What is Horizontal Gene Transfer, and Does it Support Evolution?
|by||Joe Deweese, Ph.D.|
[EDITOR’S NOTE: The following article was written by A.P. auxiliary staff scientist Dr. Deweese who holds a Ph.D. in Biochemistry from Vanderbilt University.]
One of the foundational principles of biology and genetics is the concept that parents pass genetic material (DNA) to offspring. This parent-to-offspring transmission is also called vertical gene transfer (VGT). For many years, scientists have also known that genetic information can also be passed between organisms (usually between bacteria) in a process called horizontal gene transfer (HGT), previously known as lateral gene transfer (LGT). For example, HGT allows bacteria to share genes such as those that confer antibiotic resistance, which helps explain how resistance can spread in populations of bacteria. Another form of HGT takes place when a parasite is able to incorporate genetic information into a host organism (Dunning Hotopp, et al., 2007).
Over the last two decades, with the completion of entire genome sequences for numerous organisms, scientists have been comparing DNA sequences for similar genes found in different organisms. These comparisons have shown that not all genes can be explained through a typical evolution-by-common-descent model where all genetic information is passed through VGT. In other words, some organisms share similar genes that are not found in their alleged “common ancestors,” as they would be expected to if Darwinian evolution is true. As Institute for Creation Research geneticist Dr. Jeff Tomkins notes, these genes are referred to as “orphan” genes (Tomkins, 2013). The question then arises, where did these genes come from, if not from an ancestor? Thus, in an effort to explain the presence of genes that are not found in supposed evolutionary ancestors, some researchers have utilized the concept of HGT to explain these genes even in multicellular eukaryotes. It should be noted that HGT is a controversial topic and not all evolutionists accept the HGT hypothesis for the origin of genes. Additionally, utilizing HGT to explain the presence of these genes is a large step beyond the actual “observed” cases of HGT. Again, while HGT is observed in bacteria and in a few parasite-host relationships, there is no observed mechanism for genes to spread between multicellular organisms in a horizontal fashion (Tomkins, 2015).
Interestingly, a recently published report suggested that HGT may be needed to explain dozens if not hundreds of genes in humans and non-human primates (Crisp, et al., 2015). In fact, this study identifies dozens of “foreign” genes in the human genome. Note that the criteria for deciding whether genes are “foreign” or not relies on the presupposition of evolution by common descent. In other words, “foreign” genes are those that cannot be explained by standard VGT from alleged “common ancestors.” Thus, these authors suggest that such genes must be explained by HGT (Crisp, et al.).
This study has been reviewed by Tomkins, who has been part of bacterial HGT studies in the past (Dunning Hotopp, et al., 2007). He observes several problems with this current study, including the way genes are compared using only homologous sequence segments rather than the entire gene sequence (Tomkins, 2015). In addition, the genes identified in the study include a number of essential enzyme activities—including enzymes involved in amino acid, lipid, and nucleotide metabolism (Crisp, et al.). These enzymes are integral components of metabolic pathways rather than expendable transplants from distant organisms. Further, the authors do not suggest any mechanism for how HGT would supposedly work between multicellular organisms. As Tomkins points out, HGT between multicellular organisms would require the new genes to be brought into the germline cells, incorporated into the genome, and then transmitted to offspring (2015). There are barriers to these events happening at many levels. Finally, these genes would need to be expressed, regulated, and become incorporated into the existing metabolic networks. Each of these barriers poses significant challenges to the use of HGT to explain the spread of genes.
The fact is, there is currently no evidence that HGT can occur in the wild between multicellular organisms. Further, there are no observed mechanisms for this transfer to take place. The fact that the genes identified in the study quoted above impacted important enzymes and metabolic pathways implies that these genes are part of complex and integrated networks—they do not represent minor functions in many cases. Taken together, it is clear that relying on HGT to explain the spread of “foreign” genes is a stretch, at best, and currently is lacking key pieces of evidence. This is not the first time—and will not be the last time—that evolutionists strain to interpret straightforward evidence. The fact that genes cannot be attributed to VGT and common descent could, instead, be interpreted to mean that these genes were placed there by design, which would be the simplest and most obvious explanation.
Crisp, A., C. Boschetti, M. Perry, A. Tunnacliffe, and G. Micklem (2015), “Expression of Multiple Horizontally Acquired Genes Is a Hallmark of Both Vertebrate and Invertebrate Genomes,”Genome Biology, 16:50.
Dunning Hotopp, J.C., M.E. Clark, D.C. Oliveira, et al. (2007), “Widespread Lateral Gene Transfer from Intracellular Bacteria to Multicellular Eukaryotes,” Science, 317:1753-1756.
Tomkins, J.P. (2013), “Newly Discovered ‘Orphan Genes’ Defy Evolution,” http://www.icr.org/article/newly-discovered-orphan-genes-defy.
Tomkins, J.P. (2015), “Another Horizontal Gene Transfer Fairy Tale,” http://www.icr.org/article/8673.