Anatomical and Evolutionary ConceptMa.Luisa V. Cuaresma

Biological Sciences Department

Taxonomic Principles

• Biologists classify organisms into different categories mostly by judging degrees of apparent similarity and difference that they can see. 

What if they discovered an

unknown organism? How do they begin

with their classification?

Assumption: The greater the degree of physical similarity = the closer the biological relationship.

Researchers begin their classification by:

• looking for anatomical features that appear to have the same function as those found on other species. 

• determining whether or not the similarities are due to an independent evolutionary development or to descent from a common ancestor. – If the latter is the case, then the two

species are probably closely related and

– should be classified into the same or near biological categories.

Similarities: Homology, Analogy and HomoplasyHomology features of two or more organisms sharing common

ancestry. anatomical features, of different organisms, that have a

similar appearance or function because they were inherited from a common ancestor that also had them. 

•The wing of a cat, bat, whale and your arm have the same functional types of bones as did our shared reptilian ancestor.   Therefore, these bones are homologous structures.•The more homologies two organisms possess, the more likely it is that they have a close genetic relationship.

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.

Analogy• 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

Analogous structures: wing of an insect, bird bat and pterosaur

Homoplasy• features of two or more organisms are

related by similarity of appearance • similarities cannot be explained by

either homology or analogy• nonhomologous 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)

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Ancestry

AppearanceFunction

Legend:1-Same ancestor; Diff. fxn.; Diff. appearance

2-Same ancestor; same fxn.; diff. appearance

3-Same ancestor; same fxn.; same appearance

4-Same ancestor; diff. fxn.; same appearance5-Diff. ancestor; same fxn.; diff. appearance6-Diff. ancestor; same fxn.; same appearance7-Diff. ancestor; diff. fxn.; same appearance

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

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. 

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. 

order

family family

genus genus genus genus

  species  

  species  

  species  

  species  

  species  

  species  

  species  

  species  

•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. 

Cladistics 

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

Development: Ontogeny & Phylogeny

Phylogenesis• 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 cranialInferior – structures lower or going caudadPosterior – structures located dorsally or back partAnterior – structures located ventrally or front (belly) part* In a 4-legged animal (anterior-cranial; posterior-caudal; dorsal-vertebral location; ventral-belly location)

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• 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 RegionsHead/CraniumNeck/CervicalThorax / Pectoral regionAbdomen/PeritoneumHip/PelvisUrogenital/PerineumUpper extremity

Appendages of pectoral or chest region

Lower extremityAppendages of pelvic or hip region

Cranial cavity-Oral/buccal cavity;

Nasal cavity; Orbits; Middle-ear cavity (auditory ossicles / ear bones)Vertebral cavityThoracic / Pectoral cavity

-Mediastinum – breast plate

-Pleural cavity – encases the lungs

-Pericardial cavity – encases the heartAbdominal or Peritoneal cavityPelvic / Hip cavity – encases

reproductive partsPerineum – encases urogenital parts

Body Cavity