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population genetics Population genetics is a subfield of genetics that deals with genetic differences within and between populations, and is a part of evolutionary biology. Studies in this branch of biology examine such phenomena as adaptation, speciation, and pop ...
overlapping generations refers to mating systems where more than one breeding generation is present at any one time. In systems where this is not the case there are non-overlapping generations (or discrete generations) in which every breeding generation lasts just one breeding season. If the adults reproduce over multiple breeding seasons the species is considered to have overlapping generations. Examples of species which have overlapping generations are many mammals, including humans, and many
invertebrate Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chordate ...
s in seasonal environments. Examples of species which consist of non-overlapping generations are
annual plant An annual plant is a plant that completes its life cycle, from germination to the production of seeds, within one growing season, and then dies. The length of growing seasons and period in which they take place vary according to geographical l ...
s and several insect species. Non-overlapping generations is one of the characteristics that needs to be met in the Hardy–Weinberg model for
evolution Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...
to occur. This is a very restrictive and unrealistic assumption, but one that is difficult to dispose of.


Overlapping versus non-overlapping generations

In population genetics models, such as the Hardy–Weinberg model, it is assumed that species have no overlapping generations. In nature, however, many species do have overlapping generations. The overlapping generations are considered the norm rather than the exception. Overlapping generations are found in species that live for many years, and reproduce many times. Many birds, for instance, have new nests every (couple of) year(s). Therefore, the offspring will, after they have matured, also have their own nests of offspring while the parent generation could be breeding again as well. An advantage of overlapping generations can be found in the different experience levels of generations in a population. The younger age group will be able to acquire social information from the older and more experienced age groups. Overlapping generations can, similarly, promote altruistic behaviour. Non-overlapping generations are found in species in which the adult generation dies after one breeding season. If a species for instance can only survive winter in the juvenile state the species will automatically consist of non-overlapping generations. The group of species lacking overlapping generations mostly consists of
univoltine Voltinism is a term used in biology to indicate the number of broods or generations of an organism in a year. The term is most often applied to insects, and is particularly in use in sericulture, where silkworm varieties vary in their voltinism. ...
insects, and some annual plants. One example of univoltine insects, only breeding once a year, is Dawson's burrowing bee, ''
Amegilla dawsoni ''Amegilla dawsoni'', sometimes called the Dawson's burrowing bee, is a species of bee that nests by the thousands in arid claypans in Western Australia. It is a long tongued bee, of the tribe Anthophorini and genus ''Amegilla,'' the second ...
''. Although annual plants die after one season, not all annual plants truly lack overlapping generations. Many annual plants have seed banks containing dormant seeds that remain dormant for at least one year. This makes overlapping generations possible in annual plants. N.B domestication of annual plants has led to a reduction of seed dormancy. These domesticated annual plants, therefore, have non-overlapping generations.


Effects of overlapping generations


Genetic diversity

Whether a species has overlapping generations or not can influence the
genetic diversity Genetic diversity is the total number of genetic characteristics in the genetic makeup of a species, it ranges widely from the number of species to differences within species and can be attributed to the span of survival for a species. It is dis ...
in the new generation. Changes in the genetic variance in populations due to
genetic drift Genetic drift, also known as allelic drift or the Wright effect, is the change in the frequency of an existing gene variant (allele) in a population due to random chance. Genetic drift may cause gene variants to disappear completely and there ...
have been shown to been twice as great when they are overlapping generations as opposed to there being non-overlapping generations.


Assumptions in population genetic models

Effective population size The effective population size (''N'e'') is a number that, in some simplified scenarios, corresponds to the number of breeding individuals in the population. More generally, ''N'e'' is the number of individuals that an idealised population w ...
is an essential concept in
evolutionary biology Evolutionary biology is the subfield of biology that studies the evolutionary processes (natural selection, common descent, speciation) that produced the diversity of life on Earth. It is also defined as the study of the history of life form ...
and notoriously hard to estimate. In models estimating this figure, it is often assumed that the species has non-overlapping generations. This can bias the estimate of the effective population size, because temporal fluctuations in allele frequencies follow complicated patterns when generations overlap. In the
Neutral theory of molecular evolution The neutral theory of molecular evolution holds that most evolutionary changes occur at the molecular level, and most of the variation within and between species are due to random genetic drift of mutant alleles that are selectively neutral. The ...
, it is shown that the
rate of evolution The rate of evolution is quantified as the speed of genetic or morphological change in a lineage over a period of time. The speed at which a molecular entity (such as a protein, gene, etc.) evolves is of considerable interest in evolutionary biolog ...
(substitution rate) in neutral genes is not influenced by fluctuations in population size. This, however, is only true for species having discrete generations. In this case, the substitution rate is equal to the
mutation rate In genetics, the mutation rate is the frequency of new mutations in a single gene or organism over time. Mutation rates are not constant and are not limited to a single type of mutation; there are many different types of mutations. Mutation rates ...
. When generation overlapping is incorporated in this model, the substitution rate does change with population size fluctuations. The substitution rate increases when the population size transits from small to large, with a high survival probability and when the population size transits from large to small, with a low survival probability.


Experimentation

In many experiments species are assumed to only consist of non-overlapping generations. For instance, when a scientist wants to look at genetic mutations in a strain of bacteria. He will look at all the offspring (F1) of the current generation (P). For a further look into genetic mutations in the strain he will then look at the next generation (F2) which consists only of offspring from generation F1 while the first generation P will not be used in the experiment any longer.{{citation needed, date=July 2021


References

Population genetics