Conservation genetics uses the principles of population genetics and conservation biology to provide tools to manage the long-term viability of species and ecological systems. The aim is to maintain the biological diversity, including genetic variation, essential for functional ecosystems.
Plants at risk often experience a rapid decline in number and distribution. They frequently occur in small fragmented populations. In isolated plant populations, small population size and the consequent low genetic diversity may reduce the ability of populations to adapt to environmental change. Inbreeding is frequently a consequence of population fragmentation and can manifest as reduced reproductive success.
Knowledge of the population genetics of rare and threatened species allows for planned interventions that maximise the possibility that species will survive and flourish in the wild. Population genetics also provides the scientific underpinning for maintaining healthy ex situ populations and for selecting appropriate plants for reintroduction.
Population genetics is the study of the genetic characteristics of populations using allele (gene variant) frequencies and changes to them. It takes into account population subdivision and structure and provides a theoretical framework to explain adaptation and speciation. The origin, amount and patterning of genetic variation and its fluctuation in space and time provides the basis for microevolution.
Population genetics developed largely as a response to the theories of Darwin and Mendel. In the 1920s and 1930s, R.A. Fisher, J.B.S. Haldane and Sewall Wright integrated the principles of Mendelian genetics and Darwinian natural selection. They developed a mathematical basis for measuring the effect of Darwinian selection on a population obeying Mendel's rules of inheritance.
Population genetics relies on formal mathematical models to describe fluctuations in allele frequencies under particular sets of assumptions. Models assist us in drawing conclusions about complex patterns present in empirical data, although the inferences made from a model can be very sensitive to the assumptions of that model. Modelling allele frequency change allows a quantitatively precise exploration of the consequences of different evolutionary hypotheses and the influence of the four evolutionary forces: selection, mutation, drift and migration.
Natural selection occurs when some individuals in a population leave more reproductive offspring than other members of the population because they are better suited to that environment. Provided that the relative fitness levels are due partly to genetic differences, the population's genetic makeup will alter over time.
Mutation is a change in DNA sequence and is the ultimate source of genetic variation. If the change is heritable the frequency of a mutation can change over time as a result of selection or genetic drift.
Random genetic drift refers to chance fluctuations in allele frequencies that can arise in populations. Many evolutionary models make an assumption of infinite population size to minimise the effect of chance but chance factors will always affect survival and reproductive success and result in changes to allele frequencies. Within species, random drift can cause a decrease in heterozygosity. Over time, different subpopulations may also diverge genetically as alleles accumulate differently in each, particularly if the alleles do not provide a selective advantage or disadvantage.
Migration into a population can alter the genetic composition by introducing new alleles or changing their frequency depending on the degree of difference between the source and destination populations. Migration is significant in evolutionary terms because even occasional migration can provide gene flow that limits the divergence of subpopulations that are largely isolated.
Using population genetics in conservation
Small population size, which is often the case for threatened species, leads to reduced fitness. Consequences of reduced fitness include reduced reproductive output, poor seedling performance, low pollen viability or a change in reproductive strategy. Small populations are also more at risk of extinction from random and unpredictable events.
Detrimental effects of small population size may be alleviated by increasing population size and/or increasing genetic variation by introducing new individuals or improving the connectivity between populations. Introduction of new genetic material can potentially increase offspring fitness, reproductive output and the ability of the species to cope with changing environmental pressures but there is also a risk that such intervention could be harmful.
Information on the breeding system and pollinators, the distribution of genetic variation and the impact of management regimes such as fire response all act to improve persistence of threatened species. Interventions can be modified in accordance with knowledge of the biology of the conservation target.
Conservation genetics at Royal Botanic Gardens Melbourne
Current research projects at Royal Botanic Gardens Melbourne that utilise studies of population genetics to assist in plant conservation include: