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By the end of this section, you will be able to:
  • Describe horizontal gene transfer
  • Illustrate how prokaryotes and eukaryotes transfer genes horizontally
  • Identify the web and ring models of phylogenetic relationships and describe how they differ from the original phylogenetic tree concept

The concepts of phylogenetic modeling are constantly changing. It is one of the most dynamic fields of study in all of biology. Over the last several decades, new research has challenged scientists’ ideas about how organisms are related. New models of these relationships have been proposed for consideration by the scientific community.

Many phylogenetic trees have been shown as models of the evolutionary relationship among species. Phylogenetic trees originated with Charles Darwin, who sketched the first phylogenetic tree in 1837 ( [link] a ), which served as a pattern for subsequent studies for more than a century. The concept of a phylogenetic tree with a single trunk representing a common ancestor, with the branches representing the divergence of species from this ancestor, fits well with the structure of many common trees, such as the oak ( [link] b ). However, evidence from modern DNA sequence analysis and newly developed computer algorithms has caused skepticism about the validity of the standard tree model in the scientific community.

 Image a shows Charles Darwin’s sketch of lines branching, like those on a tree. Photo b shows a photo of an oak tree with many branches.
The (a) concept of the “tree of life” goes back to an 1837 sketch by Charles Darwin. Like an (b) oak tree, the “tree of life” has a single trunk and many branches. (credit b: modification of work by "Amada44"/Wikimedia Commons)

Limitations to the classic model

Classical thinking about prokaryotic evolution, included in the classic tree model, is that species evolve clonally. That is, they produce offspring themselves with only random mutations causing the descent into the variety of modern-day and extinct species known to science. This view is somewhat complicated in eukaryotes that reproduce sexually, but the laws of Mendelian genetics explain the variation in offspring, again, to be a result of a mutation within the species. The concept of genes being transferred between unrelated species was not considered as a possibility until relatively recently. Horizontal gene transfer (HGT), also known as lateral gene transfer, is the transfer of genes between unrelated species. HGT has been shown to be an ever-present phenomenon, with many evolutionists postulating a major role for this process in evolution, thus complicating the simple tree model. Genes have been shown to be passed between species which are only distantly related using standard phylogeny, thus adding a layer of complexity to the understanding of phylogenetic relationships.

The various ways that HGT occurs in prokaryotes is important to understanding phylogenies. Although at present HGT is not viewed as important to eukaryotic evolution, HGT does occur in this domain as well. Finally, as an example of the ultimate gene transfer, theories of genome fusion between symbiotic or endosymbiotic organisms have been proposed to explain an event of great importance—the evolution of the first eukaryotic cell, without which humans could not have come into existence.

Questions & Answers

what diseases and mode of sickness it causes and vector
Biola Reply
what is Biology
Sheku Reply
what is biology
Abel
biology is the study of life basically
Ajipha
bio means life and logy means study
Biola
what is biology?
William Reply
the study of living things
Joe
what's marketing
Mirabel Reply
what are hormones
Mirabel
what is meant by th word photosynthesis
MARTHA Reply
it is the process where by chloropherious plant manufacture their food with the presence of sunlight , chlorophyll and water etc.
Kosoe
it is the process by which green plant manufacture their food through sunlight and water ,chlorophyll.
Biola
what is biology
stallon Reply
is the study living things
Zhayma
and non living things
Zhayma
what is water circle?
Faith Reply
please i asks whether this biology is for university
Sky Reply
for secondary
stallon
Copulation means coming together of male and female in the present of sexual Intercourse.
LEKAN Reply
what are the adaptive features of nervous system
Ridhwan
Please what is ovulation
Adusei Reply
What is cell division?
Adusei
What is copulation?
Adusei
bhaiya didi Gate mat
qaisar
bajali ji ka matlab Aur Payal Chaudhary ka matlab
qaisar
please tell me adaptive features of nervous system
Ridhwan
what is a metaborism
Beatrice Reply
this is a specialized part of the cell eg Nucleus
David Reply
what are adaptive features of nervous system
Ridhwan
what are the organelles?
Faith Reply
what is reproduction
mana Reply
reproduction is the process by which living organisms give rise to young ones of their own kind
Miriam
What is evolution
Wengelawit
the production of new forms of life over time as documented in the fossil record.
mana
hmm
Marvin
give two parasites where secondary host is water snail?
Kevin Reply

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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