Environment of Earth

February 23, 2016

Concepts of diversity and stability


A very perplexing ecological problem is unravelling the nature of relationship between diversity and stability of ecosystem. The problem here is that neither the meaning of stability is entirely clear, nor what aspects of diversity are being consided. Are only the number of species in the community and its relation to stability, or the relationship of evenness to stability, or some combination of both is to be considered? Most of the theoretical and empirical studies for long focused only on species diversity, i.e., the number of species present. Only recently works have focussed on the degree of functional diversity represented by species diversity. Advocates of conserving biological diversity have always invoked the diversity-stability hypothesis to justify concern about the loss of individual species. However, if other aspects of diversity also play important roles in the structure and function of ecosystems, a focus on the number of species alone may hinder proper appreciation of the role that evenness plays in the ability of ecosystems to respond to changes in energy and nutrient inputs.
There are at least three ways in which ecosystem stability might be defined:

1. Constancy- The ability of a community to resist changes in species composition and abundances in response to any disturbance.
This is not a particularly useful concept of stability for conservationists because few, if any, ecosystems could be described as truly constant. Even the ecosystems having powerful mechanisms for reacting to environmental fluctuations do so through internal changes that as quickly as possible bring it back to a stable state. But these surely involve responses and changes. These may more appropriately be regarded as examples of resilience than of constancy.
2. Resilience- The ability of a community to return to its pre-disturbance characteristics after changes induced by a disturbance.
Resilience corresponds to stability in the way it is studied in mathematical models. If deviations from an equilibrium are reduced with time, system is stable or if these are amplified with time, system is unstable. This approach still has little applicability to actual ecosystems. It measures a system’s tendency to return to a single stable point, but many ecological sytems appear to have multiple stable points. If disturbance remains below a particular threshold, ecosystem will return to its predisturbance configuration. If it exceeds that threshold, it may move to a new configuration. Furthermore, most ecological systems change not only in response to disturbance but also in response to natural, successional change. There is little evidence that ecological communities ever represent an equilibrium configuration from which it would make sense to study perturbations. Common between the constancy and resilience is focus of both on species persistence and abundance as measures of stability. For example, Selmants et al.[1] show that with decline in species diversity of serpentine grasslands in California, their susceptibility to invasion by exotic species increases. Put differently, diverse grasslands were more resilient than those with lesser diversity.
3. Dynamic stability- A system’s ability to determine its future states largely by its own current state with little or no reference to outside influences.
In many ways, this approach corresponds with our intuitive notions of stability and seems to make sense of the relationship between diversity and stability. It recalls saying of Commoner [2]: “The more complex the ecosystem, the more successfully it can resist a stress.” A dynamically stable system is relatively immune to disturbance like a rapidly spinning gyroscope is dynamically stable because the gyroscopic forces generated by it resist external forces that would alter is plane of rotation. This approach reflects the hope that stable systems would be able to maintain themselves without intervention.
A biological system with high diversity is more likely to be dynamically stable than one that has low diversity. The reason is very important role played by biotic interactions in ecosystem dynamics. This has increasingly been appreciated through many studies. In diverse communities, biotic interactions may often play a larger and very important role in the success of a species than its interactions with the physical environment. To the extent that changes in the system are driven by biotic interactions, it is dynamically stable, since characteristics of the system itself are determining its future state.
However, this formulation of the diversity-stability hypothesis is also not free from problems. How to identify systems whose future state depends primarily on their own internal characteristics?[3] Without a method to identify a dynamically stable system, even testing the truth of this approach to diversity-stability hypothesis is not possible. It seems to verge on circularity. The larger (more diverse) the system considered, fewer things are left out of it. Fewer the things left out, smaller the possible outside influences on the system. Smaller the possible outside influences, greater the degree of dynamic stability. Thus, dynamic stability is (almost) a necessary consequence of diversity simply because diverse systems include many components.[4] Moreover, the argument as presented says nothing about the types of diversity present, e.g., a diverse community assembled from non-native species would be as good as one composed solely of natives.

Ives and Carpenter [5] have suggested a different approach to understanding community stability. Their approach may be quite useful, because it points out firstly, that systems move to a region different from the one from which they were perturbed[6] and secondly, that things other than diversity (like the frequency and character of perturbation) may also affect the stability of ecosystems. A new concept in relation to stability is ‘Biological integrity’ that refers to a system’s wholeness, including presence of all appropriate elements and occurrence of all processes at appropriate rates.[7] But this approach too poses problems.
1. What are ‘appropriate elements’?
2. What are ‘appropriate rates of processes?’
By definition, naturally evolved assemblages possess biological integrity but random assemblages do not. It, therefore, provides justification for management of ecosystems focusing on native species rather than introduced ones. This seems like the logical fallacy of affirming the consequent. However, species composition of lakes exposed to nutrient enrichment or acidification responds more quickly and recovers more slowly than processes like primary production, respiration, and nutrient cycling. Shifts in biotic composition don’t necessarily lead to changes in process rates. These observations mean a focus on integrity rather than diversity makes sense but it makes more sense to conclude that species changes are a more sensitive indicator of what is going on than the process changes. Loss of native species from a system is truly a warning of process changes that may have consequences much larger than are suspected.

From the point of view of conservation, there are still many  problems.

1. Can it be psossible that constancy or resilience based approaches to diversity-stability hypothesis probably are not true and may not provide a solid conceptual basis for arguing that conservation of biological diversity is an important goal.
2. Is it possible that a less specific version defining stability as a dynamic property related to the degree that the components of a system determine their own future state, provides a plausible basis for the hypothesis. Unfortunately, this version of the hypothesis verges on circularity and is almost immune to empirical investigation. It may also be pointed out that a system that is “stable” with respect to some perturbations like hurricanes, drought, or other extreme weather events may not be stable to others like invasion by exotic plants or animals, extinctions of component species, or other biotic changes. From the point of view of practical conservation applications, the diversity-stability hypothesis seems to provides merely a useful heuristic.
There seems something more useful for practical applicability in the idea of biological integrity. Easily observable changes in species composition and community structure may act as pointer to underlying changes in ecosystem processes more quickly than attempts to directly measure these processes. Diverse systems provide more indicators of change in these underlying processes and if the systems are managed so that they are protected then the underlying processes will remain intact too. Chapin et al. [8] summarized that:

1. High species richness maximizes resource acquisition at each trophic level and the retention of resources in the ecosystem. 2. High species diversity reduces the risk of large changes in ecosystem processes in response to directional or stochastic variation in the environment. 3. High species diversity reduces the probability of large changes in ecosystem processes in response to invasions of pathogens and other species. 4. Landscape heterogeneity most strongly influences those processes or organisms that depend on multiple patch types and are controlled by a flow of organisms, water, air, or disturbance among patches.

Wang and Loreau [9] developed the last point more formally and suggested that when thinking about a meta-community or meta-ecosystem it is useful to decompose the variability in response across the entire system into components analogous to those used in partitioning species diversity i.e.
1. Variation within the individual components of a meta-community or meta-ecosystem. 2. Variation among different components of a meta-community or meta-ecosystem. 3. Variation across the entire system, the sum (or product) of alpha and beta variation.

Thinking about ecosystem functioning at various scales, as Wang and Loreau suggest, leads to recognition that the experimental focus on diversity and variation in functioning leaves out a vital component for those trying to manage ecosystems that includes a variety of different habitats. Stability at the whole-system level may depend as much or more on retaining those distinct components as it does on stability within any one of them.

Bibliography
1. Paul C Selmants, Erika S Zavaleta, Jae R Pasari, and Daniel L Hernandez. Realistic plant species losses reduce invasion resistance in a California serpentine grassland. Journal of Ecology,
100(3):723-731, 2012.
2. B Commoner. The Closing Circle. Alfred Knopf, New York, NY, 1972. 3. R MacArthur. Fluctuations of animal populations and a measure of community stability. Ecology, 35:533-536, 1955.
4. B G Norton. Why Preserve Natural Variety? Princeton University Press, Princeton, NJ, 1987.
5. Anthony R Ives and Stephen R Carpenter. Stability and Diversity of Ecosystems. Science, 317(5834):58-62, 2007.
6. Laurie J Raymundo, Andrew R Halford, Aileen P Maypa, and Alexander M Kerr. Functionally diverse reef-fish communities ameliorate coral disease. Proceedings of the National Academy of Sciences,
106(40):17067-17070, 2009.
7. Noah A Rosenberg, Jonathan K Pritchard, James L Weber, Howard M Cann, Kenneth K Kidd, Lev A Zhivotovsky, and Marcus W Feldman. Genetic structure of human populations. Science, 298(5602):2381-2385, 2002.             8. F S Chapin III, O E Sala, I C Burke, J P Grime, D U Hooper, W K Lauenroth, A Lombard, H A Mooney, A R Mosier, S Naeem, S W Pacala, J Roy, W L Steffen, and D Tilman. Ecosystem consequences of changing biodiversity: experimental evidence and a research agenda for the future. BioScience, 48:45-52, 1998.
9. Shaopeng Wang and Michel Loreau. Ecosystem stability in space: , and variability. Ecology Letters, 17(8):891-901, 2014.

April 12, 2012

Plant Diversity in the Indian Gene Centre

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January 12, 2011

Diversity of domesticated plants

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Plant Genetic Resources: General

Perspective – R.S. Paroda and R.K.

Arora

http://www2.bioversityinternational.org/publications/Web_version/174/ch05.htm#TopOfPage


Summary

The importance of plant genetic resources as basic materials for crop improvement has been highlighted. The circumstances leading to settled agriculture, and the dynamics of plant domestication resulting into changes in plants from wild to cultivated forms has been discussed. A broad picture has been presented on the centres of origin/diversity in crop-plants and recent views on this topic are expressed. The spectrum of genetic diversity covering different categories of genetic resources is indicated. The importance of crop plant diversity for increased food production is stressed, both in terms of its collection and conservation. The concern on genetic uniformity and genetic vulnerability vis-a-vis genetic erosion has been emphasized. Finally, the subject of genetic conservation has been introduced, both for in-situ and ex-situ systems.

Introduction
Change from nomadic life to settled agriculture
Dynamics of plant domestication
Regions of crop plant diversity
Spectrum of genetic resources
Value of crop plant diversity
Threat to genetic diversity
Conservation of genetic diversity
Summary
References
Appendix I. World centres of diversity of cultivated plants (Hawkes, 1983)
Appendix II. Cultivated plants and their regions of diversity (refer Fig. 3)

Plant Diversity in India

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Plant Diversity in the Indian Gene

Centre – R.K. Arora

http://www2.bioversityinternational.org/publications/Web_version/174/ch06.htm

Introduction
Antiquity of Indian agriculture
Indian subcontinent as gene centre
Diversity in other economic plants
Summary
References
Appendix I (a). Crops and areas where rich diversity in landraces and primitive cultivars occurs (Mehra and Arora, 1982; with additions by the author)
Appendix I (b). Rice varieties from Kerala with useful genes (Khoshoo, 1986)
Appendix II (a). Distribution of important wild relatives and related types in different phyto-geographical zones (Arora and Nayar, 1984)
Appendix II (b). Wild relatives and related endemic and/or rare species including endemic cultigens (Arora and Nayar, 1984)

Summary

India is one of the centres/regions of crop plant diversity. It is equally rich, unique and interesting in its floristic wealth. About 15,000 species of higher plants occur, of which over 30 percent are endemic. These also include the wild relatives of crop plants. An effort has been made to briefly deal with the distribution and extent of this diversity located in different phyto-geographical/agro-ecological zones of the country. 166 cultivated plant species, of which about 50 are truly of Indian origin, exhibit rich diversity in this subcontinent. Further, about 320 species of wild relatives of crop plants occur and their distribution and diversity is discussed. Besides, the indigenous diversity in medicinal plants, forest trees, wild forage legumes and grasses, and in native ornamental plants has also been listed, thus pointing to overall richness of plant resources of India. Antiquity of Indian agriculture and its rich heritage, which is even evident today by the prevalence of ethnic diversity and traditional cultivation as in the north-eastern and peninsular regions, has been highlighted. It is pointed out that climate apart, cultural and historical factors have effectively contributed to the introduction of several crops of African, American, European and South-east/East Asian origin. The Indian subcontinent, thus holds prominence as one of ‘the twelve regions of diversity in crop plants in global perspective.

August 17, 2010

BIODIVERSITY INDICES

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Diversity index – Wikipedia, the free encyclopedia

A diversity index is a statistic which is intended to measure the diversity of a set consisting of various types of objects. Diversity indices can be used ...

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May 26, 2008

GENETIC DIVERSITY

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Biological diversity is usually identified solely with the number of species present i.e the species diversity but the concept can also be applied at intraspecific level. The members of a species show considerable variation, which is partly environmentally induced and partly is of genetic origin. The presence of genetic diversity is a basic characteristic of the species. Phenotypic expression of genetic variation may vary between classical polymorphisms with limited number of distinct phenotypes, and quantitative characters with a continuous distribution. The genetic variation is of fundamental importance from an evolutionary viewpoint. It provides the necessary short-term adaptation to the prevailing abiotic and biotic environmental conditions for the permanence of a species. It also enables long-term changes in the genetic make up to cope up with environmental changes.

Level of genetic diversity

The knowledge of the level of genetic diversity is required for understanding the role of intraspecific variation in the survival and extinction of a species i.e. in changes in biodiversity. Study of the level of genetic diversity includes following aspects of the genetic variants:

  1. determination of polymorphic loci,

  2. the number of alleles,

  3. dominance relationships,

  4. genetic architecture

  5. spatial distribution.

1. GENETIC VARIATION AT INDIVIDUAL LEVEL

The number of different genotypes is potentially tremendous. With x varying loci, each with a alleles, number of genotypes (g) is given by:

g = [a(a+1)/2]^2

2. NUMBER OF VARIABLE LOCI

The number of variable loci is crucial to the discussion of genetic diversity. Two contrasting views have existed on this matter.

  • Classical hypothesis: It assumes that most populations have individuals that are homozygous at nearly all loci because only the ‘best’ genotype for a particular set of environmental variables would be maintained. Therefore, adaptation to the prevailing environment would lead to genetic uniformity. Genetic diversity in a species would be found among populations adapted to different environments.

  • Balance hypothesis: This opposite view assumes that individuals would be heterozygous at many loci and a lot of genetic diversity would be maintained by some kind of balancing selection e.g. heterozygote superiority.

Detection of genetic diversity

Up to two decades ago, it was virtually impossible to decide about the reality of aforementioned two models. Recent techniques like DNA-sequencing, RFLP-assays etc. now provide the means of quantitative screening of genetic diversity. However, the use of these techniques has just started and most of the knowledge up to now has come from the older technique of protein electrophoresis. This method allows detection of genetic variation at molecular level.

  1. After electrophoresis and staining, specific proteins can be identified. Genetic variation at DNA level for a particular protein can be detected by differences in electrophoretic migration distance. This type of genetic variation is called allozyme variation.

  2. Genetic differences at the DNA level, generally caused by single base substitutions, can be detected by electrophoresis. As only those base substitutions which lead to differently charged protein molecules can be detected, the method provides an underestimation of the genetic diversity present at DNA level.

  3. Since individual proteins can be identified by using specific staining techniques, genetic diversity can be detected at separate loci. By sequencing large number of individuals for several loci, estimates of genetic diversity can be obtained.

  4. This method of screening a random sample of structural loci has been extremely helpful in qunatifying genetic variation. Measures generally used are fractions of polymorphic loci (P) and heterozygosity (H). Many species have been surveyed for such allozyme variation. Following table gives a survey of allozyme variation in many species of major groups of living organisms.

GROUP

P

n

H

N

Plants

0.342 (0.012)

468

0.113 (0.005)

468

Invertebrates

0.375 (0.011)

371

0.1 (0.005)

361

Vertebrates

0.226 (0.006)

596

0.054 (0.003)

551

n = Number of species surveyed, standard error in brackets. (From Hamrick and Godt, 1990 and Nevo et al. 1984)

  • Nearly all species have considerable allozyme variation but differences among systematic groups are also present. For example, insects and gymnosperms have high while birds and large carnivores have low levels of allozyme variation. Some species e.g. Mirounga angustirostris (Northern elephant seal), Acinonyx jubalus (Cheetah),Phoca vitulina (Harbour seal) show no allozyme variation despite large number of loci screened. Reasons for this lack of genetic diversity are not clear in these species.

  • Caution should be excersised in equatic allozyme variation with other types of genetic variation e.g. variation for fitness characters. However, allozyme variation is a reliable tool for comparision of the levels of genetic diversity at a random sample of neutral genes.

Factors acting on genetic diversity

Hardy-Weinberg rule formly represnts the phenomenon of preservation of genetic variation in populations when a number of conditions are fulfilled. It also indicates that fixed genotype proportions based on allele frequencies are to be expected. However, one or more conditions most often do not hold and changes in genetic composition do occur. Following table shows the effects of various forces acting in populations on the level of genetic diversity.

Force

Genetic variation

within populations

Genetic variation

between populations

Mutations

Increase

Decrease

Gene flow

Increase

Decrease

Asssortative mating

Increase or decrease

Increase or decrease

Balancing selection

Increase

Decrease

Directional selection

Decrease

Increase or decrease

Genetic drift/inbreeding

Decrease

Increase

  • New genetic variants are introduced through mutations and gene flow from genetically different populations. Assortative mating mainly affects the genotypic proportions.

  • Selection model underlying classical hypothesis discussed above postulates that selection against (partly) rrecessives eliminates alternative alleles and leaves the population monomorphic at a particular locus.

  • Overdominance model, however, postulates that in case of heterozygote being most fit genotype, a stable allele frequency equilibrium is attained. Under this model, as in some other forms of balancing selection (such as in some cases of frequency-dependent selection), genetic diversity is protected. Loss of genetic diversity occurs when genetic drift and inbreeding are predominant.

1. GENETIC DRIFT

Impact of genetic drift is strongly dependent on population size. Genetic drift occurs when population size is small.

Due to genetic drift, allele frequencies fluctuate in the course of generations and finally each subpopulation is fixed by chance for a particular allele and total homozygosity is reduced.Genetic drift thus leads to the loss of genetic diversity in an isolated population.

At genotypic level, an important observation is that the proportion of homozygotes in metapopulation exceeds the expectations based on Hardy-Weinberg proportions associated with particular allele frequencies. In case of very small population, effects of genetic drift are extreme. In larger populations, the effects are moderate in terms of the number of generations needed for complete fixation of subpopulations and the rate of increase in homozygosity of metapopulation.

The chances of loss of an allele through drift are related with its frequency. Rare alleles are lost readily through drift.

Actual population size, as determined in a natural population, is generally not indicative of the level of drift to be expected. Theoretical considerations show that two factors are important in relationship between population size and the level of genetic drift:

(A) Bottlenecks i Bottlenecks in population size: The bottlenecks in population size in past have profound effects on the level of drift to be expected in present population of considerable size. For determination of the level of genetic drift in such situation, estimate of the effective population size (Ne) is required. In case of fluctuations in population size, relation between Ne and population sizes N1, N2, …, Nt in generations 1, 2, …, t respectively is given by:

1/Ne = 1/t [1/N1 + 1/N2 + … + 1/Nt]

This implies that Ne can be much smaller than N measured in a particular generation.

(B) Unequal sex ratio: Another cause of Ne being smaller than N is an unequal ratio of males and females contributing to next the generation. Such a situation generally results in effective population sizes which are much lower than the actual number of individuals.

In such cases of Ne being smaller than N, the effects of genetic drift are stronger than expected from actual population sizes.

2. INBREEDING

Inbreeding is closely related to genetic drift and is predominantly associated with limited population sizes.

  • Inbreeding cofficient (F): It indicates the level of inbreeding and is defined as the probability that two alleles at a locus in a diploid organism are copies of one and same ancestral allele (Wright 1969). Relatedness of inbreeding to genetic drift is shown by the use of F to quantify genetic drift processes.

Level of inbreeding in a population, indicated by F, is strongly determined by N. In a population of constant size, F will increase each generation and following relationship holds:

F = 1/2Ne

In inbreeding populations, fraction of heterozygotes is lower than expected under ideal Hardy-Weinberg conditions i.e. inbreeding results in increase in homozygosity and, therefore, loss of genetic variation.

  • Inbreeding depression: Increase in homozygosity with inbreeding is often accompnied by inbreeding depression i.e. loss of fitness due to inbreeding. Inbred individuals often have lower fitness than outbreds.

Partial dominance theory assumes that heterozygotes are more fit because in populations of sexual outcrossing organisms, recessive deleterious alleles occur at many loci though in low frequency. Due to increased homozygosity accompanying inbreeding, such alleles become homozygous and the fitness of individuals involved is reduced.

  • Niche width hypothesis (Van Valen 1965): The hypothesis proposes that species with greater genetic diversity i.e. with higher level of heterozygosity occupy larger, more variable habitats than the species with low genetic variation i.e. with low levels of heterozygosity.

3. BREEDING SYSTEMS AND DEGREE OF INBREEDING

Breeding system of a species has profound effect on the degree of inbreeding and consequently on the level and organization of genetic diversity.

  • Self-fertilization: Especially in self-fertilizing organisms, inbreeding levels increase very rapidly. Many plant species exhibit high levels of selfing. As predominantly selfing plant species flourish equally well as obligate outbreeders, apparently no or only moderate deleterious effects occur. The probable reason for this is that deleterious alleles in selfing species have been eliminated in the past (Lande and Schemske 1985, Charlesworth et al. 1990). Study of the effects of inbreeding in two populations of Eicchornia paniculata, one predominantly outcrossing and other predominantly selfing has confirmed the above view.

  • Outcrossing: On the other hand, many hermaphrodite plant species possess self-incompatiability systems that prevent selfing. This is commonly viewed as a mechanism to prevent deleterious effects of selfing.

4. GENETIC DIVERSITY AND EXTINCTION OF SPECIES

Species that are exposed to habitat destruction or habitat fragmentation often consist of isolated populations of small population sizes. Such small populations have a relatively high chance of becoming extinct due to purely demographic reasons. Such small populations generally carry high risk of attaining zero population size due to fluctuations in their size than larger populations. The genetic processes described above may also contribute to increased risk of extinction in case of small populations. The lowered individual fitness expected in such populations may increase the vulnerability to current and future environmental stresses. In this respect, following two aspects of genetic drift and inbreeding are highly relevant.

  • Loss of specific alleles at one or restricted number of loci: When such variants provide individuals with tolerance to specific environmental stresses, their loss will lead to deterioration of individual adaptedness. A population consisting of individuals which have lost the potency for adaptation will be more vulnerable to extinction when exposed to a specific stress.

With respect to the relation of particular genetic variants to specific stresses, numerous examples are known where adaptation has been strongly increased in natural populatons e.g. industrial melanism in moths, sickle-cell polymorphism in humans, warfrain resistance in rats, insecticide resistance in insects and heavy-,metal tolerance in plants. It is possible that the gain in individual fitness in a population may also increase the overall population fitness. However, the concept of such increased population fitness is much less well defined than individual fitness and is often disputed.

Short-term population fitness may be defined as the existence of a population over a limited number of generations. Fitness differences among populations are then reflected in differential survival of populations or in differences in population sizes.

Long-term population fitness has been defined by Thoday (1953) as the ‘probability that a contemporary group of individuals will survive for a given long period of time, such as 10^8 years, that is to say, will leave descendents after the lapse of that time’. In this context, Lewontin (1957, 1961) has proposed an all-or-none fitness concept which states that if a population can survive and reproduce in more environments than another population, the fitness of former population is greater than that of the latter population.

It can be argued that a population that survives in a greater number of environments than others also has a greater chance to survive in future environments. This enables an experimental approach to long-term fitness by testing populations in a great number of environments.

The importance of the presence or absence of particular genetic variants is demonstrated by an experiment in which populations of Drosophila melanogaster difffering in genetic composition for alcohol dehydrogenase (Adh) locus were subjected to various types of stresses. Adh locus is polymorphic for two alleles, Fast (F) and Slow (S). The Adh genotypes differ considerably in survival on medium containing toxic concentrations of alcohols. The F-homozygote survives more often than the S-homozygote while heterozygote is intermediate in survival. Survival is positively correlated with ADH-activities of the genotypes. When large numbers of vial populations, either monomorphic for S-allele or for F-allele or popymorphic, were exposed to toxic concentrations of ethanol, some populations did not survive in the course of several generations. However, extinction fractions differred considerably among population types. Monomorphic S-populations had higher extinction rates than the other population types, apparently because they did not have F-allele which gives higher individual fitness to its carriers when exposed to ethanol. Thus, individual fitness differences are transferred to the population level. This results in differential extinction probabilities. In another set of experiments, populations were exposed to varying stress factors in successive generations. The populations were exposed during one generation to one particular stress condition (e.g. high temperature, low humidity, ethanol addition to food etc.) and in the next generation to another factor (randomly chosen from a set of conditions) and so on. The differential extinction was again observed. In this case, however, the polymorphic populations had the lowest extinction rates. This experiment points to the importance of polymorphism in varying environments.

  • General loss of fitness associated with inbreeding depression: In this case, the probability of extinction is not limited to one particular stress factor but will occur generally. However, the effects of inbreeding depression will generally be more severe under harsh conditions.

In this case, the cause for increased rate of extinction is genetic diversity following inbreeding associated with inbreeding depression. For several species of mammals kept in captivity, juvenile mortality is one of the most important traits affected by inbreeding depression. In Drossophila melanogaster, population size and productivity are severely reduced following inbreeding.

Considerable improvement is obtained in such populations when genetic variation is increased artificially by X-irradiation. Merely the reduction in population size will increase the probability of extinction of local, isolated populations.

It is well established that when lines of Drossophila are inbred, a considerable number of lines are lost in the course of generations. Some of these extinctions would occur because of homozygosity for recessive lethals, others will be due to increased chance of extinction associatedwith reduced population size. Similar observations in inbred strains of mice have been made. Litter size decreased with about 0.53 young per 10% increase in F. Only 3 out of 20 lines survived by the time F had increased to 0.76.

5. PROSPECTS FOR NATURAL POPULATIONS

The expectations of the relations between population size, levels of genetic variation and fitness based on theoretical considerations laboratory experiments and data from animal and plant breeding are well established. However, the significance of these results for natural populations is still largely unclear. Studies in this field are comprehensive and time-consuming because information is needed on the demography of populations (both in the present and in the past) as well as on levels of genetic diversity and fitness and fitness. Only few such studies have been undertaken recently.

In the present times, many species in nature are increasingly being faced with destruction of their habitat and consequently in reduction in numbers of individuals together with fragmentation and isolation of the populations. The predicted loss of genetic diversity under such a scenario may have profound effects on biodiversity. Therefore, much interest is being shown for the studies related to relation between population size, genetic diversity and extinction of species. However, much theoretical and experimental work is needed in these areas. More information is needed on the effects of bottlenecks in population size on the genetic variance for quantitative characters. The relation between population extinction, genetic variation and demographic stochasticity also needs to be explored. In addition, important items for serious consideration and further study are the general effects of gene flow and concept of metapopulation levels in relation to demographic and genetic equilibrium.