Evolutionary Developmental Biology

Background

Embryos are hard to distinguish

• Developmental processes are shared between species
• When and how do they diverge? Between sexes, species, parts of the body?

Proximate and ultimate causes

• Four-Question Framework
• Evo Devo generally focused on proximate causes (not ultimate)

Evo Devo

Main idea - changes in form require changes in develpoment

• Explanatory position of embryonic development within evolutionary theory
• Integrated developmental processes intothe Modern Synthesis of Darwinism

Example - webbed feet

• Many tetrapods have webbed feet as embryos
• In birds, the development of webbed feet is linked to expression changes in BMP-gremlin pathway (proximate cause)

History

• Modern synthesis of Darwinism has only started being studied with regard to developmental biology since the 1980s and 90s

Big questions of Evo Devo

• What changes in developmental mechanisms give rise to different phenotypes?
• How do genetic differences among species map to phenotypic differences?
• What is the roleof development in constraining or enhancing evolutionary change?
• How does developmental information help us identify homologous characters?
• Can understanding development help us understand the origin of novel characteristics?

Van Baers law

Species are often more similaras embryos than adults

Haeckel's recapitulation theory

Enst Haeckel (author my arm tattoo) "Ontogeny recapitulates phylogeny" > development is sort of like a history of an organism's evolutionary history

Homology

• Structures derived from a common ancestral structure are homologous
  !EvoDevo_Kratochwil, p.7

Isometric and allometric growth

• Isometry: All body parts grow around the same rate
• Allometry: Different parts grow at different rates
  • For example, baby humans have large heads which grow slowly in relation to the rest of the body

Growth Curves

• Measuring growth of different body parts on different plots can show if body parts grow at the same rate
• In the below example, we have the human brain showing allometric growth and heart showing isometric growth
• In the beetle, allometric growth of mandibles between species
  !EvoDevo_Kratochwil, p.9

Heterochrony

• Evolutionary change in timing and rates of developmental events

Example: Zebra species

Between the plains zebra and Grévy's zebra, the Grevy's zebra has more dense stripes in its adult form

• They both show isometric growth, but the timing is different, so that grevyi zebras start developing the same pattern of stripes when they are already larger, so the stripes are more dense.
  !EvoDevo_Kratochwil, p.10

Paedomorphosis

• Adult form of a species looks more like a juvenile compared to ancestor

Neoteny

• Reduced rate of development, resulting in paedomorphosis.
  • Axolotl is an example of this, retaining their gills in adulthood

Heterotopy

• Evolutionary change of the spatial position of a feature within an organism
  • Example- adventitious roots, extra thumb in panda
• "genetically easy solution to a complex problem"

Modularity

• A vertebra or a tooth are like a "module" that can be repeated within a body
• "serially homologous"

Individualization

• Distinct identities within an modular piece are individualization
  • Teeth are an example

Key observations in Developmental Biology

• All cells in an organism have the same genes (mitosis)
• Differences among cells, tissues and organs are from differences in gene expression
• Different cells have properties that affect morphogenesis
  • Growth, shape, adhesion to cells, mitosis in certain dimensions, cellmovement, programmed cell death
• Some aspects are regulated by chemical signals like horomones

Morphogenes

• A molecule that triggers specific cellular reponses depending on its concentration
• Can create regular patterns

Gene Regulation

• Many adaptations are related to small changes in how genes are controlled, rather than development of new genes
• This is why the same structures appear as part ofevolutionary processes
• I don't really understand the more technical answers given below:
  !EvoDevo_Kratochwil, p.13

Drosophila

• A mutation in Drosophila made their haltere develop into a second set of wings, like their ancestral forms
  • This couldhave been passed onto offpsring
• This mutation was caused by a hox gene (Ultrabithorax (UBX))
  • Hox genes are sorted on the geonome in the same order (from head to tail) as their affected part of development in the embryo
  • Turns out this type of gene is in mammals too and works similarly, nearly all animals have these genes

Toolkit genes

• Genes like this are sometimes called "genetic toolkit" genes, shared widely among animal and can contribute to evolutionary changes in diverse developmental roles
• this same UBX gene in water striders controls th length of the middle legs (Co-option)

Gene Regulation and Evolution

• Differences in length and depth of premaxilla in Darwin finchesare determined by gene Bmp4 during development
  • High, early Bmp4 - high beak depth and width
  • Low Bmp4 - low beak depth/width
• A similar gene regulates beak length

UBX evolution and leg loss:

• This gene suppresses development of legs on abdomen
• A certain sequence of a protein stops the legs from growing on the abdomen

Pleiotropy

Pleiotropy refers to one gene coding for multiple traits, such as multipe organs or parts of the body. When identifying the genetic basis for developmental processes this is very important to understand.