Genetic drift is common after a population experiences a population bottleneck. A population bottleneck arises when a significant number of individuals in a population die or are otherwise prevented from breeding, resulting in a drastic decrease in the size of the population. Genetic drift can result in the loss of rare alleles, and can decrease the size of the gene pool.
Genetic drift can also cause a new population to be genetically distinct from its original population, which has led to the hypothesis that genetic drift plays a role in the evolution of new species. How does the physical distribution of individuals affect a population?
A species with a broad distribution rarely has the same genetic makeup over its entire range. For example, individuals in a population living at one end of the range may live at a higher altitude and encounter different climatic conditions than others living at the opposite end at a lower altitude. What effect does this have?
At this more extreme boundary, the relative allele frequency may differ dramatically from those at the opposite boundary. Distribution is one way that genetic variation can be preserved in large populations over wide physical ranges, as different forces will shift relative allele frequencies in different ways at either end.
Migration is the movement of organisms from one location to another. Although it can occur in cyclical patterns as it does in birds , migration when used in a population genetics context often refers to the movement of individuals into or out of a defined population. What effect does migration have on relative allele frequencies? If the migrating individuals stay and mate with the destination individuals, they can provide a sudden influx of alleles. After mating is established between the migrating and destination individuals, the migrating individuals will contribute gametes carrying alleles that can alter the existing proportion of alleles in the destination population.
How do populations respond to all these forces? As relative allele frequencies change, relative genotype frequencies may also change. Each genotype in the population usually has a different fitness for that particular environment. In other words, some genotypes will be favored, and individuals with those genotypes will continue to reproduce. Other genotypes will not be favored: individuals with those genotypes will be less likely to reproduce. What type of genotype would be unfavorable?
Unfavorable genotypes take many forms, such as increased risk of predation, decreased access to mates, or decreased access to resources that maintain health. Overall, the forces that cause relative allele frequencies to change at the population level can also influence the selection forces that shape them over successive generations.
For example, if moths with genotype aa migrate into a population composed of AA and Aa individuals, they will increase the relative allele frequency of a. However, if the aa genotype has a clear disadvantage to survival e. This page appears in the following eBook.
Aa Aa Aa. Genetic variation describes naturally occurring genetic differences among individuals of the same species. Other chapters in Help Me Understand Genetics. Genetics Home Reference has merged with MedlinePlus. Learn more. The information on this site should not be used as a substitute for professional medical care or advice.
Contact a health care provider if you have questions about your health. How are gene variants involved in evolution? From Genetics Home Reference. Topics in the Variants and Health chapter What is a gene variant and how do variants occur?
How can gene variants affect health and development? Do all gene variants affect health and development? What kinds of gene variants are possible? Can a change in the number of genes affect health and development?
Can changes in the number of chromosomes affect health and development? There may be potential for the population to expand their range into other waterways on the island and thus increase the total population size. Within their current distribution, however, the population is unlikely to grow beyond individuals. The small number of founders, genetic isolation, and sustained small population size will continue to impact on the genetic diversity of this island population.
Both microsatellite and mitochondrial data only provide support for the contribution of Victorian founders suggesting the initial Tasmanian founders were unsuccessful.
This limits the founder population to a maximum of 16 Victorian individuals and potentially fewer effective founders those that have contributed genetically to subsequent generations. Today's Kangaroo Island platypus population remains small and the number of individuals contributing to subsequent generations is likely to be even smaller. Estimates of the effective population size of Kangaroo Island obtained from both population size modeling and genetic data were similar, indicating an N e of approximately 10—11 individuals.
This is extremely low and potentially unsuitable for long-term population persistence. An N e of at least Franklin , but more likely greater than Willi et al. Recently bottlenecked populations are likely to experience a reduction in allele number, particularly due to the loss of rare alleles, but can still maintain reasonable levels of heterozygosity in the short-term Luikart et al.
Isolation since in combination with a small effective population size is likely to have elevated the impact of genetic drift, contributing to the observed decrease in allele number particularly the loss of rare alleles and decrease in heterozygosity. With a small population size and continued isolation of the Kangaroo Island population, genetic diversity is likely to continue to diminish through time Fig.
Long-term isolation and low N e are also likely to have led to inbreeding on these island populations. Islands typically exhibit increased levels of inbreeding with the effective inbreeding F e estimated from equation 1 from Frankham ,. For O. Low levels of genetic diversity and high levels of inbreeding are likely to have important consequences for the long-term survival of island populations. Both measures correlate with population persistence as well as various fitness indicators including reproductive success, survival, and parasite resistance Ralls et al.
Despite the occurrence of long-term inbreeding on King Island, the genetic load is still likely to be high Frankham a ; Ballou ; Eldridge et al. Indeed, King Island individuals harbor low diversity in an important immune response gene family, the major histocompatability complex MHC Lillie The occurrence of the fungal mucormycosis on the Tasmanian mainland Gust and Griffiths is of particular concern and could have potentially devastating consequences should it emerge within the island population.
High levels of genetic diversity are required to maintain adaptive potential and minimise the risk of extinction Reed and Frankham Islands can act as an important reservoir for Australian wildlife.
They are often free from introduced predators or competitors. There are numerous examples of species persisting on islands despite extinction on the mainland Van Dyck and Strahan Island populations can provide a valuable source of individuals for conservation programs, including captive breeding, reintroductions, or translocations.
However, island populations also present many problems that must be effectively managed if they are to be employed in conservation efforts. Currently, genetic diversity in King Island O. Consequently, their long-term viability is likely to be under threat.
Levels of genetic diversity on Kangaroo Island are reduced in comparison to mainland populations and are predicted to decrease to levels as low as King Island within approximately 50 generations Fig. To maintain adaptive potential and minimise the risk of extinction Reed and Frankham , levels of genetic diversity need to be maintained in the case of Kangaroo Island or ideally, increased. If additional suitable platypus habitat can be found on Kangaroo Island, increasing the total population size through population range expansion can slow the loss of genetic diversity.
Human-mediated migration provides an alternative means to mitigate the negative effects of lowered genetic diversity with the transfer of individuals from suitable source populations.
The contribution of just one effective migrant per generation is presumed sufficient to alleviate the effects of drift and reduce inbreeding depression while maintaining local adaptation Wang The long-term persistence of these populations will ultimately depend on adequate levels of genetic diversity Bouzat and the addition of new genetic material provides a means to achieve that goal.
In particular, we would like to thank Stuart McColl and the many volunteers for help collecting samples. This work was carried out with ethics approval DPI National Center for Biotechnology Information , U.
Journal List Ecol Evol v. Ecol Evol. Author information Article notes Copyright and License information Disclaimer. Tel: 61 3 ; Fax: 61 3 ; E-mail: ua. Published by Blackwell Publishing Ltd. This is an open access article under the terms of the Creative Commons Attribution Non Commercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.
This article has been cited by other articles in PMC. Abstract Genetic diversity generally underpins population resilience and persistence. Keywords: Conservation, genetic diversity, island populations, platypus, Ornithorhynchus anatinus. Introduction Genetic diversity has been identified as an important factor influencing a population's long-term potential for survival Bouzat Open in a separate window.
Figure 1. Methods Genetic analysis Platypus live-capture surveys were conducted using either gill nets [refer to Grant and Carrick for methodologies] or fyke nets. Population size modeling—Kangaroo island Intensive platypus mark-release-recapture studies were carried out along Rocky River in Kangaroo Island. Results Genetic diversity A total of alleles were detected across 13 polymorphic microsatellite loci.
Table 1 Thirteen microsatellite loci screened, primer sequences, annealing temperatures, and the number of alleles found in the contemporary populations of upper Yarra Victoria, northwestern Tasmania; Kangaroo Island, South Australia; and King Island, Tasmania.
Table 2 Ornithorhynchus anatinus population genetics statistics for individuals sampled from the upper Yarra Victoria, northwestern Tasmania, King Island Tasmania, and South Australia.
Figure 2. Figure 3. Figure 4. Population size estimates—Kangaroo island Each data set females only and males only was tested for closure Table 3. Table 3 Closure tests for the female and male data. Table 4 Various parametric models fitted to female and male Kangaroo Island platypus data with the body weight covariate for M h -type models.
Figure 5. Discussion Levels of genetic diversity in platypus populations from Tasmania and Victoria are similar to those observed in mainland populations of other Australian mammalian species Taylor et al. Conclusion Islands can act as an important reservoir for Australian wildlife. References Akiyama S. Bundoora, Australia: La Trobe University; Handbook of capture-recapture analysis. Princeton, New Jersey: Princeton Univ. Press; Koalas on Kangaroo Island.
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