An internal node of a phylogenetic tree is described as a polytomy or multifurcation if (i) it is in a rooted tree and is linked to three or more child subtrees or (ii) it is in an unrooted tree and is attached to four or more branches. A tree that contains any multifurcations can be described as a multifurcating tree.

Soft polytomies vs. hard polytomies

Two types of polytomies are recognized, soft and hard polytomies. Soft polytomies are the result of insufficient

phylogenetic In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical dat ...

information: though the lineages diverged at different times – meaning that some of these lineages are closer relatives than others, and the available data does not allow recognition of this. Most polytomies are soft, meaning that they would be resolved into a typical tree of dichotomies if better data were available. In contrast, a hard polytomy represents a true divergence event of three or more lineages.

Applications

Interpretations for a polytomy depend on the individuals that are represented in the phylogenetic tree.

Species polytomies

If the lineages in the phylogenetic tree stand for species, a polytomy shows the simultaneous speciation of three or more species. In particular situations, they may be common, for example when a species that has rapidly expanded its range or is highly panmictic undergoes

peripatric speciation Peripatric speciation is a mode of speciation in which a new species is formed from an isolated peripheral population. Since peripatric speciation resembles allopatric speciation, in that populations are isolated and prevented from Gene flow, ex ...

in different regions. An example is the ''Drosophila simulans'' species complex. Here, the ancestor seems to have colonized two islands at the same time but independently, yielding two equally old but divergently evolved daughter species

Molecular polytomies

If a phylogenetic tree is reconstructed from DNA sequence data of a particular gene, a hard polytomy arises when three or more sampled genes trace their ancestry to a single gene in an ancestral organism. In contrast, a soft polytomy stems from branches on gene trees of finite temporal duration but for which no substitutions have occurred.

Recognizing hard polytomies

DNA sequence A nucleic acid sequence is a succession of bases within the nucleotides forming alleles within a DNA (using GACT) or RNA (GACU) molecule. This succession is denoted by a series of a set of five different letters that indicate the order of the nu ...

evolution is usually much faster than evolution of complex

phenotypic In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology (physical form and structure), its developmental processes, its biochemical and physiological propert ...

traits, it may be that genetic lineages diverge a short time apart from each other, while the actual organism has not changed if the whole ancestral

population Population is a set of humans or other organisms in a given region or area. Governments conduct a census to quantify the resident population size within a given jurisdiction. The term is also applied to non-human animals, microorganisms, and pl ...

is considered. Since few if any individuals in a population are genetically alike in any one population – especially if

lineage sorting Incomplete lineage sorting (ILS) (also referred to as hemiplasy, deep Coalescent theory, coalescence, retention of ancestral Gene polymorphism, polymorphism, or trans-species polymorphism) is a phenomenon in evolutionary biology and population gene ...

has not widely progressed – it may be that hard polytomies are indeed rare or nonexistent if the entire

genome A genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding genes, other functional regions of the genome such as ...

of each individual organism is considered, but rather widespread on the population genetical level if entire

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

are considered as interbreeding populations (see also

species concept A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of classification and ...

). "Speciation or lineage divergence events occurring at the same time" refers to evolutionary time measured in

generation A generation is all of the people born and living at about the same time, regarded collectively. It also is "the average period, generally considered to be about 20–⁠30 years, during which children are born and grow up, become adults, and b ...

s, as this is the only means that novel traits (e.g.

germline In biology and genetics, the germline is the population of a multicellular organism's cells that develop into germ cells. In other words, they are the cells that form gametes ( eggs and sperm), which can come together to form a zygote. They dif ...

point mutation A point mutation is a genetic mutation where a single nucleotide base is changed, inserted or deleted from a DNA or RNA sequence of an organism's genome. Point mutations have a variety of effects on the downstream protein product—consequences ...

s) can be passed on. In practical terms, the ability to distinguish between hard and soft polytomies is limited: if for example a

kilobase A base pair (bp) is a fundamental unit of double-stranded nucleic acids consisting of two nucleobases bound to each other by hydrogen bonds. They form the building blocks of the DNA double helix and contribute to the folded structure of both DNA ...

s that mutate approximately 1% per million years is analyzed, lineages diverging from the same ancestor within the same 100,000 years cannot be reliably distinguished as to which one diverged first.

Founder effect In population genetics, the founder effect is the loss of genetic variation that occurs when a new population is established by a very small number of individuals from a larger population. It was first fully outlined by Ernst Mayr in 1942, us ...

s and

genetic drift Genetic drift, also known as random genetic drift, allelic drift or the Wright effect, is the change in the Allele frequency, frequency of an existing gene variant (allele) in a population due to random chance. Genetic drift may cause gene va ...

may result in different rates of evolution. This can easily confound

molecular clock The molecular clock is a figurative term for a technique that uses the mutation rate of biomolecules to deduce the time in prehistory when two or more life forms diverged. The biomolecular data used for such calculations are usually nucleot ...

algorithms to the point where hard polytomies become unrecognizable as such.

References

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External links

*http://biology.fullerton.edu/biol404/phylo/polytomies.html Phylogenetics