Comparative anatomy reviewer Part 1

Evolution and speciation

  • Evolution – a change in the genetics of a population over time (generations).
  • Population – all individuals of the same species living in a defined area at the same time.

2 types

Microevolution – small genetic changes within a population

  • Occurs through several mechanisms
  • The best known is natural selection
  • Natural selection – is evolution that occurs because individuals with some traits survive and reproduce better than do individuals with other traits
  • Fitness – refers to the degree which individuals with certain traits are expected on average to survive and reproduce

Natural selection is simply the logical result of 4 features of living system:

  1. Variation – individuals from a population vary from one another
  2. Inheritance – parents pass on their traits to their offspring genetically
  3. Selection – some variants reproduce more than others
  4. Time – successful variations accumulate over many generations

Darwin’s 4 postulates on natural selection:

  1. More young are produced each generation that can survive to reproduce
  2. Individuals in a population vary in their characteristics
  3. Differences among individuals are based on genetic differences
  4. Individuals with few strong characteristics survive and reproduce better


  • A series of micro evolution
  • Speciation – the formation of new species when one ancestral species evolves more than 1 typical descendants
  • Since speciation occurs when one species evolve into more than 1 new species it increases the number of species that exist


  • Hereditary traits expressed (phenotype) in the organism’s morphology increases it’s chance for survival – mutation of genes
  • Evolutionary process of part modification by an organism to fit to its mode of life in a particular environment
  • Environmental factors that have influenced this mutation is through natural selection, mutant genes have a survival value


  • Arise of new species from primitive ancestor as a result of geographical isolation of a population that leads to genetic drift of species
  • (Ex: Palawan species may have the same Borneo species).
  • Genetic shift – short mutation (minor)
  • Genetic drift – long mutation ( major)
  • Consequence: reproductive isolation
  • Isolating populations in different environments can lead to the beginning of reproductive isolation. These results are consistent with the idea of geographic isolation is an important step of some speciation event.


  • Or niche, if habitat alters rapidly without giving time for species to adapt to new condition, the habitat disappears as species remains
  • Unsatisfactory habitat – the old way of life weakens
  • Some species may suit in the offers of the new habitat. Selection pressure is strengthened, shifting towards the new habitat. (+) Evolved species in a habitat.
  • Change in form – result of the interplay between changing environment and adapting organisms
  • Habitat – nature as living conditions, acts as selection pressure for the screening process of evolution, the genetic mutation and inheritance produce the adapting model.

2 types:

  • Linear evolution – habitat alters as 1 unit = change is in a straight line. (adaptation is dependent to each other)
  • Branching evolution –if the habitat subdivides into several units = original population of organism becomes isolated from one another (adaptation is independent).

Structures that arise from evolution and habitat formation can lead to…

Structures Specialized General
Morphology Modified part; uncommon Suited adequately; common
Function Restricted/limited Less restricted
Effectivity Maximum but specific Various uses


Adaptability Rarely adjusts; more pressured for change Flexible; less pressured for change


Evolutionary Trend or Morphocline

  • Gradual adaptive change in the evolution of a feature within a phyletic line. -> moderate change
  • (+) large population
  • Prolonged by selection pressure
  • Traces evolutionary path
  • Constant directional change -> progress at constant rate with temporary arrests advantageous for survival

Parallelism and Convergence


  • Evolutionary change in 2 or more lineages with common ancestor
  • Corresponding features undergo equivalent similarities.
  • Independent similarities within species share common ancestry.
  • Descendants appearance is same to their ancestors but exactness is not present due to effects of natural selection. Animals with identical functional requirements lead to similar structural adaptation.
  • Ex: Rats (non-flying) & Bats (flying) mammals
  • Kangaroo rat of North America and Jerboa of Africa
  • Similarities:
  • Long limbs and Short forelimbs
  • Loss of lateral toes and Long tail with white tip,
  • Large eyes and Compacted cervical vertebrae
  • Parallelism – The ancestor is common but both A and B have evolved a primitive trait independently.


  • Evolutionary change in 2 or more lineages that have similar features (ancestry is not common).
  • Two or more different phyletic lines had increasing similarity in features.
  • Descendants were more alike compared to their ancestor since ancestors are more remote.
  • Influenced by similar climates and habitats.
  • Ex: Sharks, Icthyosaur, Dolphins (similar habitats but different ancestors) more become similar in functional structures than phylogenetic relationship.

Divergent evolution – is when two different species share the same ancestral origins but have evolved differently.

  • Both the wooly mammoth and the elephant originated from a common ancestor, but the common ancestor eventually diverged, leading to two new species.

Convergent evolution – is when species with different ancestral origins have developed similar features.

  • These two species look very similar but are not closely related. Flying squirrels are placental mammals like whales, dogs, and humans,where as sugar gliders are marsupials like kangaroos and possums. The species Current similarity is an example of convergent evolution; they have begun to look more similar due to similar adaptations, not because of common ancestry.
  • The common ancestor was a primitive armored fish unlike any of them; shark has no terrestrial ancestor, ichthyosaur and dolphin have dissimilar terrestrial ancestors, nevertheless they have remarkable resemblance.


Geologists divide the history of the Earth into eras and periods, the boundaries of which were times of rapid change in the Earth’s crust and in its biota.

At least five times in the past 500 MY there have been many cases of mass extinction when as many as 60% of all genera died out within the span of 5 MY.

When do species become extinct?

Species become extinct when they cannot adapt to sudden shifts in their environment as:

  • Climatic change
  • Increase in competition for resources
  • Misbalance in predator-prey relationships
  • Alteration in host-parasites relation


  • Entire assemblages of animals become extinct when the scale of the environmental change is extreme major change in vegetation, or significant shift in sea level.
  • Catastrophic events such as impacts by asteroids and devastating volcanic activity could cause mass extinction.
  • Different phyletic lines evolve at different rates, each line evolves at different rates at different times, and different characters of one line evolve at different rates at the same time




  • Study of phylogenetic relationships
  • Active area of evolutionary biology
  • CLADISTICS –special area of systematics that studies phylogenetic relationships based on shared or derived traits.


  • Provides in-depth knowledge about evolution of traits within groups.
  • Traces the origin & spread of diseases especially zoonotic diseases (animal human).
  • Relation of species helps in formulation of advocacy programs for conservation of species.

2 Approaches in studying relationships of species…

  • Fossils –provide a preserved record of the history of life forms; portrays the phylogeny of life.
  • Hierarchical pattern of homology –different species that share the same structures depicts that they may have evolved from the same ancestor. (common features / traits shared close relationship of species; less traits shared distant relationships)
  • CLADISTICS –answers those gaps in systematics that do not rely in the number of shared characteristics.

Phyletic Line

  • A lineage that is relatively continuous & complete in the fossil record.
  • Genera (sing. Genus) are related by evolution (linear & branching) and progressive change from extinct organisms.
  • Each phyletic line evolves at diff. rates diff. times while diff. characters of 1 phyletic line evolve at diff. rates simultaneously.


  • Shows the pattern of evolutionary relationships or history of speciation.
  • Represented by a TREE that shows where points of ancestors speciated into 2 new species.


Dendograms & Cladistics

  • Modern vertebrates differ from their ancestors; evolutionary hx. Can be traced using similarities in morphological characteristics thus, relationships between groups would imply close relationships.
  • Primitive condition – ancestral
  • Derived condition-descendants


  • Summary representation of evolutionary course or phylogeny
  • Branched connections between related groups like a tree PHYLOGENETIC TREE
  • Illustrates the evolutionary history of related organisms
  • It also shows abundance & diversity of species



  • Legend: Every line that branches into species above the branch (descendants) arise from species below the branch point (ancestor).
  • Ex: Trees on the right shows the relationship between mouse, bird, lizard, snake.
  • Trees on the left shows relationship between salamander and frogs.



  • Primitive Traits (plesiomorphic characters) are characters of organisms that were present in the ancestor of a certain group of related organisms
  • –Ex: Ancestor of lizard, bird, alligator = scaly skin; 3-chambered heart; (+ )teeth; (-) wings
  • Derived Traits (apomorphic characters) are characters of organisms that have evolved within the group or related organisms that were not present in the ancestor.
  • Ex: birds, lizard, alligator = gizzard; 4-chambered heart, feathers, (-) teeth; wings


  • Character/s is present in immediate ancestor only but not in the earliest ancestor.
  • Derivative traits


  • Character/s is present in immediate ancestor and earlier ancestor.
  • Primitive traits
  • Ex: Birds developed wings but lost the primitive 4 legs that have been present with the birds’ ancestor.


  • Method in determining primitive vs. derived traits.
  • Determine 1 or more species that are relatives of the group of interest (ingroup), and the species equally related to all members of the group of interest (outgroup).
  • Character/s of comparison found common in both groups is considered Primitive trait, while, character/s found common only in one group but absent to other is considered as Derived trait.


Outgroup comparison

  • Bony skeleton
  • (+) trout, frog, mouse; (-) shark
  • Therefore, ancestor B had evolved a bony skeleton.
  • In general, provides evidence of ancestry that bony skeleton as a phylogenetic trait traced from ancestor B to trout, frog, mouse.


  • A paraphyletic group (incomplete clade); stage of evolutionary attainment (adaptation) expressed by an evolving group. (Ex: jawless to jawed fishes)
  • An incomplete clade lacks one or more component lineages.
  • If more distinct derived characters are present shared in groups a new GRADE is attained.


  • A natural evolutionary lineage which includes an ancestor +all and only descendants.
  • Descendants may have high similarity or difference from their ancestors (variation in morphology is not restricted).
  • CLADOGRAMS – a hypothetical diagram of lineages & evolutionary relationships (geological time scale is not included).


Cladogram (family tree) of a biological group. The red and blue boxes represent clades (i.e., complete branches). The green box is not a clade, but rather represents an evolutionary grade, an incomplete group, because the blue clade descends from it, but is excluded.

Types of Clades

Monophyletic clade

  • Includes an ancestor + all descendants

Paraphyletic clade

  • Includes more than one ancestor + but not all descendants

Polyphyletic clade

  • Does not share an immediate common ancestor



Anatomical and Evolutionary Concept

Taxonomic Principles

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


  • 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 characters — characters in different organisms that are similar because they were inherited from a common ancestor that also had that character.


An example of homologous characters is the four limbs of tetrapods Birds, bats, mice, and crocodiles all have four limbs. Sharks and bony fish do not. The ancestor of tetrapods evolved four limbs, and its descendents have inherited that feature — so the presence of four limbs is a homology.

Not all characters are homologies. For example, birds and bats both have wings, while mice and crocodiles do not. Does that mean that birds and bats are more closely related to one another than to mice and crocodiles?





Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s