Origins of Eusociality in Insects
Tree of Life - Subphylum
of Insect Eusociality
generally affords a great survival advantage to any group of
animals, and is likely best exemplified in insects. Behavioral
characteristics include the cooperative care of offspring,
distinct castes of workers (often sterile) that do not sexually
reproduce and groups that do, and well defined divisions of
labor. The behaviors of distinct groups are genetically coded,
and one caste has lost the ability to perform functions of
other castes. Eusociality is highly refined and abundant in
insect Order Hymenoptera (the ants, bees and wasps), as well
as in Infraorder Isoptera, Order Blattodea (the termites).
There are also eusocial, aphids, and thrips. However, there
is considerable differentiation between species in extent and
type of eusocial behavior. Among the hymenopterans, ants are
far and away the most social (almost all species), most having
highly refined labor division, and have even been noted for
collective colonial problem solving. Bees and wasps as a group
are far less social, with some highly social lineages, and
other lineages having entirely solitary behavior. The Isopterans
are highly eusocial, where a single king and queen perform
all reproduction, and there are castes of workers and soldiers.
Darwin's Problem and the Genetic Basis of Eusocial Traits
high prevalence of eusociality among hymenopterans compared
to its general rarity within the animal kingdom has been
an area of debate in evolutionary biology, and the new genomics
technologies are enabling its study in detail. In a simple
view of Darwinian evolution, such altruistic self-sacrificing
own genes could be viewed as contradictory. But, according
to Hughes (et al., 2008), eusociality evolved eight to ten
within Hymenoptera. In The Origin of Species, Darwin
described sterile worker castes in the social insects as "the
one special difficulty, which at first appeared to me insuperable
and actually fatal to my whole theory". The dilemma
has been explained by the concept of “inclusive fitness” a
combination of individual reproductive success and reproductive
success of a group having similar genes. In simple mathematical
terms, the portion of the altruist’s genes that are
passed on exceeds those that would be passed on in an individual
to procreate. We now know it’s more complex than this
due to females having diploid cells (having two homologous
of each chromosome, one from each parent), and males having
haploid cells (with but one chromosome). Consequently,
males share only
25% of their sisters' genes, whereas females on average share
50% of their sister's genes, which is the same as it would
be with their own offspring. Evolution will select for altruistic
more efficient to raise siblings than offspring, providing
a sustainable selective
advantage of eusociality because the collective expends less
energy per offspring by its cooperative behaviors and division
of labor. The relatedness of sisters diminishes should the
queen not be sexually monogamous. Interestingly, many ants,
wasps have evolved behavior of lifetime monogamy, where the
queen mates with one and only one male, who dies afterwards.
This monogamous trait is an ancestral characteristic
in all eusocial Hymenopteran lineages (Hughes, 2008).
Natural Selection in Evolution of Eusocial Altruism
above explains the genetic basis for eusocial behavior being
in a lineage, but how did it arise? That would
fairly straightforward natural selection. First, some advantage
was accrued by common nesting, and selected for. Mutations
and selection led to selective silencing of genes for individual
wandering (e.g., loss in wings in worker castes). Behavior
to the queen’s reproduction is reinforced. Finally,
colonial lineages that are eusocial outcompete those that
Evolutionary Biology Studies of Eusocial Behavior, Enter
research has been powered by modern high-throughput molecular
technology that can quickly and inexpensively measure the
expression of all genes, or all protein, or entire genome
sequences, leading to dramatics new insights. Epigenomics
that studies the interaction of genes and environment has
been particularly important, as well as roles played by DNA
methylation, and microRNA-guided, novel post-transcriptional
modifications (i.e., same gene, but different resulting proteins).
of finding have emerged in the last decade, such as:
1) Each eusocial insect lineage evolved from a solitary common
ancestor a species in which a single genome produced a single
phenotype; 2) Transcriptomes studies support wasps as the
oldest eusocial group, with bees and ants having
the wasp lineage around 145 million years ago at the very
lowest Cretaceous Period; 3) Sociality started simple, but
became increasing complex as genomes were
further shaped and thus eusociality refined by natural selection,
where today some 14,000 eusocial
species span a wide spectrum of eusocial behavioral traits;
4) some genes important in the eusocial phenotype lack homology
across species, indicating recent rapid evolution. As of
2014, entire genomes were available for 11 eusocial hymenopteran
determination of which genes are likely involved in modulating
eusocial behavior, and which genes have been modified through
selection for different behavior.
termite (or Alate) in amber.
Liaoning Province, China
termites in Colombian amber
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