Lecture 15: Evolution, Development, and Macroevolution

4 types of changes in development that affect body plans:

1. allometry

2. heterochrony

3. heterotopy

4. changes in modularity

allometry

• Evolutionary change in the rate of growth of different parts

• Changes in relative growth rate can lead to different forms

heterochrony

• Evolutionary change in the timing or rate of development

• This can occur via a change in the overall rate or duration of development

• Timing of development can shift in two directions:

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

  • Peramorphosis – delayed maturity leads to exaggerated adult forms

heterotopy

• Change in the spatial position of a feature in an organism

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

modularity

• The addition, subtraction, or differentiation of modules

• Some modules are repeated at various sites on the body and lack distinct individual identities

• individualization

individualization

The acquisition of distinct identities by such modules

early biologist discoveries

1. All cells in an organism have the same set of genes

2. The differences among cells, tissues, and organs must result from differences in the activity of certain genes.

3. Different cells have properties that affect morphogenesis (the development of form). These include cell growth rate, shape, adhesion, mitosis, movement, and programmed cell death.

4. Many aspects of growth and differentiation are affected by chemical signals, especially hormones.

In eukaryotes, transcription is initiated when polymerase binds to an upstream region, the promoter. This occurs when ….

regulatory proteins that determine transcription of a gene (transcription factors) bind to a short upstream region called an enhancer.

Cis-regulatory elements

modify the expression of a gene on the same stretch of DNA (e.g., enhancers)

Trans-regulatory elements

modify the expression of distant genes (e.g., transcription factors)

Hox genes

• discovered and studied in the 1970-1980s

• responsible for the specific patterning of body structures during development.

• master regulators that control the expression of other genes

• define where each structure is placed in developing flies

• ~500 million years old and exist in all animals

• evolved through duplication from a common ancestor

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

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

evolutionary changes in: _____ of phenotypic traits can be understood as consequences of evolutionary alterations in gene regulation

1. presence versus absence

2. size

3. form

4. location on organism

phenotypic plasticity

when a single genotype can produce different phenotypes in response to environmental stimuli

reaction norm

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

Many differences among species are due to _____ and _____ changes in developmental rates. Some characteristics have evolved by heterotopy or through changes in modularity.

heterochronic and allometric

Evolutionary change in the _______ among genes is believed to underlie much of the phenotypic diversity seen in nature.

regulatory connections

Microevolution

mutation (generation of new variation) followed by allele frequency change (drift, gene flow, natural selection)

Macroevolution

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

drift, gene flow, and natural selection are all processes that lead to …

changes within populations of a species (microevolutionary change)

Several patterns have emerged from studies of macroevolution:

• Evolution is gradual

• New characters evolve as modifications of ancestral ones

• Stasis is common

• Complex characters evolve via intermediate steps

• Most trends are passive

• Evolution is not teleological

____ is a hallmark of many evolutionary transitions

Gradualism

______ is common over time

Stasis (lack of phenotypic change)

What can explain stasis in a lineage over great spans of time?

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

Complex characters evolve via ____

intermediate steps

There is no trend towards ____

increased complexity

Active or 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

• can cause the illusion of progress where none exists

most evolutionary trends are ____

passive

Evolution is not teleological, which means …

it has no plan or purpose

Natural selection can only favor or disfavor traits based on what

the current environment

Features can evolve towards increased complexity, increased fitness, better fit for the environment. but if the environment changes or if natural selection becomes ineffective…

any of these changes can be reversed

common patterns of macroevolution:

• Evolution is gradual

• New characters evolve as modifications of ancestral characters

• Stasis is common

• Complex characters evolve via intermediate steps

• Most trends are passive