Genes, Development, and Evolution by Natural Selection

Chapter 21: Genes and Development II

Segment Identity in Drosophila

Developmental genes, especially Hox genes, play a crucial role in determining the identity and morphology of body segments in animals such as Drosophila (fruit flies).

• Hox genes are expressed in specific combinations along the embryo, determining the structure of each segment.

• These genes are organized in clusters, and their order on the chromosome matches their spatial expression in the embryo.

• Conservation of Hox gene function is observed across species, such as flies and mice, indicating their ancient evolutionary origin.

Example: Figure 21.13 shows that Hox genes in flies and mice are similar in organization and expression, supporting the idea of evolutionary conservation.

Conservation of Hox Gene Function

Hox genes are homologous across animal species, meaning they are derived from a common ancestor and have retained similar functions.

• Experiments introducing the mouse Hoxb6 gene into fruit flies resulted in flies developing legs in place of antennae, similar to the effect of the Antp gene in flies.

• This demonstrates that Hox genes arose early in animal evolution and have been highly conserved.

Regulators Can Be Used in Different Ways

Developmental regulatory genes and signaling molecules are often reused in various contexts during development.

• wingless (Wnt) gene in Drosophila is essential for wing formation, segment boundary specification, and other developmental decisions.

• The gene product is a signal protein that controls the development of multiple organs and tissues.

• Related genes, called tool-kit genes, are found in different organisms and direct similar developmental processes.

• Mutations in these genes can lead to new structures, such as bat wings or whale flippers.

Changes in Developmental Gene Expression Drive Evolutionary Change

Alterations in the expression of developmental genes can lead to significant evolutionary changes in morphology and function.

• If early developmental processes are disrupted, the embryo may not survive.

• Modifications in developmental processes can result in new phenotypes, affecting size, shape, or activity of structures.

• Evolutionary developmental biology (evo-devo) studies how changes in developmental genes drive the evolution of new forms.

Case Study: Snake Limb Loss

• Snakes evolved from ancestors with four limbs.

• The signaling protein Sonic hedgehog (SHH) is essential for limb development, regulated by an enhancer sequence.

• In snakes, mutations in the SHH enhancer reduce or eliminate its function, leading to limb loss.

• SHH is a tool-kit gene; changes in its regulation can cause dramatic morphological changes.

Change in Regulatory Genes

Genetic switches, such as regulatory elements, determine where and when genes are expressed, influencing both development and evolutionary differences among species.

• In arthropods, the Hox gene Ubx is involved in the development of thoracic and abdominal appendages.

Hox Genes and Morphological Variation: Wings

• Dragonflies have similar forewings and hindwings.

• Beetle forewings are modified into protective coverings.

• Butterfly wing color patterns vary between species.

• Flies have one pair of wings; hindwings are specialized into halteres (balancing organs).

Changes in Hox-Protein Responsive Elements of Downstream Genes

Evolution of wing patterns in insects is influenced by changes in the regulatory elements that respond to Hox proteins.

• For example, the evolution of Ubx-regulated hindwing patterns explains why butterflies have four wings while flies have only two.

Segments Differentiate under Control of Genetic Switches

Expression of Hox genes in specific segments can repress or activate the development of certain structures.

• In Drosophila, expression of Ubx in thoracic segment 3 represses wing gene expression and induces haltere gene expression.

The Amount, Timing, and Location of Gene Expression and Morphological Evolution

Three key concepts describe how gene expression changes can drive morphological evolution:

• Heterometry: Change in the quantity of gene expression.

• Heterochrony: Change in the timing of gene expression.

• Heterotopy: Change in the spatial pattern of gene expression.

Heterometry: Change in Level

Modularity allows genes to evolve independently in form or expression. Heterometry refers to changes in the amount of gene expression.

• Example: Beak shape in Darwin's finches is influenced by the expression levels of BMP4 and CaM during development.

Heterochrony: Change in Timing

Heterochrony refers to shifts in the timing of developmental processes.

• Example: Giraffes have long necks not because of more vertebrae, but because the growth period of their neck vertebrae is extended.

Heterotopy: Changing Spatial Patterns

Heterotopy involves changes in where a gene is expressed during development.

• Example: Ducks and chickens both express BMP4 in their developing feet, but ducks also express Gremlin, a BMP inhibitor, in webbing cells, preventing apoptosis and resulting in webbed feet.

Concept	Definition	Example
Heterometry	Change in the amount of gene expression	Beak shape in Darwin's finches (BMP4, CaM)
Heterochrony	Change in the timing of gene expression	Giraffe neck length (extended growth period)
Heterotopy	Change in the spatial pattern of gene expression	Webbed feet in ducks (Gremlin expression)

Chapter 22: Evolution by Natural Selection

Introduction to Evolution by Natural Selection

Evolution by natural selection is a foundational theory in biology, explaining how populations adapt to their environments over time.

• Formulated independently by Charles Darwin and Alfred Russel Wallace.

• Darwin's On the Origin of Species (1859) provided extensive evidence for evolution and natural selection.

• Scientific theories consist of a pattern (observations about the natural world) and a process (mechanism producing the pattern).

22.1 The Rise of Evolutionary Thought

Understanding the development of evolutionary theory requires knowledge of earlier ideas about biological diversity.

• Typological thinking (Plato): Each organism is an unchanging example of a perfect type.

• Scale of Nature (Aristotle): Species are fixed and organized in a hierarchy from simple to complex, with humans at the top.

• Lamarckian evolution: Species evolve by moving up the scale of nature, with acquired traits passed to offspring (e.g., giraffes stretching their necks).

• Darwin and Wallace: Proposed that species change over time, share common ancestry, and that natural selection is the mechanism of evolutionary change.

Comparison of Evolutionary Thought

Thinker	Key Idea	Mechanism
Plato	Typological thinking (unchanging types)	Divine creation
Aristotle	Scale of Nature (hierarchy of complexity)	Fixed species
Lamarck	Change through time, inheritance of acquired traits	Use/disuse, acquired traits passed on
Darwin & Wallace	Change through time, common ancestry	Natural selection

Key Terms and Concepts

• Homologous genes: Genes derived from a common ancestor, often retaining similar functions.

• Tool-kit genes: Genes that control key developmental processes and are reused in different contexts and species.

• Enhancer: A DNA sequence that regulates the expression of a gene, often controlling when and where a gene is active.

• Apoptosis: Programmed cell death, a normal part of development.

• Modularity: The concept that different developmental processes can evolve independently.

Additional info: The notes emphasize the importance of regulatory changes (not just gene sequence changes) in driving evolutionary diversity, a central theme in evolutionary developmental biology (evo-devo).