Evolution of eusociality
Encyclopedia
Eusociality is used to describe a high level of order within a group of individuals of a species. The highly structured colonies of ants, bees, and wasps are the most famous cases of eusociality
, though other examples exist (termite, naked mole rat, etc). Eusociality has evolved numerous times in the order Hymenoptera
(ants, bees, and wasps), as well as a few times in other arthropod
groups, and once in mammals. That it should have evolved so many times in Hymenoptera, and remain so rare throughout the rest of the Animal Kingdom, has made eusociality a topic of interest and debate to many. More striking still, is that eusocial organisms at first appear to behave in stark contrast with what Darwinian evolution predicts. The workers in most eusocial colonies are sterile, and rather than reproduce, they help take care of their mother and siblings. Why an individual would surrender her reproductive rights in a eusocial colony remains a hot topic of debate.
could have evolved proved to be a problem for Charles Darwin
. He described, in The Origin of Species
the existence of 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." In the next paragraph of his book, Darwin describes a solution. If the trait of sterility can be carried by some individuals without expression and those individuals that do express sterility help reproductive relatives, the sterile trait can persist and evolve. Thus, a queen in a social insect colony carries but does not express the trait of sterility. Her sterile daughters help feed and maintain the queen and her brood, so the workers are essentially aiding in the maintenance of the sterile trait. Darwin illustrated that "A breed of cattle always yielding oxen [castrates] with extraordinary long horns could be slowly formed by carefully watching which individual bulls and cows, when matched produced oxen with the longest horns; and yet no one ox could ever have propagated its kind" .
and kin selection
became important theories explain many social behaviors in animals. Inclusive fitness describes how an individual's behavior affects the combination of an individual's direct fitness (the number of offspring it produces) and the fitness of other individuals with similar genes. Kin selection is a subset of inclusive fitness pertaining specifically to individuals related by descent, and epitomized by Hamilton's Rule. Hamilton's Rule states that:
where
Hamilton's Rule suggests that if the benefit of a behavior to a recipient, weighted by the relatedness to the altruist, outweighs the costs of the behavior to the altruist, then it is in the altruist's genetic advantage to perform the altruistic behavior.
R.A. Fisher described situations in which individuals could benefit from the success of its relatives. Aposematic coloration in poisonous caterpillars makes the caterpillars more conspicuous to predators and warns them of the caterpillar's poisonous taste. However, predators must learn to avoid these brightly colored caterpillars by tasting one early in life. Fisher explained how the genes of this first caterpillar can benefit by being eaten by a predator. If the caterpillars travel in sibling groups, the individual that gets eaten will teach the predator to avoid the sibling group and the individual's sacrifice will have saved the lives of its siblings
J.B.S. Haldane revealed his early understanding of inclusive fitness
by famously remarking that he would not lay down his life for his brother, but that he would for two brothers or eight cousins .
William D. Hamilton proposed that eusociality arose in social hymenoptera
by kin selection
because of their interesting genetic sex determination trait of haplodiploidy. Because males are produced by parthenogenesis
and females are produced from fertilized eggs, sisters are essentially 75% related to each other whereas mothers are only 50% related to their daughters. Thus, sisters will benefit more by helping their mothers to raise more sisters than to leave the nest and raise their own daughters . Hamilton's proposal of the importance of haplodiploidy in the evolution of eusociality reigned supreme amongst the scientists who study social hymenoptera for the rest of the century.
, aphids, thrips, brine shrimp, and a species of beetle (amongst some other examples) . The most striking example is that of the social Isoptera, the termites which have highly sophisticated colonies but do not display haplodiploidy.
Hamilton's argument also does not take into account species that are haplodiploid but are not eusocial, such as solitary wasps and bees.
There also exist several cases of eusocial haplodiploid species that have decreased levels of relatedness so that sisters no longer share an average of 75% of their genes. This can be due to having multiple queens in the colony or by multiple matings by a queen prior to starting a colony .
The kin selection model may be too constrained to aptly describe the reality of a eusocial colony. It assumes that fitness can be described by additive isolated interactions and that these interactions are binary. However, social insects often benefit from cooperation between many individuals.
. While selection would still work on the gene level and the individual level, it would simultaneously act upon groups of individuals. Nowak, et al. (2010) outline a path by which eusociality could evolve by means of multi-level selection in five steps:
, will push the group further toward eusociality.
Nowak, et al (2010) did not settle the debate over how eusociality evolved. Many researchers still argue that kin selection remains the best explanation .
Eusociality
Eusociality is a term used for the highest level of social organization in a hierarchical classification....
, though other examples exist (termite, naked mole rat, etc). Eusociality has evolved numerous times in the order Hymenoptera
Hymenoptera
Hymenoptera is one of the largest orders of insects, comprising the sawflies, wasps, bees and ants. There are over 130,000 recognized species, with many more remaining to be described. The name refers to the heavy wings of the insects, and is derived from the Ancient Greek ὑμήν : membrane and...
(ants, bees, and wasps), as well as a few times in other arthropod
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
groups, and once in mammals. That it should have evolved so many times in Hymenoptera, and remain so rare throughout the rest of the Animal Kingdom, has made eusociality a topic of interest and debate to many. More striking still, is that eusocial organisms at first appear to behave in stark contrast with what Darwinian evolution predicts. The workers in most eusocial colonies are sterile, and rather than reproduce, they help take care of their mother and siblings. Why an individual would surrender her reproductive rights in a eusocial colony remains a hot topic of debate.
Early Hypotheses
Imagining the conditions in which eusocialityEusociality
Eusociality is a term used for the highest level of social organization in a hierarchical classification....
could have evolved proved to be a problem for Charles Darwin
Charles Darwin
Charles Robert Darwin FRS was an English naturalist. He established that all species of life have descended over time from common ancestry, and proposed the scientific theory that this branching pattern of evolution resulted from a process that he called natural selection.He published his theory...
. He described, in The Origin of Species
The Origin of Species
Charles Darwin's On the Origin of Species, published on 24 November 1859, is a work of scientific literature which is considered to be the foundation of evolutionary biology. Its full title was On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the...
the existence of 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." In the next paragraph of his book, Darwin describes a solution. If the trait of sterility can be carried by some individuals without expression and those individuals that do express sterility help reproductive relatives, the sterile trait can persist and evolve. Thus, a queen in a social insect colony carries but does not express the trait of sterility. Her sterile daughters help feed and maintain the queen and her brood, so the workers are essentially aiding in the maintenance of the sterile trait. Darwin illustrated that "A breed of cattle always yielding oxen [castrates] with extraordinary long horns could be slowly formed by carefully watching which individual bulls and cows, when matched produced oxen with the longest horns; and yet no one ox could ever have propagated its kind" .
Inclusive Fitness and Kin Selection
During the twentieth century, Inclusive fitnessInclusive fitness
In evolutionary biology and evolutionary psychology, the inclusive fitness of an organism is the sum of its classical fitness and the number of equivalents of its own offspring it can add to the population by supporting others...
and kin selection
Kin selection
Kin selection refers to apparent strategies in evolution that favor the reproductive success of an organism's relatives, even at a cost to the organism's own survival and reproduction. Charles Darwin was the first to discuss the concept of group/kin selection...
became important theories explain many social behaviors in animals. Inclusive fitness describes how an individual's behavior affects the combination of an individual's direct fitness (the number of offspring it produces) and the fitness of other individuals with similar genes. Kin selection is a subset of inclusive fitness pertaining specifically to individuals related by descent, and epitomized by Hamilton's Rule. Hamilton's Rule states that:
where
-
is the reproductive cost to the altruist, -
is the reproductive benefit to the recipient of the altruistic behavior, and -
is the probability, above the population average, of the individuals sharing an altruistic gene – commonly viewed as "degree of relatedness".
Hamilton's Rule suggests that if the benefit of a behavior to a recipient, weighted by the relatedness to the altruist, outweighs the costs of the behavior to the altruist, then it is in the altruist's genetic advantage to perform the altruistic behavior.
R.A. Fisher described situations in which individuals could benefit from the success of its relatives. Aposematic coloration in poisonous caterpillars makes the caterpillars more conspicuous to predators and warns them of the caterpillar's poisonous taste. However, predators must learn to avoid these brightly colored caterpillars by tasting one early in life. Fisher explained how the genes of this first caterpillar can benefit by being eaten by a predator. If the caterpillars travel in sibling groups, the individual that gets eaten will teach the predator to avoid the sibling group and the individual's sacrifice will have saved the lives of its siblings
J.B.S. Haldane revealed his early understanding of inclusive fitness
Inclusive fitness
In evolutionary biology and evolutionary psychology, the inclusive fitness of an organism is the sum of its classical fitness and the number of equivalents of its own offspring it can add to the population by supporting others...
by famously remarking that he would not lay down his life for his brother, but that he would for two brothers or eight cousins .
William D. Hamilton proposed that eusociality arose in social hymenoptera
Hymenoptera
Hymenoptera is one of the largest orders of insects, comprising the sawflies, wasps, bees and ants. There are over 130,000 recognized species, with many more remaining to be described. The name refers to the heavy wings of the insects, and is derived from the Ancient Greek ὑμήν : membrane and...
by kin selection
Kin selection
Kin selection refers to apparent strategies in evolution that favor the reproductive success of an organism's relatives, even at a cost to the organism's own survival and reproduction. Charles Darwin was the first to discuss the concept of group/kin selection...
because of their interesting genetic sex determination trait of haplodiploidy. Because males are produced by parthenogenesis
Parthenogenesis
Parthenogenesis is a form of asexual reproduction found in females, where growth and development of embryos occur without fertilization by a male...
and females are produced from fertilized eggs, sisters are essentially 75% related to each other whereas mothers are only 50% related to their daughters. Thus, sisters will benefit more by helping their mothers to raise more sisters than to leave the nest and raise their own daughters . Hamilton's proposal of the importance of haplodiploidy in the evolution of eusociality reigned supreme amongst the scientists who study social hymenoptera for the rest of the century.
Failures of the Kin Selection Argument
Though Hamilton's argument appears to work nicely for ants, bees, and wasps, it excludes eusocial organisms that are not haplodiploid. These organisms include the mammalian naked mole ratNaked Mole Rat
The naked mole rat , also known as the sand puppy or desert mole rat, is a burrowing rodent native to parts of East Africa and the only species currently classified in the genus Heterocephalus...
, aphids, thrips, brine shrimp, and a species of beetle (amongst some other examples) . The most striking example is that of the social Isoptera, the termites which have highly sophisticated colonies but do not display haplodiploidy.
Hamilton's argument also does not take into account species that are haplodiploid but are not eusocial, such as solitary wasps and bees.
There also exist several cases of eusocial haplodiploid species that have decreased levels of relatedness so that sisters no longer share an average of 75% of their genes. This can be due to having multiple queens in the colony or by multiple matings by a queen prior to starting a colony .
The kin selection model may be too constrained to aptly describe the reality of a eusocial colony. It assumes that fitness can be described by additive isolated interactions and that these interactions are binary. However, social insects often benefit from cooperation between many individuals.
Group Selection
An alternative theory to kin selection is that eusociality evolved by group selectionGroup selection
In evolutionary biology, group selection refers to the idea that alleles can become fixed or spread in a population because of the benefits they bestow on groups, regardless of the alleles' effect on the fitness of individuals within that group....
. While selection would still work on the gene level and the individual level, it would simultaneously act upon groups of individuals. Nowak, et al. (2010) outline a path by which eusociality could evolve by means of multi-level selection in five steps:
1. Formation of groups
Groups could consist of parent-offspring groups or unrelated groups (in situations where cooperation is beneficial) living in a structured nest.2. Pre-adaptations
Pre-adaptations for social living, such as progressive provisioningProgressive provisioning
Progressive provisioning is a term used in entomology to refer to a form of parental behavior in which an adult feeds its larvae directly after they have hatched, feeding each larva repeatedly until it has completed development...
, will push the group further toward eusociality.
3. Mutations
Mutations will arise and be selected. Some genes are known to have been silenced in social insect history, leading to the reduction of dispersal behavior and the origin of the wingless caste4. Natural Selection acts on Emergent Traits
The interactions of the individuals can be considered as part of the extended phenotype of the queen. These interactions produce emergent properties upon which natural selection can act.5. Multi-level Selection
More cooperative groups out-compete less cooperative groups.Nowak, et al (2010) did not settle the debate over how eusociality evolved. Many researchers still argue that kin selection remains the best explanation .