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Section 23_3: The Molecular Processes that Underlie Evolution

Page history last edited by Neil Patel 11 years, 2 months ago

A. Learning Objectives


23.3  The Molecular Process that Underlie Evolution 

  • Describe several ways that a species might acquire new genes.
  • Examine how molecular changes in the genetic material is associated with evolution. 


B. Section Summary


     Innovations in technology helped us analyze DNA sequences and revolutionized the field of evolutionary biology. We can now examine changes in genetic material and see how these changes are linked with changes in phenotype. Molecular changes in genetic material is associated with evolution.

     Homologous genes are two or more genes that come from the same ancestral gene. Orthologs are homologous genes in different species. Studying and analyzing homologous genes reveals information about evolutionary change at the molecular level. So how are homologous genes formed? Homologous genes come from a common ancestral gene. The ancestor slowly diverges into new species. These additional species contain the same gene (orthologs), but these orthologs develop random mutations over time. These mutations slightly alter the sequences in each gene. Even though these homologous genes are similar because they come from a common ancestor, they vary slightly due accumulation of random mutations. 

     Paralogs are two or more homologous genes found in a single species. Paralogs arise from gene duplications, which produce multiple copies of a gene. These multiple copies then can lead to a gene family. A gene family is two or more copies of paralogous genes within the genome of a single species. For example, the globin gene family encodes protein subunits that function in oxygen binding. This gene family consists of 14  genes that are derived from a single ancestral globin gene. The globin gene first duplicated around 500 to 600 million years ago. Over time, more duplications occurred, as well as chromosomal rearrangements. These events have led to the current number of 14 different globin genes. Gene families are advantageous because they lead to genes encoding for proteins with specialized functions. For example, hemoglobin is specialized in binding and transporting oxygen via red blood cells. On the other hand, myoglobin is specialized in binding and storing oxygen in muscle cells. Also, different globin genes are expressed at different stages of life. This has helped us evolve and adapt to our environment. Because we have globin genes that are expressed during the embryonic and fetus stage, we are now allowed to develop our fetus internally.  We adapted to our environment by evolving internal gestation. 

     Another molecular process that leads to evolution is called exon shuffling. Exon shuffling is a form of mutation in which exons and part of their flanking noncoding introns of one gene is inserted into another gene. This ultimately creates proteins with additional functional domains. Exon shuffling involves gene duplication and rearrangement of exons. If this new, diverse protein is beneficial, it will be naturally selected and the new modified gene will be seen in offspring generations. One possibility exon shuffling could occur is by a double crossover. A double crossover would promote the insertion of an exon into another gene, which is known as nonhomologous recombination. Another possibility is transposable elements (refer to chapter 21) may promote the insertion of exons into another gene. 

     The formation and evolution of paralogs, orthologs, and the process of exon shuffling are all referred to as vertical evolution. Vertical evolution involves a lineage from a series of ancestors that pass on the evolutionary genetic changes. Evolution cannot only occur vertically, but also horizontally. Horizontal gene transfer  is a process in which one organism incorporates genetic material from another organism, which can be from the species or from a completely different species, without resulting in offspring. For example, a bacteria cell gets engulfed by a eukaryotic cell via endocytosis. The eukaryotic cell then degrades this foreign object and while doing so, the bacterial chromosome escapes to endocytic vesicle. The bacterial chromosome then enters the nucleus and inserts itself into an eukaryotic chromosome. This is a an example of horizontal gene transfer in which an eukaryotic cell incorporated the genetic material from a bacterial cell. Researchers have seen this common phenomenon occur from prokaryotes to eukaryotes, from eukaryotes to prokaryotes, between different prokaryotes, and between different eukaryotes. Gene transfer in bacteria is widespread and about 20% to 30% of genetic variation in prokaryotes is caused by gene transfer. Bacteria carry out gene transfer through conjugation, transformation, and transduction. These newly acquired genes and are usually beneficial. Some genes are R factors, degradative plasmids, Col-plasmids, virulence plasmides, and F factors. 

     Changes in chromosome structure and number may also cause evolution. For example, when chromosomes of humans were compared to chimpanzees, gorillas, and orangutans, the banding pattern were very similar. However, there were some significant differences in chromosome number and orientation. These changes could have occurred due to evolution. 


C. Useful Material


http://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter16/animation_-_exon_shuffling.html  Short animation that covers the topic of exon shuffling. Very useful and helps you understand the topic better with visuals. 
http://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter24/evolution_of_homologous_genes.html  Short animation that covers the topic of the evolution of homologous genes. Very useful and helps you understand the topic better with visuals. 
http://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter24/transposons__shifting_segments_of_the_genome.html  Short animation that covers the topic of transposons. Very useful and helps you understand the topic better with visuals. 
http://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter24/changes_in_chromosome_structure.html  Short animation that covers the topic of chromosomal changes. Very useful and helps you understand the topic better with visuals. 
http://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter24/horizontal_gene_transfer.html  Short animation that covers the topic of horizontal gene transfer. Very useful and helps you understand the topic better with visuals. 
http://cid.oxfordjournals.org/content/27/Supplement_1/S12.full.pdf+html?sid=1082e4ab-be61-4979-888f-7462e9a2fed2   This article talks about a strain of bacteria that acquired a new, antibiotic resistant gene by horizontal gene transfer. This is really helpful as it takes abstract ideas and puts them into a concrete form. We see a bacterial cell donates a R factor via conjugation.  
http://jzhang.public.iastate.edu/transposition.html   This page shows the process of transposition in a animation. It also provides a detailed overview of the different ways transposition can occur. This is a great tool and gives you so much to help understand how transposition can play an important roll in changing genetic material and leading to evolution. Also, it provides possible consequences to this process.

This guy is awesome! This video is perfect and underlines the concept of exon shuffling. It talks about how exon shuffling leads to new genes and could ultimately lead to evolution.  



D. Primary Literature


http://www.scientificamerican.com/article.cfm?id=gene-genesis-scientists - Gene Genesis: Scientists Observe New Genes Evolving from Mutated Copies *Main Article*


     This article talks about how the evolution of genes has led to life on Earth. Gene duplication and modification has led to the advancement of species from amoebas to humans. The two copies of each gene are essential as one functions normally, and the other is able to evolve. For example, the venom that is released by a platypus contains three peptides. The gene then evolved through random duplications and this resulted in an immune compound to be produced. Also in humans, gene duplications in a opsin gene led to a gene family of opsin genes. This is great and it really relates to section 23.3! If you click on the hyperlink in the third paragraph of the article, you can really see the connection to this section. This hyperlink talks about how the opsin gene evolved through duplications from a lineage of different species. At first, there was only a small eye-spot, which then evolved into the complex eyes we possess today. This additional link is a great read to further understand the concept of gene duplications and gene families. The direct link to the article is http://www.nyas.org/publications/detail.aspx?cid=93b487b2-153a-4630-9fb2-5679a061fff7 . 

     Coming back to the original article,  Geneticist Susumu Ohno says gene duplications is vital to evolution. These mutations also need to be passed down many generations, prove to be beneficial and then it will finally be naturally selected. However, vast duplications can lead to function loss which is harmful. His model suggested organisms should not duplicate the spare gene until the organism has a better chance to evolve. However, this theory was problematic to two other scientists. These two scientists believed the beneficial mutation should come first. They believed genes had two functions: a primary function and a secondary function. They believed the weak, secondary function would mutate under certain circumstance to become beneficial. 

      In a lab experiment, Salmonella enterica was grown without the gene for producing tryptophan.  The bacteria, however, had one gene with a primary function, and a weak function that partially did the missing gene's work. After many duplications and divisions, the bacteria had the one gene with the original primary function, as well as a second primary function that was produce tryptophan! These studies continue to happen and provide us information about how genes evolve and create new functions that are beneficial. This is a great article that really ties up all the concepts in section 23.3.


E. Virtual Lectures




F. PowerPoint Slides



G. Exam Questions


H. Practice Quiz


Practice Quiz on Section 23.3



Grading Sheet (Neil)

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