<h2>Looking at the Diversity of Life</h2>
<p>It is a fact that biology, as a science, has such a broad scope. Basically, this is because of the tremendous diversity of <a href="https://app.ulearngo.com/tutorials/introducing-study-of-life/what-is-life">life</a> on earth. The source of this diversity is <strong>evolution.</strong> In general, evolution is the process of gradual change during which new species arise from older species. As a matter of fact, evolutionary biologists study the evolution of <a data-lesson="principles-of-biology" data-subject="biology" data-tutorial="biology-foundations" href="https://app.ulearngo.com/biology/biology-foundations/principles-of-biology">living things</a> in everything from the microscopic world to ecosystems.</p>
<p>We can summarize the evolution of various life forms on Earth in a phylogenetic tree (<a href="#attachment_95063">see image below</a>). A <strong>phylogenetic tree</strong> is a diagram showing the evolutionary relationships among biological species based on similarities and differences in genetic or physical <a href="https://app.ulearngo.com/tutorials/introducing-genetics/mendelian-inheritance">traits</a> or both.</p>
<p>A phylogenetic tree consists of nodes and branches. The internal nodes represent ancestors. In addition, these nodes are points in evolution when, based on scientific evidence, an ancestor is thought to have diverged to form two new species. The length of each branch is proportional to the time elapsed since the split.</p>
<div class="wp-caption alignnone" id="attachment_95063" style="width: 800px"><a href="https://data.ulearngo.com/assets/d85c691d-4447-4a72-b6a1-a35f16d6d779"><img alt="phylogenetic-tree" class="wp-image-95063 size-full" height="540" src="https://data.ulearngo.com/assets/d85c691d-4447-4a72-b6a1-a35f16d6d779" width="800"/></a><p class="wp-caption-text">Microbiologist Carl Woese constructed this phylogenetic tree using data obtained from sequencing ribosomal RNA genes. The tree shows the separation of living organisms into three domains: Bacteria, Archaea, and Eukarya. Bacteria and Archaea are prokaryotes, single-celled organisms that lack intracellular organelles. Image Attribution: Eric Gaba; NASA Astrobiology Institute.</p></div>
<h2>Carl Woese and the Phylogenetic Tree</h2>
<p>In the past, biologists grouped living organisms into five kingdoms: animals, plants, fungi, protists, and bacteria. The organizational scheme was based mainly on physical features. This is as opposed to physiology, biochemistry, or molecular biology, all of which are used by modern systematics.</p>
<p>The pioneering work of American microbiologist Carl Woese in the early 1970s has shown that life on Earth has evolved along three lineages, now called domains—<strong>Bacteria</strong>, <strong>Archaea</strong>, and <strong>Eukarya</strong>. The first two are <a data-lesson="prokaryotic-cells" data-subject="biology" data-tutorial="cell-structure" href="https://app.ulearngo.com/biology/cell-structure/prokaryotic-cells">prokaryotic cells</a> with <a data-lesson="who-are-microbiologists" data-subject="biology" data-tutorial="cell-structure" href="https://app.ulearngo.com/biology/cell-structure/who-are-microbiologists">microbes</a> that lack membrane-enclosed nuclei and organelles.</p>
<p>On the other hand, the third domain contains the eukaryotes. In fact, this domain includes unicellular microorganisms together with the four original kingdoms (excluding bacteria). Woese defined Archaea as a new domain, and this resulted in a new taxonomic tree (<a href="#attachment_95063">see image above</a>).</p>
<p>Many organisms belonging to the Archaea domain live under extreme conditions and we call them extremophiles. An <strong>extremophile</strong> is an organism that thrives in physically or geochemically extreme conditions that are detrimental to most life on Earth. In contrast, organisms that live in more moderate environments may be termed <strong>mesophiles</strong> or <strong>neutrophiles</strong>. To construct his tree, Woese used genetic relationships rather than similarities based on morphology (shape).</p>
<div class="wp-caption alignnone" id="attachment_95064" style="width: 800px"><a href="https://data.ulearngo.com/assets/8f21b4a2-a7ae-4ec7-8a96-1bbda75bd9c8"><img alt="domain-lineages" class="wp-image-95064 size-full" height="827" src="https://data.ulearngo.com/assets/8f21b4a2-a7ae-4ec7-8a96-1bbda75bd9c8" width="800"/></a><p class="wp-caption-text">These images represent different domains. The (a) bacteria in this micrograph belong to Domain Bacteria, while the (b) extremophiles (not visible) living in this hot vent belong to Domain Archaea. Both the (c) sunflower and (d) lion are part of Domain Eukarya. Image Attribution: a:- modification of work by Drew March; b:- modification of work by Steve Jurvetson; c:- modification of work by Michael Arrighi; d:- modification of work by Leszek Leszcynski</p></div>
<h3>Using Genetic Relationships to Differentiate Organisms</h3>
<p>Woese constructed his tree from comparative sequencing of genes. So, he used universally distributed genes present in every organism. Also, he made sure to use conserved genes. Generally, this means that these genes have remained essentially unchanged throughout evolution.</p>
<p>Woese’s approach was revolutionary. This is because comparisons of physical features are insufficient to differentiate between the prokaryotes that appear fairly similar in spite of their tremendous biochemical diversity and genetic variability (<a href="#attachment_95064">see image above</a>). The comparison of homologous <a href="https://app.ulearngo.com/tutorials/introducing-genetics/dna">DNA</a> and RNA sequences provided Woese with a sensitive device that revealed the extensive variability of prokaryotes, and which justified the separation of the prokaryotes into two domains: bacteria and archaea.</p>

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Diversity of Life

Biology studies life and living organisms. Biology can be divided into different sections that cover the morphology, physiology, anatomy, behavior, and so on of living organisms.

Biology

Explore the foundations of biology as a science, including scientific reasoning, the scientific method, controlled experiments, and reporting scientific work to gain an understanding of the properties of life, levels of organization, and diversity of living things.


Diversity of Life

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