Studying how populations (of different species) are evolving in time and space is of great importance concerning economical, ecological and fundamental research.
In fact, knowing how populations evolve and have evolved can help scientists to predict how populations will be affected by future changes in their environment. For example, considering economically important species like oysters, being able to predict what will become of the different populations if the number of available habitats decreases or if the temperature changes will be of great interest to industrials.
Scientists decipher scenarios of past, present and future evolution by studying the life history traits of populations, the dispersal ability of the species, the population and response to selective pressure thanks to population genetics.
In fact, at the population level, in order to study the evolution, scientists have to know about the life history traits of the concerned population. The life history traits of a species describe its reproduction and life cycles and all the phenomena interfering with them.
Knowing about these characteristics is important to study evolution as the way genes are passed from an individual to the other will depend on the way these individuals reproduce. For example, bacteria which have a much shorter life cycle than most organisms living in the ocean will reproduce much faster and will therefore get adapted faster to their environment. For example, species that are able to do clonal propagation or selfing reproduction will be able to colonize new habitats with a limited number of founder individuals.
Looking at the gene flow (or gene exchanges) between different populations is also possible. In fact, looking at the genetic material of organisms of different populations can tell scientists whether or not these populations are still in contact, thus exchanging genes between them. If they are not, the lack of gene flow will conduct to the separation of the original population into two independent populations that will eventually no longer be of the same species (if there is total isolation and independent evolution). It is interesting to know if individuals from populations living at different places can reproduce. For example, if one population is affected by an important change in its environment, like oil pollution that endanger the survival of this population, knowing its reproduction potential with another population will enable to save the endangered population from disappearing through reintroduction. Knowing how populations interact and can respond to changes in their environment is linked to direct economical and ecological concerns.
Looking at genetic markers is one way of studying dispersion mechanisms for which introduced species as Crepidula fornicata, are very interesting models (see link to introduced species).
Concerning the study of the geographical distribution of populations, molecular tools and genetic analysis will also give scientists insight on how these populations evolve in space and time defining a whole field of research called molecular biogeography or phylogeography.
Life history traits of populations therefore have to be studied. Biologists studying marine organisms have put forward that an important variety of reproduction cycles and making systems exist in the sea (see the link to alternation of generation). For instance, progeny-array analysis (like paternity analysis: comparison of the genotype of parents and their offsprings) provide insights into the reproduction pattern in the field and the reproductive success of individuals. Demographic studies are carried out in the natural environment of the population studied to see how the individuals reproduce in the wild by opposition with the artificial conditions of the laboratory. It has been discovered, for example, that the brown algae, Ascophyllum nodosum, can live 200 years long which is comparable to an oak tree. Their generation time is about 25 years. Knowing the life history traits of a population is indispensable to understand how it can evolve in time and space but also to understand the data generated by population genetics.
The gene flow between populations is measured by looking at the proportion of alleles shared by several populations. For example, among the genetic markers that can be used, microsatellites are interesting because they mutate often and therefore have a high level of polymorphism facilitating the analysis of the resemblance between populations.
Other types of genetic markers, the mitochondrial markers, are interesting because they can give information on much a larger spatial and temporal scale like the one studied in phylogeography. Among the mostly used genetic markers are the genetic sequences present in organelles: in animals, mitochondrial genetic material is often used and in plants and algae, it is the genetic material present in the chloroplast that is often used. In fact, mitochondria, for example, are organelles present in each animal cell. Their major role is procuring the cell energy. These organelles contain genetic material and what makes this mitochondrial genome special is that, in a large number of species, it is almost only passed to descendents through the female. This genetic material will not be mixed with the male’s genes during reproduction. This can facilitate phylogeography analysis although selective effects can also be played. In plants, markers on the genetic material contained in the plasts will be analysed as this genetic sequence is also transmitted by the female only (see chloroplast at http://hyperphysics.phy-astr.gsu.edu/hbase/biology/chloroplast.html).
The genetic markers of populations living in different environments can also be sequenced in order to compare them across different environments and see what the influence of the environment on their genome is and how they have evolved. This kind of study is part of environmental genomics.
Scientists around the world are collaborating in order to understand the evolution of populations better and see how they could be affected by environmental changes due to global change for instance. For example, collaboration is taking place between the Centre of Marine Sciences Faro, CCMAR (partner #21) and the Biological Station of Roscoff (partner #2) in order to study the reproduction cycles of populations of different species of Fucus (Fucus spiralis and Fucus vesiculosus). They found out these species were able to hybridize and were therefore close species. Scientists are trying to understand what the link is between the environmental conditions and the reproduction cycle. Scientists in the Biological Station in Roscoff are also collaborating with the Pontificia Universidad Catolica de Chile, PUCCH (partner # 43) on species that belong to the same families or even genus of species in both places. In fact, the environmental conditions in some parts of Chile and in Brittany are close although geographically very distant. It is therefore interesting to compare the evolution of these species.
Researches carried out on population evolution can enable scientists to predict what could be the consequences of climate change or the settlement of invasive species on the local species. Worldwide collaborations enable to have a global view on changes and how marine populations react to changes. In fact, changes in temperature, for example, can cause populations to disappear if the individuals of this population cannot adapt to resist to this change. It is important to predict which species or population will be affected because this species might be of economical interest and, more importantly, because of the whole ecosystem revolving around this species. If a population of Fucus (a type of brown algae) disappears, the shellfishes living under these algae will disappear as they need the freshness and habitat provided by the Fucus. This is just an example of the interactions that will be compromised by the vanishing of just one species. There can be dramatic cascade effects if populations vanish that are difficult to imagine.
Fucus vesiculosus
credit photography: Hans Kautsky, http://www2.ecology.su.se/dbbm/var/pb_smrc.htm
In Chile, the red alga, Gracilaria chilensis, is cultivated for the polysaccharides it contains that are used as gelling agents in the food industry. This alga is being cultivated by doing propagation by cuttings (the same individual is cloned a great number of times). However, by using this technique, the pool of genes in the population tends to be very poor. The population will therefore have more difficulties to survive if changes occur in the environment than a natural population would have. Studying population genetics and the evolution of populations can also enable to point out problems of this type.
Gracilaria chilensis
credit picture: http://www.guiamarina.com/chile/02%20plants/Rhodophyceae/Gracilaria%20chilensis.htm
Studying population evolution can also be very interesting to understand how some types of parasites have evolved to develop such efficient techniques to take advantage of their host. Some parasites of the red algae, the adelphoparasites (adelpho meaning brother) are so close phylogenetically from their host that they are not recognised by the host as being foreign. The parasites are therefore able to penetrate the host’s cells and use all of the host’s cellular machinery without it even noticing. Studying the genes of these populations and their history could enable to explain how such a phenomenon is possible and how it can last in time.
Contributed by Stephanie Riès
