population genetics Population genetics is a subfield of genetics that deals with genetic differences within and among populations, and is a part of evolutionary biology. Studies in this branch of biology examine such phenomena as Adaptation (biology), adaptation, s ...

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 animals that neither develop nor retain a vertebral column (commonly known as a ''spine'' or ''backbone''), which evolved from the notochord. It is a paraphyletic grouping including all animals excluding the chordata, chordate s ...

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. Globally, 6% of all plant species and 15% of herbaceous plants (excluding trees and shrubs) are ...

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 the change in the heritable Phenotypic trait, characteristics of biological populations over successive generations. It occurs when evolutionary processes such as natural selection and genetic drift act on genetic variation, re ...

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. Amegilla dawsoni

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''. 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 correlated to the span of survival for a species. It is d ...

in the new generation. Changes in the

genetic variance Genetic variance is a concept outlined by the England, English biologist and statistics, statistician Ronald Fisher in his Fisher's fundamental theorem of natural selection, fundamental theorem of natural selection. In his 1930 book ''The Genetica ...

in populations due to

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 ...

have been shown to be twice as great when there are overlapping generations as opposed to when generations do not overlap.

Assumptions in population genetic models

Effective population size The effective population size (''N'e'') is the size of an idealised population that would experience the same rate of genetic drift as the real population. Idealised populations are those following simple one- locus models that comply with ass ...

is an essential concept in

evolutionary biology Evolutionary biology is the subfield of biology that studies the evolutionary processes such as natural selection, common descent, and speciation that produced the diversity of life on Earth. In the 1930s, the discipline of evolutionary biolo ...

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, it is shown that the rate of evolution (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, nucleotide sequence, 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 mu ...

. 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