Question

1. In the context of genetic variation and gene duplication, briefly describe the significance of the human hemoglobin genes.

2. Define, compare, and contrast “biological evolution” and “natural selection”

Answer 1

1. Insights into the evolution of hemoglobins and their genes are an abundant idea source regarding hemoglobin function and regulation of globin gene expression. This article presents the multiple genes and gene families encoding human globins. It summarizes major events in the evolution of the hemoglobin gene clusters, and discusses how these studies provide insights into regulation of globin genes. Although the genes in and around the α-like globin gene complex are relatively stable, the β-like globin gene clusters are more dynamic, showing evidence of transposition to a new locus and frequent lineage-specific expansions and deletions. The cis-regulatory modules controlling levels and timing of gene expression are a mix of conserved and lineage-specific DNA, perhaps reflecting evolutionary constraint on core regulatory functions shared broadly in mammals and adaptive fine-tuning in different orders of mammals.

A wide range of animals, vertebrate and invertebrate, use hemoglobins to transport oxygen, carrying it from lungs, gills, or other respiratory organs to peripheral tissues which need the oxygen for efficient metabolism. Therefore it is natural to compare the structure and function of hemoglobin proteins between species both to explore adaptation and to discover aspects of biochemistry and physiology that are conserved. Comparative studies also have been conducted on the genes and gene clusters that encode the hemoglobins, revealing a rich history of gene duplications and losses as well as translocations. One motivation for comparative studies has been to use the insights from the evolutionary analyses to better understand mechanisms of gene regulation. Many human hemoglobinopathies result from inadequate expression of globin genes, and attempts to modulate globin gene expression are a fundamental approach to seek novel avenues to therapy.

The diversity of hemoglobins, their functions, their exquisite regulation, and the pathological consequences of some mutations make it a fascinating family of proteins and genes. Exploration of these genes in many different species continues to illuminate some and challenge other evolutionary models. Production of different forms of hemoglobin at progressive developmental stages is widespread in vertebrates and beyond, and studies of hemoglobin switching are pursued in several non-human species as models of the process in humans. The evolutionary comparisons summarized here illustrate the power of this approach, but they also remind us that such studies are best done while embracing both interspecies conservation of some elements and lineage-specific changes for others. Indeed, this can lead to important insights, such as the impact of differences in expression pattern of a key transcription factor driving a change in developmental timing of expression in humans.

2.

Evolution:
Darwin proposed that species can change over time, the new species come from pre-existing species, and the all species share a common ancestor. In this model, each species has its own unique set of heritable or genetic differences from the common ancestor, that have accumulated gradually over very long time periods. Repeated branching events, in which new species split off from a common ancestor, produce a multi-level "tree" that links all living organisms.

Darwin referred to this process, in which groups of organisms change in their heritable traits over generations, as “descent with modification." Today, we call it evolution.

Natural selection:

Importantly, Darwin didn't just propose that organisms evolved. If that had been the beginning and end of his theory, he wouldn't be in as many textbooks as he is today! Instead, Darwin also proposed a mechanism for evolution: natural selection. This mechanism was elegant and logical, and it explained how populations could evolve in such a way that they became better suited to their environments over time.

Darwin's concept of natural selection was based on several key observations:
1. Natural selection doesn't favor traits that are somehow inherently superior. Instead, it favors traits that are beneficial in a specific environment. Traits that are helpful in one environment might actually be harmful in another.
2. Natural selection needs some starting material, and that starting material is heritable variation. For natural selection to act on a feature, there must already be variation for that feature. Also, the differences have to be heritable, determined by the organisms' genes.
3. The original source of the new gene variants that produce new heritable traits, such as fur colors, is random mutation. Random mutations that are passed on to offspring typically occur in the germline, or sperm and egg cell lineage, of organisms. Sexual reproduction "mixes and matches" gene variants to make more variation.
Darwin's model of evolution by natural selection allowed him to explain the patterns he had seen during his travels. For example, if the Galápagos finch species shared a common ancestor, it made sense that they should broadly resemble one another. If groups of finches had been isolated on separate islands for many generations, however, each group would have been exposed to a different environment in which different heritable traits might have been favored, such as different sizes and shapes of beaks for using different food sources. These factors could have led to the formation of distinct species on each island.
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