HOMOLOGY( Methods in identification

of homology species concepts )

Presented by,

Guru P. N.Ph. D. 015/39

Dept. of Agril. Entomology,PGI, MPKV, Rahuri.

Species Concepts  

1.The Typological Species Concept (TSC, Linnaeus)

  

A group of individuals that differ from other groups by possessing constant diagnostic characters.  

2. The Biological Species Concept (BSC, Dobzhansky, Mayr)

  

Mayr (1940):

species are groups of actually or potentially interbreeding natural populations that are reproductively isolated from other such groups.

Dobzhansky (1937):

species are the largest and most inclusive reproductive community of sexual and cross-fertilizing individuals that share a common gene pool.

3. The Evolutionary Species Concept (ESC, Simpson, 1951)

 

An evolutionary species is a lineage evolving separately from others with its own unitary evolutionary role and tendencies.

4. The Phylogenetic Species Concept (PSC, Cracraft, 1983)

  

The smallest diagnosable monophyletic group of populations within which there is a parental pattern of ancestry and descent.

Homology species concept

Universal ancestor

Comparative study reveals that they are based upon a common prototype or ground plan, which varies with the organism’s particular environment

The interpretation of anatomy requires an understanding of homologous versus analogous structures:

COMPARATIVE ANATOMY

wings Vertebrate forearms

Homology

• Similarity resulting from common ancestry.

e.g., the forelimb bones of a bird, bat, and cat.

Homoplasy (analogy)

• Similarity not due to common ancestry

• Reversal – loss of new (apomorphic) feature, resembles ancestral (old) feature.

• Convergence (parallelism) – gain of new, similar features independently.

Comparison of insect and bird wings to show their analogy

a. HomologyTwo species bearing the same phenotype caused by common ancestry for the same genotype.

c. ConvergenceTwo species with the same phenotype whose common ancestor is very far in the distant past

b. ParallelismTwo species with the same phenotype descended from a common ancestor with a different phenotype and genotype

Species 1

Species 2

Phenotype a

Phenotype b

Species 1

Species 2

Species 1

Species 2

Ancestor phenotype a

Ancestor phenotype b

Phenotype c

Homology is assessed regardless of shape or function. Only morphological equivalence in terms of relative position, structure, and connections with nerves and blood vessels counts.

HOMOLOGY FORMS

Phylogenetic or evolutionary homology: provided the organisms are members of different species.

commonly used in systematics and phylogenetics

Iterative homology, i.e. homology between parts of the same organism, like the homology between foliage leaves and petals on flowering plants.

(homonomy, or serial homology, if the structures are arranged along the main body axis)

1. The Idealistic Homology Concept : non-historic causes

• Two characters are thought to be homologous because they are built according to the same plan or archetype.

2. The Historical Homology Concept

• Relationship between those parts of different organisms that correspond to an equivalent organ in a common ancestor.

3. The Biological Homology Concept (Roth, 1984)

• “Homology is resemblance caused by a continuity of information”

• Roth: "sharing of pathways of development, ( . . . ) controlled by genealogically related genes"

DIFFERENT HOMOLOGY CONCEPTS

Vertebrate Limbs & Common Ancestor

Can Genes be Homologous?

Which limb is NOT homologous in each set? Why?

B

Bird

B

Whale

Grasshopper

Lizard

Human

A Human

LizardOctopus

Human

LizardLizard

Human

LizardOctopus Lizard

Human

Octopus Lizard

A Human

Octopus Lizard

A Human

Octopus Lizard

1. Are the wings of a bat (mammal) and the wings of a robin (bird) homologous?

No (they do not share a common

ancestor with wings)

2. Fins of a shark (fish) and the fins of a dolphin mammal?

No (they do not share a common

ancestor with fins)

3. The limbs of an eagle and the limbs of a penguin?

Yes (both are birds that have adapted

different uses for their wings)

4. Wings of a dragon fly and the wings of a butterfly?

Yes (both are insects that evolved from

insects with wings)

5. Are all of these different leaves Homologous?

Yes (all are types of modified leaves

from different plants)

Thank you all….

Presented by,Guru P. N.Ph.D. 015/39