The Evolution of Dispersal
Dispersal is an important part of life history for a significant percentage of eukaryotes.
Though the means vary from plant seeds ranging far from the parent plant on the wind to
juvenile mammals leaving the natal group and joining a different group, dispersal - movement of
individuals and therefore gene flow from one location to another - is widespread (Ronce 2007).
Since it exists in so many different organisms, there must be pressures favoring its evolution and
continuation. A few possibilities are often suggested: avoiding inbreeding and avoiding kin
competition are two of them (Handley and Perrin 2007). These benefit the species as a whole.
But dispersal risks reduced fitness for the individual involved. There is a high mortality rate
among dispersing individuals, and they often lose fitness in the new environment, especially if
knowledge of the area or kin coworking is important in the allocation of resources (Handley and
Perrin 2007). This paper investigates the factors that lead to the evolution of dispersal and its
continuation in so many different types of organisms, and to the variation that exists between
organisms.
Dispersal occurs in many different forms. Plants disperse when their seeds are carried far
from the parent plant, by the wind, in the digestive organs of animals, or by insects (Nathan
2006). Insects disperse as well; most hatch in a large group and separate to find their own niches
(Roff and Fairbairn 2007). Reptiles also hatch in groups and separate, eventually finding their
ways to colonies (Warner 2008). Mammals vary widely in the timing and prompting of dispersal
(Handley and Perrin 2007). Most common is dispersal as juveniles, where social mammals enter
new social groups before breeding. This dispersal is usually sex-biased; males commonly
disperse while females remain in the natal group. But there are many other variations on this
plan. Some species practice female-biased dispersal. Secondary dispersal can take place long
after adulthood is reached. Other individuals in a group may force one individual to leave, or
another group entice an individual to join them. Social structure has a large impact on the
process of dispersion among mammals.
For evolution to act on dispersal, it must be a heritable trait, there must be variety in that
trait, and that variation must lead to differences in fitness. Dispersal fits this pattern. Studies
have shown that there is variation in dispersal between individuals of a species (Chesson 2000).
One source of variation is the distance that they disperse. In a plant, seed size and shape varies
greatly and affects the distance that they can be carried by the wind (Nathan 2006). Another
factor is the likelihood of dispersal. Some ndividuals disperse and others stay in the natal area.
This trait is heritable as shown by common-garden experiments. Offspring of parents who
dispersed are more likely to disperse themselves (Ronce 2007). And dispersal changes fitness
and reproductive success for the individuals who disperse; the risk of mortality during dispersal
and difficulty in settling into a new environment are countered by increased fitness when
inbreeding is avoided and the dispersed organism's kin have greater sucess without increased
competition (Handley and Perrin 2007). These factors will be examined in more detail
throughout the rest of the paper.
Dispersal is a risky activity. There is a high mortality rate during dispersal,
particularly in plants and rodents (Ronce 2007). Seeds being dispersed have no control over
their site of deposition and ofen fall into hostile environments, where they do not survive
(Nathan 2006). Most rodents ordinarily live in burrows out of range of most of their predators,
particularly birds. Travelling from one burrow to another places them in a vulnerable position
(Ronce 2007). Even for those who survive, there are often disadvantages to being the newcomer
in a group (Handley and Perrin 2007). If gathering food requires knowledge of the area,
newcomers are at a disadvantage and risk their fitness through malnutrition or being unable to
court a mate properly. In highly organized social groups, newcomers are often at the bottom of
the pecking order, with little chance of mating. And dispersal traits are often tradeoffs for other
life history traits (Ronce 2007). In plants, smaller seeds disperse farther, but have less nutrients
stored to help the plant develop. In animals, dispersing usually uses up energy reserves that
could have been used for reproduction. The stamina to disperse well may require more resources
to support throughout life, leading to reduced fitness when shortages occur.
With all of these disadvantages, it seems strange that dispersal should be so common.
Past researchers posited several reasons for dispersal to persist. The first is inbreeding avoidance
(Handley and Perrin 2007). Inbreeding causes a high rate of delitorious recessive alleles being
expressed, increasing the mortality rate of the offspring. Dispersal promotes gene flow,
preventing the delitorious effects of inbreeding. Individuals have more choice of mates and are
less likely to end up mating with close kin. In some species this advantage has led to a
preference by the choosing sex (usually females) for mates who have come from another natal
group, over their closer kin (Gandon 1999). Then sexual selection influences the other sex to
disperse, and sex-biased dispersal becomes the norm.
This concept was very popular when dispersal was first studied, in the seventies. Models
were created to show how dispersal counteracts inbreeding to maintain the strength of a
population (Johynson and Gaines 1990). However, data from the field did not always follow the
model. Inbreeding is not a fixed problem, as it was commonly thought earlier (Ronce 2007).
Changes in mutation rate and their effects on the species, and size of the population, all affect the
amount of harm inbreeding does and how much it is necessary to avoid it. And dispersal is not
the only counter to inbreeding; species with highly developed kin-recognition techniques may
not need dispersal to avoid inbreeding (Gandon 1999). However, such species often still
disperse. Also, the inbreeding avoidance model predicts that dispersal would tend toward all of
one sex dispersing, while all members of the opposite sex stayed in place. This would provide
the very least chance of inbreeding. However, this is never the case. Even in extremely sexbiased dispersal, not all members of the normal dispersing sex disperse. And in all species, some
members of the non-dispersing sex also leave for new groups (Handley and Perrin 2007). Other
factors than inbreeding avoidance are at work on the evolution of dispersal.
Another suggested cause for the evolution of dispersal is the avoidance of kin
competition (Gandon 1999). This is highly related to inbreeding avoidance; dispersal avoids
both negative effects of having family members in too close proximity. If due to dispersal,
siblings are in different areas, they will not compete with each other for resources, allowing each
of them to maximize their fitness (Gandon 1999). Since a sibling has the same relatedness as
offspring, maximizing siblings' reproductive success is as useful for overall fitness as one's own
children's success. In this model, the evolution of dispersal is related to altruism. Individuals
might lose direct reproductive success, due to the mortality risks of dispersal, but indirect
reproductive success could actually be higher if their siblings had greater success due to their
actions (Ronce 2007).
If this model is accurate, dispersal should be most common when resources are strained
and there is overcrowding in the natal area. However, the data do not support this prediction. In
rodents, dispersal has been shown to be largely unresponsive to population pressures (Ronce
2007). In fact, individuals often join together in dispersal to form regions of high population
density. Also, siblings often disperse together, keeping them in competition with each other.
For instance, in primates natal groups of juvenile males often remain together and join a new
group as a body (Handley and Perrin 2007). This implies that kin competition alone cannot be
responsible for the evolution of dispersal.
Yet another factor in dispersal is its potential for exploiting more ecological niches
(Ronce 2007). If the habitat has a great deal of variation, dispersal of offspring might maximize
the chance that some of them will find areas with many resources and few competitors. One
individual might be more suited than another for a certain habitat, and dispersal allows this
variation to increase each individual's fitness. If the environment is constantly changing, a high
degree of mobility may mean that offspring can find new areas to which they are adapted when
the natal area changes to be less appropriate for the species. If the habitat is deteriorating over
time, dispersal will be favored, and offspring can find new, untouched areas of habitat.
However, all of these factors interact in each population. One population may respond to
changing habitat by decreasing reproductive rates, and the lack of overcrowding might counter
dispersal (Ronce 2007). Another population may have very little inbreeding due to kin
recognition, but lack of resources leads to widespread dispersal of both sexes. The factors that
affect dispersal are too complicated to be accurately modeled at this point.
One reason for this problem is that dispersal is a highly plastic trait, particularly in
mammals. Though offspring of dispersed parents are more likely to disperse themselves, the
chance is mediated by many other factors. Most organisms practice conditional dispersal;
whether or not dispersal takes place, where and how far an individual goes, and the timing of
dispersal are all affected by the situation (Bowler and Benton 2005). Organisms can make the
choice to disperse or to stay at the natal site, which neighboring group to join, and when to
move. These choices are moderated by resource availability, actions of other members of the
group, and interactions with non-group members. For instance, among rodents, dispersal is
much more common when resources around the natal burrow are scarce (Ronce 2007). If an
individual burrow is important for reproduction and the mother bequeaths her burrow to one of
the daughters, the others are more likely to disperse to find their own burrows. Among primates,
a major cause of post-juvenile male dispersal is aggression from other males. Losers in fights
can be forcibly driven from the group, making them disperse regardless of other factors (Handley
and Perrin 2007). Juvenile male gibbons are attracted to other groups by the actions of the other
females; when two gibbon populations come into contact, females attract males and invite them
into their group (Handley and Perrin 2007). All of these factors affect the rate of dispersal
among a population. They make dispersal difficult to model and are responsible for the variety
of forms of dispersal that exist in different organisms.
Mammals with complex social structures are particularly difficult. Social structure
heavily affects who disperses, when, and why (Bowler and Benton 2005). This has been most
studied in primates (Hammond et al 2006). Most primates live in harems where one male
controls many females. Juvenile males leave the group as they approach maturity in order to join
other groups where they may someday become the breeding male. However, this is not always
the case. In gibbons, it is juvenile females who leave the group. This seems to follow the
inbreeding avoidance theory; the females leave in order to avoid breeding with their potential
father. In some groups, juveniles leave as a group and attempt to take over another group as a
gang. This is explained by kin selection; the benefit may be greater for a weak male to help a
stronger brother reproduce than to attempt to reproduce himself.
Females are not inactive in this process, even when the males usually disperse and
control breeding rights by aggression (Hammond et al 2006). Females can lure males from one
group into another when the two groups come into contact. In other groups, females have the
final choice over whether or not a male is allowed to join the group. Some mothers drive
juvenile sons from the group, rather than aggression from other males driving them out.
Dispersal is an important source of gene flow in populations (Ronce 2007). It is
important to understand dispersal in order to see how small populations avoid inbreeding and
continue to persist. Understanding the pressures that led to the evolution of dispersal and its
persistance allows researchers to understand how species survive the pressures of a constantly
changing environment. It also allows practical applications in conservation biology. Many
populations are becoming isolated by human activity, and the lack of gene flow leads to
inbreeding depression. In order to prevent this populations from dying out, conservation
biologists need to introduce or reintroduce a process of dispersal between these isolated
populations. Understanding how this process evolved and is moderated in each species allows
them to better mimic natural dispersal and help the populations in question survive.
Dispersal is also an important life history trait. Life history traits are insufficiently
understood and their evolution is very complex. Investigation into the evolution of
dispersal relates many other life history traits, like reproductive rate and age and which
individuals mate. Dispersal is essential to understanding how populations are organized.
Though dispersal is so important, it is still only slightly understood. Models that do not
take into account the conditional nature of dispersal can hint at some of the causes, but cannot
truly explain the variation in dispersal that the data show (Johnson and Gaines 1990). Newer
techniques are required, integrating data with models and changing concepts based on
observation. For instance, the difference between long and short distance dispersal has not been
investigated. Some researchers believe that one grew out of the other, while others believe that
they are due to entirely different evolutionary processes (Handley and Perrin 2007). The impact
of social structure on dispersal is insufficiently understood and provides many fascinating new
avenues of research. Most dispersal is sex-biased, but which sex disperses and in what
percentage is much more complex that was previously believed. In particular, female-biased
dispersal among mammals is rare but does occur (Hammond et al 2006). This subject offers a
great deal of potential research.
Advances in computer science provide the capacity to better model the systems involved.
Conditional dispersal is still a highly complex phenomenon, but computers now can model
several conditions at once and show their interactions (Ronce 2007). With these new models,
better understanding of the many pressures that drive dispersal can be achieved. They also offer
much more potential for integrating observed data into the models. There has always been a lag
between amounts of data and models based upon that data. As more data is collected, there may
be sufficient amounts for models to accurately describe what is seen.
Dispersal is a very complex life history trait, and its evolution is not well understood.
However, it is very common among plants and animals. Since it is heritable, this implies that
selection is acting upon it so that it persists. There are many pressures that can favor dispersal,
but there are also high risks associated with dispersal. Understanding these selective pressures
has allowed some basic models to be formed, but they do not take into account the variety of the
data. Dispersal is a highly plastic trait and depends upon many conditions. Modeling all of these
conditions is not possible at the moment, but understanding other life history traits and social
structures allows researchers to come closer to understanding why dispersal may evolve in so
many different forms. With new techniques and the support of computer science, the evolution
dispersal may be better linked to its contexts and its persistance better understood.
Works Cited
Bowler DE, Benton TG. Causes and consequencese of animal dispersal strategies. Biological
Reviews. 2005; 80(2):205-225.
Chesson P. Mechanisms of maintenance of species diversity. Annual Review of Ecology and
Systematics. November 2000; 31:343-366.
Hammond RL, Handley LJL, Winney BJ, Bruford MW, Perrin N. Genetic evidence for femalebiased dispersal and gene flow in a polygynous primate. Proceedings of the Royal Society
of Biological Sciences. 2005; 273(1585):479-484.
Nathan R. Long-distance dispersal of plants. Science. August 2006; 313(5788):786-788.
Roff DA, Fairbairn DJ. The evolution and genetics of migration in insects. BioScience. February
2007; 57(2):155-164.
Ronce O. How does it feel to be a rolling stone? Annual Review of Ecology, Evolution, and
Systematics. December 2007; 38:231-253.
Warner DA, Shine R. Determinants of dispersal distance in free-ranging juvenile lizards.
Ethology. April 2008; 114(4):361-368.