Population ecology: Origins of genetic variability in seals

Date:
June 10, 2020
Source:
Ludwig-Maximilians-Universita"t Mu"nchen
Summary:
A new study shows that fluctuations in population sizes in the past
have had a significant effect on contemporary seal populations,
and estimates the risk of genetic impoverishment in the species
investigated.



FULL STORY ==========================================================================
A new study led by Ludwig-Maximilians-Universitaet (LMU) in Munich
researchers shows that fluctuations in population sizes in the past have
had a significant effect on contemporary seal populations, and estimates
the risk of genetic impoverishment in the species investigated.


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In the course of Earth's history, evolution has given rise to an enormous
range of biological diversity, which in turn enabled the emergence of
complex, species-rich ecosystems. The availability of adequate levels of genetic variation is a basic prerequisite for evolution. Higher levels
of genetic variability therefore increase the probability that any
given population will be able to adapt to new environmental conditions
and remain evolutionarily flexible. Scientists led by LMU evolutionary biologist Jochen Wolf have examined the genetic variability of multiple
seal species and show that a large part of today's variation is due to historical fluctuations in population sizes. In addition, the authors use
the results of their genomic analyses to derive a parameter that allows
them to assess the risk that genetic impoverishment and inbreeding pose
to seal populations today. The new study appears in the journal Nature
Ecology & Evolution.

Genetic variation is the product of random mutations, which are passed
down from generation to generation. However, mutations can also be
lost, owing to the effects of 'genetic bottlenecks', for instance. Such bottlenecks can occur when a large fraction of the population is lost. "It
is generally assumed that populations that are made up of many individuals
are likely to exhibit high levels of genetic variability," says Wolf. "We
have now tested this assumption for 17 species of seals, by analyzing the genetic differences between 458 animals from 36 populations." Since the genetic variation found in present-day populations can tell us a great
deal about the genetic make-up of their ancestors, the authors of the
study were able to deduce from their data how different populations have changed with time. "Genetic data are like a microscope that allows us to
peer into the past," says Wolf. "The greater the differences between the genomic sequences, the farther back in time their last common ancestor
lived. Our analyses enable us to look back thousands and even millions
of years, and we can see that many populations must have gone through
very narrow genetic bottlenecks -- in other words, were drastically
reduced in size -- while others experienced significant expansions."
The researchers use the 'effective population size' as a measure of
the extent of genetic variation within a population. This parameter is
defined as the number of individuals that, under theoretically ideal conditions, would be expected to exhibit the same level of genetic
variance as the real population of interest. The effective population
size is related to, but much smaller than, the actual size of the real population, because the parameter includes the effects of factors such as reproductive behavior. Male seals in some species compete aggressively for females. That implies that the less dominant males may have no chance to reproduce, which in turn reduces the range of genetic variation in the following generation. "We assessed the impact of such effects, but our
analyses indicate that the amounts of genetic variation in modern seals
have been influenced mainly by historical fluctuations in population
sizes, which are probably related to changes in the climate," says Wolf.

The ratio of the effective to the actual population size is often used
to infer whether or not a given population possesses enough genetic
variability to survive in the longer term. A very low quotient serves
as a warning signal, since populations with low levels of variation are especially susceptible to inbreeding effects which, among other things, increase the risk of disease.

"Most genetic studies undertaken in the context of conservation assess
the level of genetic variability only across a few generations," says
Wolf. "Our investigation, on the other hand, extends much further back
in time. So we were able to take fluctuations in population sizes into
account, and could calculate the population sizes we would expect to find
today due to the genetic variability." The expected population sizes were
then compared with their actual sizes by means of a complex statistical procedure, which reveals whether the extant population is larger or
smaller than the expected value. "This then tells us if a population
is at risk because its current size is much too small to sustain that particular species in the longer term," says Wolf. In this context,
the absolute number of individuals can be misleading. For instance, only
400 Saimaa ringed seals survive in the wild, and the species is regarded
as endangered." From a genetic point of view, however, despite their
small number, we do not expect them to run into problems in the near
future, as the animals are highly variable," says Wolf. The indications
are that they settled in their present habitat only a short time ago --
in evolutionary terms -- and they retain the full range of variation
that characterized their ancestors. The situation in the Galapagos is
quite different. There too, seal and sea lion populations are small,
but their levels of genetic variability are also low -- a factor which
is not reflected in the value of the conventional ratio of effective to
actual population size. The study shows that comparative genomic analyses
of animal populations constitute an important tool for the identification
of vulnerable populations in order to take protective measures.


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========================================================================== Journal Reference:
1. Claire R. Peart, Sergio Tusso, Saurabh D. Pophaly, Fidel
Botero-Castro,
Chi-Chih Wu, David Aurioles-Gamboa, Amy B. Baird, John W. Bickham,
Jaume Forcada, Filippo Galimberti, Neil J. Gemmell, Joseph
I. Hoffman, Kit M.

Kovacs, Mervi Kunnasranta, Christian Lydersen, Tommi Nyman, Larissa
Rosa de Oliveira, Anthony J. Orr, Simona Sanvito, Mia Valtonen,
Aaron B. A.

Shafer, Jochen B. W. Wolf. Determinants of genetic variation across
eco- evolutionary scales in pinnipeds. Nature Ecology & Evolution,
2020; DOI: 10.1038/s41559-020-1215-5 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/06/200610135022.htm

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