phylogenetics
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What is phylogenetics?
Phylogenetics is a field of science that focuses on phylogeny, which is the study of the evolutionary history of a species or a group, especially regarding lines of descent and the relationships among broad groups of organisms. A key assumption in phylogenetics is that different species of plants and animals have descended from common ancestors.
Phylogenetic trees are constructed using models that seek to reconstruct the evolutionary history of taxa (species or other groups of organisms). These trees show branching relationships to reflect the relationships of ancestry.
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Why do scientists study how living things are related?
Scientists study the relationships between living things to understand the history of life and the processes of evolution. This approach, known as phylogenetics, helps in constructing evolutionary trees that illustrate how species have diverged and changed over time. By examining similarities and differences in physical characteristics, development, and molecular data such as DNA, scientists can infer the degree of relationship and descent from common ancestors. Understanding these relationships is crucial for taxonomy, the science of classifying organisms, and for conservation efforts.
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What is a phylogenetic tree?
A phylogenetic tree is a diagram that shows the evolutionary relationships between different groups of organisms that are derived from a common ancestor. The ancestor is represented as the trunk of the tree, and the organisms that have evolved from it are placed at the ends of the branches. The distance between the groups indicates how closely related they are.
Evolutionary trees reconstruct the evolutionary history of taxa, such as species, genera, families or orders. The trees show branching relationships that reflect ancestry, or cladogenesis, and the changes that have occurred along each lineage, or anagenesis.
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How do scientists use DNA to compare different species in phylogenetics?
Scientists use DNA to compare different species in phylogenetics by examining the degree of evolutionary relatedness at the genetic level. This involves comparing homologous DNA sequences from different species and measuring the number of nucleotide substitutions that have occurred over time. Also, by counting differences in the sequence of units that make up DNA molecules, researchers have devised a tool for measuring the degree to which different species have diverged since evolving from a common ancestor. These comparisons help in constructing cladograms and evolutionary trees that illustrate evolutionary relationships and ancestry. This approach is similar to using radioisotopes in geologic dating. By using molecular data, systems are constructed to compare features common to different species.
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What can phylogenetics tell us about the history of life on Earth?
Phylogenetics is the study of the ancestral relationships between groups of organisms. It is based on the idea that different species descend from common ancestors, and it helps to understand the history of evolution and the relationships among broad groups of organisms.
Phylogenetic relationships are determined through cladogenesis, where a lineage splits into two or more separate lines, and anagenesis, where changes occur within a lineage. Scientists use data from paleontology, comparative anatomy, comparative embryology, and molecular genetics to determine phylogenies. Molecular data, like DNA and protein sequences, help measure how much species have diverged since evolving from a common ancestor. Evolutionary trees are models that display the relationships of ancestry, or cladogenesis.
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How do phylogenetic studies help scientists understand diseases and their spread?
Phylogenetic studies help scientists understand the origins and transmission of diseases like AIDS, influenza, and dengue. Modern phylogenetic methods use genetic data to trace the source of an infection and an individual's genetic heritage. They also help identify evolutionary distinctiveness of endangered species, parentage in captive breeding programs, and levels of hybridization and inbreeding across species. Phylogenetic analyses can even serve as evidence in criminal court cases involving purposeful viral transmission. Mathematical models informed by evolutionary biology have implications for public health, especially concerning antibiotic resistance.
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phylogenetics, in biology, the study of the ancestral relatedness of groups of organisms, whether alive or extinct.
History
Classification of the natural world into meaningful and useful categories has long been a basic human impulse and is systematically evident at least since time of ancient Greece. Dominant for close to 2,000 years in the West was the notion of a “Great Chain of Being,” or scala naturae, which emphasized a static notion of reality and depicted a hierarchy that started from matter and nature (such as rocks) and moved upward to humans, angels, and eventually and highest of all, God. Since Carolus Linnaeus, and especially since Charles Darwin, classificatory schemes have not only quickly put aside such notions as the scala naturae but also slowly moved away from postulating relationships between species based on either presumed essential traits or based on general physical similarity. The field of phylogenetics takes a functional and more scientific turn in its attempts to construct an objective depiction of evolutionary relationships between organisms based on genetic, molecular, archaeological, and historical studies and with the specific purpose of explaining, predicting, and testing similarities and differences between organisms.
The “tree of life”
Ancestral relationships among species are commonly represented as phylogenetic trees (also called cladograms or dendrograms). A cladogram is a physical diagrammatic representation of a hypothesis of inferred relationship between species. The evolutionary relatedness between species is reflected by branching pattern of the tree and the relative distance between species on the tree. Any two species will have a common ancestor at the point where their respective branches are traced back to intersect. The more recent a common ancestor to humans, the more closely related it is to humans. For example, the common ancestor of modern humans and Neanderthals would be traced back to approximately 500,000 years ago. Similarly, the common ancestor of humans/Neanderthals and chimpanzees existed approximately 6.5 million years ago. Thus, since humans and Neanderthals share a common ancestor not shared by chimpanzees, we have reason to believe that humans are more closely related to Neanderthals than to chimpanzees. Similar retracing can be done farther and farther back on the branches of the tree of life to connect the common ancestry humans have with unicellular species some billions of years ago.
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Estimating relatedness
Cladograms can be constructed with the aid of technologies that estimate molecular divergences in key sequences of DNA or protein amino acids. Similar to the progress seen in estimating the age of organic substances with the use of radioactive decay technologies and carbon dating, the advent of molecular biological technologies in the later half of the 20th century have increasingly allowed scientists to more accurately estimate the degree of evolutionary relatedness at the genetic level. Taking two homologous DNA sequences in different species, one can estimate evolutionary distance by measuring the number of nucleotide substitutions that have occurred over time. Alternatively, using protein products of DNA expression, one can measure the number of amino acid substitutions that have occurred between homologous protein sequences.