Homology/Analogy
Natalia AlvarezKevin Coleman
2006
Botany 940
Evidence for evolution
Homolog structure: Similar structure and position, but different function
Courtesy of Prof. Ken Sytsma
http://evolution.berkeley.edu
Analog structure: Similar function, but different origin
Homology
How can we explain this?
Hypotheses??
Courtesy of Prof. Ken Sytsma
Homology
• Archetypal explanation“The same organ in different
animals under every variety of form and function.” (Owen,1843)
• Common ancestry“A structure is similar among
related organisms because those organisms have all descended from a common ancestor that had an equivalent trait. “ (Darwin,1859)
Homology in character evolution. Staton, July 2000
Wells’ Critique: Circular definition
Homology/Common ancestor
“Features are homologous because they are inherited from a common ancestor”
Common ancestry is inferred using homologous features.
•Features can be tested by “Multiple ad hoc hypothesis of homology” (Kluge 1997)
Origin of arthropod compound eye. Oakley,2002.
How would you test common ancestry?
How would you test common ancestry?
• Fossil record– Structure and position– behavioral patterns
• Fossil record -Fossil intermediates– Behavioral patterns
Dinosaur
BirdAlligator
How would you test common ancestry?
• Fossil record– Structure and position– behavioral patterns
• Genetics
Wells’ Critique: Genetics
• Assumption: homologous features are programmed by similar genes
• Problems
1. Similar genes determine radically different structures.
2. Organisms with different genes produce similar structures.
Example: Pax6 in fruit flies, mice and humans
• Genetics: Homolog structures and genes
Is there a correlation between genotype and phenotype?
Pax6 in fruit flies, mice and humans:
"master regulator of eye development“ (qtd. in Displan,1997).
Downstream genes are not the same, thus determines different structures.
http://big.big.or.jp/~mastakeu/pax6.html
http://www.umich.edu/~mmgmed/faculty/glaser/glaser2-resprojects.html
How would you test common ancestry?
• Fossil record– Structure and position– behavioral patterns
• Genetics
• Developmental pathways
Wells’ Critique: Developmental Pathways
• Assumption: homologous features should develop in similar ways
• Problems1. Similar pathways may
produce very dissimilar features.
2. Similar features are often produced via very different pathways.
Haeckel’s drawings
Gilbert, S. F. 1997. http://7e.devbio.com/about.php
• Developmental pathways: shared features, shared early developmental features ,presence and sequence of development stages.
http://www.natcenscied.org/icons/icon4haeckel.html
How would you test common ancestry?
• Fossil record– Structure and position– behavioral patterns
• Genetics
• Developmental pathways
• others?
Analogy
• Different structures which perform the same function (Owen, 1843)
• Convergence: Similarities between organisms that evolved independently.
Tasmanian wolf Mexican wolfFoquieria –
FoquieriaceaeNorth America
Allauidia – DidieriaceaeMadagascar
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