no known common ancestor.
  • established through behavioral and biomechanical analysis
  • may or may not be homologous
  • Examples: insect wing & bird’s wing, Fish fin; whale flipper

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  • Analogous structures: wing of an insect, bird bat and pterosaur
  • Bat wings consist of flaps of skin stretched between the bones of the fingers and arm.
  • Bird wings consist of feathers extending all along the arm.
  • These structural dissimilarities suggest that bird wings and bat wings were not inherited from a common ancestor with wings.

The idea that bird wings and bat wings were not inherited from a common ancestor with wings is illustrated by the phylogeny below, which is based on a large number of other characters.


  • Bird and bat wings are analogous — that is, they have separate evolutionary origins, but are superficially similar because they have both experienced natural selection that shaped them to play a key role in flight. Analogies are the result of convergent evolution.
  • Birds and bats did not inherit wings from a common ancestor with wings, but they did inherit forelimbs from a common ancestor with forelimbs.



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  • features of two or more organisms are related by similarity of appearance
  • similarities cannot be explained by either homology or analogy
  • non homologous structural similarities between species.  In these cases, the common ancestor did not have the same anatomical structures as its descendants.  Instead, the similarities are due to independent development in the now separate evolutionary lines.
  • Misleading similarities.
  • Homoplastic structures can be the result of parallelism, convergence, analogies, or mere chance.
  • Ex: Sail fish and Pelycosaur ; Mimicry & camouflage


Mimicry or Camouflage


The Distinctions and Relations among Common Ancestry (Homology), Common Function (Analogy) and Common Appearance (Homoplasy)


Homoplastic structures can be the result of parallelism, convergence, analogies, or mere chance.

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  • Parallelism, or parallel evolution, is a similar evolutionary development in different species lines after divergence from a common ancestor that did not have the characteristic but did have an initial anatomical feature that led to it.
  • Convergence, or convergent evolution, is the development of a similar anatomical feature in distinct species lines after divergence from a common ancestor that did not have the initial trait that led to it.  (bat and bird wings.)


Linnaean scheme of Classification

  • Lumps organisms together based on presumed homologies.

Assumption :

  • The more homologies two organisms share, the closer they must be in terms of evolutionary distance.
  • The higher, more inclusive divisions of the Linnaean system are created by including together closely related clusters of the immediately lower divisions.


The result is a hierarchical system of classification with the highest category consisting of all living things.



  • This involves making a distinction between derived and primitive traits when evaluating the importance of homologies in determining placement of organisms within the Linnaean classification system.
  • Derived traits are those that have changed from the ancestral form and/or function.
  • An example is the foot of a modern horse.  Its distant early mammal ancestor had five digits.  The bones of these digits have been largely fused together in horses giving them essentially only one toe with a hoof.
  • In contrast, primates have retained the primitive characteristic of having five digits on the ends of their hands and feet.  Animals sharing a great many homologies that were recently derived, rather than only ancestral, are more likely to have a recent common ancestor.
  • Evolutionary trees depict clades.
  • A clade is a group of organisms that includes an ancestor and all descendants of that ancestor. You can think of a clade as a branch on the tree of life. Some examples of clades are shown on the tree below.


There are three basic assumptions in cladistics:

  1. Change in characteristics occurs in lineages over time.
    The assumption that characteristics of organisms change over time is the most important one in cladistics. It is only when characteristics change that we are able to recognize different lineages or groups. We call the “original” state of the characteristic plesiomorphic and the “changed” state apomorphic.kbbkbk
  2. Any group of organisms is related by descent from a common ancestor. This assumption is supported by many lines of evidence and essentially means that all life on Earth today is related and shares a common ancestor. Because of this, we can take any collection of organisms and hypothesize a meaningful pattern of relationships, provided we have the right kind of information.dyrjyt
  3. There is a bifurcating, or branching, pattern of lineage-splitting.
    This assumption suggests that when a lineage splits, it divides into exactly two groups.


Development: Ontogeny & Phylogeny


  • Developmental history of an organism affected by genes; emb; embryogenic changes due to aging and ends at death. Single lifetime



  • Evolutionary history of a Taxon (family or group of organisms) by relation to an evolutionary (ancestral) lineage.
  • 100T to 100M of years


Symmetry and Segmentation

  • describes the way in which the body of the animal meets the surrounding environment.
  • is the balanced distribution of duplicate body parts or shapes.
  • Body Symmetry: orientation of the animal body in relation to environment.


Radial Symmetry


  • Body is laid equally from a central axis; any several planes passing through divides the animal into equal halves.
  • Ex: Body of Starfish


Bilateral Symmetry

  • Body is laid equally from a mid-sagittal plane; divides the body into two, mirror halves.
  • Ex: Vertebrate Animal


Midsagittal and Sagittal (lengthwise)

-Divides the R & L parts

Coronal (frontal planes)

-Divides the ventral (anterior) and dorsal (posterior) parts.

Transverse (horizontal)

-Divides the body into superior (upper) & inferior (lower) parts.



Superior – structures higher or going cranial

Inferior – structures lower or going caudad

Posterior – structures located dorsally or back part

Anterior – structures located ventrally or front (belly) part

* In a 4-legged animal (anterior-cranial; posterior-caudal; dorsal-vertebral location; ventral-belly location)

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Directional Terms

Anterior: In front of, front
Posterior: After, behind, following, toward the rear
Distal: Away from, farther from the origin
Proximal: Near, closer to the origin
Dorsal: Near the upper surface, toward the back
Ventral: Toward the bottom, toward the belly


Superior: Above, over
Inferior: Below, under
Lateral: Toward the side, away from the mid-line
Medial: Toward the mid-line, middle, away from the side
Rostral: Toward the front
Caudal: Toward the back, toward the tail




  • segmentation (metamerism) – Division of the body along the anteroposterior axis into a serial succession of segments.
  • Divides the body into duplicated sections or metamerism
  • Metamere – segment or unit section
  • More evident in invertebrates (ex: worms) than vertebrates.
  • Ex: Backbone; Muscles of the fish; Teeth


Body Regions


  • Head/Cranium
  • Neck/Cervical
  • Thorax / Pectoral region
  • Abdomen/Peritoneum
  • Hip/Pelvis
  • Urogenital/Perineum
  • Upper extremity
  • Appendages of pectoral or chest region
  • Lower extremity
  • Appendages of pelvic or hip region


Cranial cavity

  • -Oral/buccal cavity; Nasal cavity; Orbits; Middle-ear cavity (auditory ossicles / ear bones)

Vertebral cavity

  • Thoracic / Pectoral cavity
  • -Mediastinum – breast plate
  • -Pleural cavity – encases the
  • lungs
  • -Pericardial cavity – encases
  • the heart

Abdominal or Peritoneal cavity


Pelvic / Hip cavity – encases reproductive parts

  • Perineum – encases urogenital parts





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