Development and Evolution

Chapter 17

I) Historical Views

Historically development was a preeminent discipline in biology. It was believed that the true nature of biology and workings of organisms can be uncovered by looking at various stages of development.

A) Ontogeny and Phylogeny – clearly, related organisms should share developmental pathways

1) Von Baer’s Law – Features common to a large/broad taxonomic group (i.e., subphylum vertebrata) are formed earlier in development than characteristics particular to a small/narrow taxonomic group (i.e., species, family, etc.)

2) Haeckel’s Biogenetic Law – Ontogeny recapitulates phylogeny or the developmental phases that an organisms goes through is a repeat of the evolutionary relationships among related taxa.

Phylogenetic Relationship:

Developmental Stages of Mammal:

a) Haeckel’s Biogenetic Law is generally not supported.

i) rarely real adult feature (i.e., gills of fish stage in human development are very different morphologically and physiologically.

ii) never ancestral morphology in its entirety (i.e., only “gills” are found in fish stage of human development, not fins, jaws, swim bladder, etc.)

iii) preadult stages in development have independent, unique adaptations not found in “parallel” phylogenetic stage.

iv) necessitates terminal addition only

II) Developmental Principles

A) Individualism and Disassociation

1) Organic bodies consist of distinct units or “modules”

2) Individualism – evolution of morphology can be viewed as an increasing degree of specialization of these modules

e.g.) worm = segment – segment – segment – segment – head

lizard = segment – leg segment – segment – leg segment – head

3) dissociation – specialization of segments that have acquired independent genetic control and can now evolve independently from other segments.

B) Heterochrony – an evolutionary change in the timing or rate of developmental effects.

1) Somatic versus Somatic – D’ Arcy Transforms

Bigeye Snapper

By pulling at various sides of the grid the morphology of the fish changes. The “pulling” is accomplished in a developmental sense by altering the rate of tissue growth along the various

axes

The modification can be seen in related taxa:

Molidae:

2) Gametic versus Somatic –The rate of development of gametic tissue relative to somatic tissue can also be modified and can result in Paedomorphosis. Paedomorphosis is when adult descendants resemble the youthful stages of their ancestors.

Ancestral:

a) Progenesis – accelerated development of gametic tissue and normal rate of somatic tissue.

b) Neoteny – normal rate of gametic tissue development and slowed rate of somatic.

C) Threshold Response – many morphological characters follow a threshold response relationship with gene expression

D) Pattern formation – many part of development rely on proper formation of patterns that are spatially differentiated developmental processes likely involving threshold traits.

III) Developmental Genetics

A) Gene expression – highly regulated (temporally and spatially) cascade of gene products are responsible for proper development.

1) Effector genes – cell or tissue specific genes that set up the basic developmental patterns

2) Regulatory genes – control effector genes as well as other regulatory genes.

B) Homeotic Genes – Hox genes are transcriptional regulation factors with spatial and temporal patterns of expression and they provide positional information during development.

1) Evolution of Hox Genes

a) primarily through gene duplication allowing individualism through disassociation

b) changes in expression levels both spatially and temporally resulting in morphological changes

c) changes in sensitivity of effector genes and new effector genes.

2) Hox and homology – Hox gene expression can modify the degree of development of an existing trait.

3) Mutations to Hox genes