1: Diversity of Life Introduction - Biology

1: Diversity of Life Introduction - Biology

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The Domains of Life

As of the 1970's, life has been classified into three domains: Bacteria, Archaea, and Eukarya (shown in Figure (PageIndex{1}) as Eukaryota). This diagram represents Archaea and Eukarya as sister taxa, more closely related than either group is to Domain Bacteria.

Figure (PageIndex{1}): "A phylogenetic tree of living things, based on RNA data and proposed by Carl Woese, showing the separation of bacteria, archaea, and eukaryotes. Trees constructed with other genes are generally similar, although they may place some early-branching groups very differently, thanks to long branch attraction. The exact relationships of the three domains are still being debated, as is the position of the root of the tree. It has also been suggested that due to lateral gene transfer, a tree may not be the best representation of the genetic relationships of all organisms. For instance some genetic evidence suggests that eukaryotes evolved from the union of some bacteria and archaea (one becoming an organelle and the other the main cell)." This vector version produced in 2006: Eric Gaba (Sting, fr:Sting), Cherkash, Public domain, via Wikimedia Commons.

Relationships Within Eukarya

Figure (PageIndex{2}): This phylogeny shows one interpretation of the relationship between organisms within Domain Eukarya. The diversity of life on Earth is immense and the evolutionary relationships between groups are often difficult to decipher. As we get new information, our classification of organisms and understanding of their evolutionary history changes. Most of the groups covered in botany appear in this phylogeny (except the Cyanobacteria). Figure from Pawlowski (2013)

Depicting Relationships

Branching tree diagrams (like the phylogenies shown above) are used to communicate a hypothesis for how organisms are related. In Figure (PageIndex{3}) and Figure (PageIndex{4}), some example trees are drawn to show how this works.

Figure (PageIndex{3}): These diagrams (cladograms) show hypotheses for four different taxa: A, B, C, and D. These taxa could represent individuals, different species, or any other level of classification. Trees 1a and 1b are drawn differently but represent the same hypothesis: C and D are most closely related (sister taxa) and A is the most ancestral lineage. Trees 2a and 2b represent a slightly different hypothesis: C and D are sister taxa, but A and B are also sister taxa; neither lineage is depicted as "older" than the other. This last relationship is also represented in the "Unrooted" tree: there is no hypothesis in this tree for which group is most ancestral (no "root"). Images drawn by Maria Morrow, CC-BY.

Figure (PageIndex{4}): In these four trees, we are again looking at hypotheses for how A, B, C, and D are related. However, in these trees we have some information about why these determinations were made: traits have been included. In trees 1 and 2, vascular tissue and megaphylls have been added to the two different hypotheses presented in Figure (PageIndex{3}). Vascular tissue appears on the root of the trees as an ancestral trait. This means that A, B, C, and D all have vascular tissue. Megaphylls appears in both trees as a shared derived characteristic (synapomorphy) of C and D. This means C and D have megaphylls, but A and B do not. Both tree 1 and tree 2 are equally parsimonious. In trees 3 and 4, a third trait has been added: heterospory. In addition to the information from trees 1 & 2, we now see that A and B are heterosporous, while C and D are not. In tree 3, heterospory appears as an ancestral trait which is lost in C and D. In tree 4, heterospory is a synapomorphy of A and B. Tree 3 has four changes, while tree 4 only has three. This means that tree 4 is more parsimonious and therefore more likely (though not necessarily correct!). Images drawn by Maria Morrow, CC-BY.

Genetic Information as Traits

Figure (PageIndex{4}): This image shows amino acid sequences for the same protein (VEGF) in humans and mice. The sequences have been aligned to find similarities and differences. The bars across the sequences show amino acids that are unchanged across the organisms tested. These positions are considered shared ancestral characteristics. At all other locations, there may be differences in the amino acids between organisms. We can use these as synapomorphies to build a tree, assuming that organisms who share more similar sequences are more closely related. This information has revolutionized the way phylogenies are built. "Alignment of the VEGF Homology Domain of representative VEGFs (all mammalian VEGFs from human and mouse plus one VEGF-E (viral) and one VEGF-F (snake venom)" Mjeltsch, CC BY-SA, via Wikimedia Commons.

Putting it Together

Video (PageIndex{1}): This video walks through how phylogenetic trees are built using genetic information. Creating a Phylogenetic Tree by Oxford Academic (Oxford Publishing Press)


Content authored and curated by Maria Morrow, CC-BY

1: Diversity of Life Introduction - Biology

An introduction to evolution

Leaves on trees change color and fall over several weeks. Mountain ranges erode over millions of years.
A genealogy illustrates change with inheritance over a small number of years. Over a large number of years, evolution produces tremendous diversity in forms of life.

Download this series of graphics from the Image library.

The definition
Biological evolution, simply put, is descent with modification. This definition encompasses small-scale evolution (changes in gene &mdash or more precisely and technically, allele &mdash frequency in a population from one generation to the next) and large-scale evolution (the descent of different species from a common ancestor over many generations). Evolution helps us to understand the history of life.

The explanation
Biological evolution is not simply a matter of change over time. Lots of things change over time: trees lose their leaves, mountain ranges rise and erode, but they aren't examples of biological evolution because they don't involve descent through genetic inheritance.

The central idea of biological evolution is that all life on Earth shares a common ancestor, just as you and your cousins share a common grandmother.

Through the process of descent with modification, the common ancestor of life on Earth gave rise to the fantastic diversity that we see documented in the fossil record and around us today. Evolution means that we're all distant cousins: humans and oak trees, hummingbirds and whales.


Develop a thorough understanding of the basis of classification schemes proposed by various scientists.

Understand the utility of acceptance of Five-Kingdom Classification scheme proposed by R.H Whittaker along with its advantages and disadvantages.

Know the differences between the prokaryotic and eukaryotic cells and cellularity in living organisms.

Examples are very crucial in the study of diversity in the living world. Having the examples written in tabular forms of point-wise will help you in quick revision.

Refer to NCERT textbook for examples. Previous years&rsquo questions have shown that the examples have come directly from the NCERT text.

From the analysis of previous years&rsquo question papers, the animal kingdom becomes very important from the perspective of the number of questions.

Solve as many questions as possible. As the unit diversity in the living world gives a lot of factual information, it will be of great help that you prepare bullet notes from questions. This manner of reverse learning helps a lot with memorizing examples and facts.

Watch the video: Εισαγωγή στην (June 2022).


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