Lecture #15 | Evolution, development and Macroevolution

Allometry

Differential growth of parts or dimensions of an organism.

change in development that affect body plans

• In crab claws: growth rate of one claw is much larger than other

• Human growth: human babies have a large head that eventually becomes proportional

Heterochrony

Changes that occur in the timing of development

change in development that affect body plans

• paedomorphosis

• peramorphosis

Heterotopy

Changes in the spatial position of a future in an organism

change in development that affect body plans

• can result in the expression of genes in novel part of a developing body

• ex: roots on stems, bone on skin

Changes in modularity

Addition, subtraction, or differentiation of modules

• distinct units of an organism that are under genetic

  control

change in development that affect body plans

ex: teeth

Paedomorphosis

developmental shift leading to early maturity and a ‘child-form’ (juvenilized breeding adult)

• example of heterochrony

biological process where an organism retains juvenile or larval characteristics into its adult stage

Peramorphosis

delayed maturity leads to exaggerated adult forms

• example of heterochrony

ex: very large antlers showing that antlers grow until breeding maturity

changes in developmental rates that result in individuals reaching adulthood with traits that are larger or more extreme than those of their ancestors.

When does transcription occur?

Initiated when polymerase binds to the promoter after regulatory proteins like transcription factors bind to an enhancer

Cis-regulatory elements

modify the expression of a gene on the same stretch of DNA

Trans-regulatory elements

Modify the expression of distant genes

Hox genes

Genes responsible for the specific patterning of body structures during development

• Master regulators that control the expression of other genes

• ex: They define where each structure is placed in developing flies

History of Hox Genes

500 million years old ad exist in all animals

• Members of a single gene family

• evolved through duplication from a common ancestor

• all have an amino acid site that binds DNA (Homeobox)

• Occur in all animals by building an evolutionary tree of the family of Hox genes we can trace back the evolution of forms to the base of the metazoans

What happens if there are mutations in Hox genes?

Can change the body composition

• ex: a mutation in a gene can transform a segment to be a duplicate of a previous segment like double wings in drosophila

Phenotypic plasticity

When a single genotype can produce different phenotypes in repose to environmental stimuli

• shows how an environment can affect the developmental expression of genotypes

• May or may not be adaptive

Reaction norms

The set of phenotypes that a genotype is capable of expressing under different environmental conditions

• tells how flexible a trait is dependent on the environment

  

Adaptive phenotypic plasticity

is the ability of an organism to adjust its physical characteristics (phenotype) in response to environmental changes, improving its ability to survive and reproduce in a specific local environment

ex: larvae hatching in different seasons look different from each other, and reflect the flowers blooming during that season

Microevolution

Mutations (new variation) followed by allele frequency change (drift, gene flow, natural selection)

• processes that lead to changes within populations of species

Macroevolution

Evolution above the species level or patterns of the origin, diversification, and extinction of higher taxa

What patterns have been observed through studying macroevolution?

1. Evolution is gradual

2. New characters evolve as modifications of ancestral ones

3. Stasis is common

4. Complex characters evolve via intermediate steps

5. Most trends are passive

6. Evolution is no teleological

Example of gradualism

Horses have evolved increased biomass over millions of years

Example of modification of ancestral characters to new characters?

Giants pandas and moles both have a 6th finger that independently evolved via the extension of the sesamoid bone

Stasis

Lack of phenotypic change

• common over time

How can stasis be explained over great spans of time?

Stabilizing selection for the same optimized character over great spans of time

• could be common with habitat tracking

How do complex characters evolve?

Via intermediate steps

• we can observe many potential intermediate steps for eyes by looking at their features across a diversity of invertebrates

There is no trend toward increased complexity

Active/Driven trends

Caused when changes in a lineage in one direction are more likely to occur than in another direction

Passive trends

Caused when a lineage is just as likely to evolve in one direction versus another, but there are morphological/evolutionary barriers that exist in one direction

• most evolutionary trends are passive

• passive trends can cause the illusion of progress where non exists

  

Cope’s rule

predicts that mammal lineages tend to increase in body size over time.

• This is mostly a passive trend. Most mammals were near their minimum size when they emerged as a lineage

What does it mean that evolution is not teleological?

Means there is no plan or purpose

• natural selection can only favor or disfavor traits based on the current environment