Chapter 20 Lecture

Homologous structures are similar structures in different organisms that arise from common ancestry.
These structures are controlled by homologous genes, which are genes with shared ancestry.
Example: The limbs of a whale, hummingbird, and other animals have different forms and functions but share a similar underlying structure due to homologous genes.
Even organisms as different as humans and dogwood trees share some similar genes, highlighting the deep connections in the tree of life. Approximately 25% of the human genome is similar to that of a banana.

Vestigial structures are remnants of organs or structures that had a function in an ancestral species but have lost their function over time.
Example: The human tailbone is a vestige of a tail that our ancestors possessed. During embryonic development (in utero), human embryos have a tail that is typically reabsorbed before birth.
In rare cases, babies are born with a small tail-like structure due to incomplete reabsorption.
The human appendix is another example of a vestigial organ with no apparent function, but it can be dangerous if it ruptures.

Hox genes are transcription factors that play a crucial role in specifying the development of body parts.
These genes are highly conserved across diverse species, indicating their importance in development.
Example: A Hox gene from one organism (e.g., human) can be inserted into the embryo of another organism (e.g., Drosophila) and still activate the appropriate downstream genes for eye development. The resulting eye will resemble a Drosophila eye, demonstrating the conserved function of Hox genes.

Hox genes are developmental genes. Mutations in these genes can result in anomalies and tumors.
Hox genes can be overexpressed or underexpressed in various human cancers like prostate, breast, and endometrial cancer.
Humans have 39 Hox genes in the Hox gene family, originating from duplications of ancestral genes.
While Drosophila has eight Hox genes, humans have a more extensive set of Hox genes, because of gene duplication events.
The homeobox, a conserved sequence, is virtually identical across different Hox genes. This conserved sequence evolves over time, but at a different rate than other areas of the gene.

Gene duplication is a crucial mechanism in evolution, allowing for the creation of new genes and functions.
Duplication events can lead to one copy of a gene retaining its original function while the other copy evolves a new function or becomes inactivated.
Gene duplication is a type of deletion, duplication, inversions, and translocations, and these things have been important in evolution. It was crucial to the evolution of just new species.

Phylogenetic trees illustrate the evolutionary relationships between different genes or organisms.
Homologous genes are sequences related by any evolutionary means. These genes can be the same and perform similar functions across organisms.
Example: Hemoglobin genes have multiple subunits (alpha, beta, gamma, delta) expressed at different times during development. These genes evolved from a single ancestral globin gene through duplication events.
Sequence analysis and alignment are used to determine the similarities and differences between genes, with closely related genes having fewer changes.

Gene Inactivation: After duplication, one copy becomes non-functional (vestigial).
Evolution of New Function: One copy acquires a completely new role.
Subfunctionalization: Duplicated genes divide the original function into different expression patterns or using different exons.

The globin gene family is an example of genes evolving through gene duplication.
Alpha and beta globin genes are active in adults and involved in oxygen transport.
Other globin genes (fetal type) are expressed during embryonic development.
These genes are located on chromosomes 16 (alpha) and 11 (beta) and share a common origin.

Genome duplication can lead to rapid evolutionary change and the origin of new species.
Yeast are similar to plant genomes, where whole genome duplications are frequently seen.
Expansion of Hox gene clusters occurs via multiple whole genome duplication events.

Variation in morphological traits allows natural selection to act upon those Evolution of traits is either adaptive or injurious.
Agouti protein is found in very different creatures that all have exactly the same gene.
Habitat changes can relax selective constraints such as in cavefish that no longer require eyes.
The paintbrush gene in fruit flies: regulatory sequence evolution. Small changes in gene expression or regulation can result in relatively large phenotypic changes. Subfunctionalization: changes in regulatory sequences.

Pre-zygotic isolation prevents mating or fertilization:
- Habitat isolation
- Temporal isolation
- Behavioral isolation (mating rituals)
- Mechanical isolation (physical incompatibility)
- Gametic isolation (incompatible eggs and sperm)
Post-zygotic isolation results in a hybrid zygote that is inviable or infertile.
An example is a point mutation in the promoter region (Duffy antigen in red blood cells) that leads to malaria resistance.