**4. Evolutionary factors influencing genetic variability**

## **4.1 Mutation**

Mutations encompass a wide range of phenomena; from a change of a single base pair in the genetic code to an inadvertent doubling of the number of chromosomes. Many mutations are deleterious or lethal, some are near neutral and a small number may be beneficial (usually exist as rare alleles). A large number of mutations are completely invisible in the phenotype and can only be detected with various genetic techniques. Hence, mutations are of concern for conservation geneticists in a couple of circumstances. First, mutations in small, remnant, or isolated populations with deleterious effects. Second, whether the emergence of new mutations will replace

variability lost due to population extinction and genetic erosion. Mutation rates are usually in the order of one per 105 cell divisions with time for the accumulation of new variants in a population taking tens of thousands of years.

Deleterious alleles (alleles that are accountable for genetic defects such as albinism) are usually very rare and have a frequency less than 0.0001. In a large population, natural selection purges very rare alleles of deleterious mutations from the population almost immediately. However, in a small, remnant, or isolated population, purging for such deleterious alleles in the context of the conservation of threatened species breeding program should be controlled or eliminated instantly artificially because natural selection is inefficient. Even though extinction due to the presence of deleterious mutations is almost unknown, but their contribution to the extinction process should not be ignored. Theoretically, the accumulation of deleterious mutations can significantly induce inbreeding depression and genetic erosion of fitness [10]. Deleterious alleles if not eliminated in a population, will gradually increase in frequency and become a serious problem when the frequency exceeds 0.05 or 1/(2Ne). Fortunately, this process took hundreds of generations.

On the other hand, conservation geneticists are often being demanded to save rare alleles including mildly deleterious alleles in threatened populations as they may be important for the population's adaptation towards environmental changes. The maintenance of desirable rare alleles including mildly deleterious alleles require very large population sizes and is simply not possible in most captive management programs. The risk of extinction due to fixation of rare alleles including mildly deleterious mutations of equal importance to environmental stochastics and can reduce the long-term viability of populations with Ne of less than a few thousand. An optimum Ne = 10,000 is required to ensure genetic and demographic factors act synergistically for avoiding inbreeding depression and for suppressing genetic erosion of fitness [11]. Small populations (Ne < 500) can decline fitness rapidly with the accumulation of mildly deleterious mutations, called mutational meltdown [11–13]. However, many threatened species currently have insufficient individuals to ensure long-term viability if Ne = 10,000 is strictly required.

Therefore, conservation geneticists are often left with conflict to design conservation plans that will further maintain rare alleles including mildly deleterious alleles, and eliminate deleterious alleles in threatened populations without jeopardizing populations' fitness. If the purpose of a conservation program is to return captive populations to the wild, then managers should maximize the genetic variability of rare alleles including mildly deleterious mutants. On the other hand, if the population cannot be returned to the wild and must be sustained in captivity for many generations, managers should either purge or rigorously control deleterious mutations and maintain rare alleles including mildly deleterious mutants as they are identified. For example, the homozygous recessive rare allele of White tigers (*Panthera tigris*) show no severe physiological defects but are needed to be strictly controlled in the captive populations and curbed from transmission to the wild populations to maintain the wild tiger populations [14].

#### **4.2 Non-random mating**

The ideal population genetic theory is based on random mating. It is widely accepted that random mating in sexual reproduction species evolved in part because of chromosomal crossing over and recombination facilitated by outbreeding. Most plants and animals species have effective immunological and behavioural mechanisms to favour outbreeding. These include asynchronous maturation of male and female gametes, sex-biased dispersal of the juvenile from their natal population, complex courtship behaviours, and the evolution of diverse self-incompatibility

#### *Conservation Genetics for Managing Biodiversity DOI: http://dx.doi.org/10.5772/intechopen.101872*

systems. Though, such mating behaviour is rarely observed in the nature of nonrandom mating species. The three extreme modes of non-random mating species are self-fertilized hermaphroditic, obligate outbreeding dioecious, and females preferentially mate (also known as selective breeding).

The most extreme consequence of non-random mating is the rise of inbreeding. Inbreeding refers to the mating of close relatives — mattings between father and daughter, brother and sister, or first cousins. Many species of plants and animals have evolved mechanisms to minimize close inbreeding. Species differ greatly in their tolerance to inbreeding; for example, some trees and dioecious plants are obligate outcrosses. In wild populations, the occurrence of gradual inbreeding allows natural selection to purge the first generation but the partially recessive near-neutral mutations continue to increase in frequency and significance. Inbreeding results in increased homozygosity of recessive partially deleterious mutants and by chance, in small isolated populations, these alleles can become fixed. In the simplest genetic example of a trait under the control of this recessive allele, there is an increased risk that the offspring of two related healthy but heterozygous individuals will inherit the harmful allele from each parent and die. Although the risk, in this case, is only one in four, this is a very strong fitness difference in which natural selection will act. Generalizing from this simplest single-locus example, geneticists discuss inbreeding depression as an overall manifestation of the genomic effects of mating between close relatives. These effects may involve outright genetic disease (congenital abnormalities) but are more often subtle and appear as decreased growth rate, behavioural abnormalities, and reduced fertility and fecundity. Inbreeding is rare in typically outbreeding populations but becomes a serious problem in small isolated populations. In small isolated populations and fragmented populations, inbreeding depressions can intimidate population viability. Animal and plant breeders have learned this lesson from their centuries of experience with artificial selection, and therefore they limit inbreeding rates to less than 2% per generation. The genetic underpinnings of inbreeding depression (i.e., reduced viability and fecundity) are best studied and understood in inbred strains of laboratory-reared Drosophila and Mice, in which recessive lethal mutations and mildly deleterious mutations arise due to non-random mating [5].

There is abundant evidence that isolated wildlife populations suffer inbreeding depressions. Inbreeding depression can be avoided in the short term if Ne > 50 [12]. The inbreeding coefficient (F) increases by 1/2Ne per generation and centuries of animal breeding experience show that a 1% increase in F per generation is tolerable. Thus, Ne = 50 is necessary to avoid inbreeding depression [12]. Jamieson and Allendorf [12] further concluded that Ne > 500 was necessary to enable a population to continue to evolve in the long term. Although this 500 number has been revised upwards, the theory behind the 50 number is still accepted [15]. But it is important to realize that its derivation was based on controlled laboratory experiments; larger Ne (Ne = 10,000) are required in nature, where environmental fluctuations are more severe and stressful.

#### **4.3 Gene flow**

One of the fundamental agents in evolution that interest conservation geneticists are the dispersal of genes (i.e., gene flow) between populations of a species. Gene flow can be either active or passive, often gender-biased and limited to certain phases of the life cycle. It may be accelerated under certain climatic conditions that occur at frequencies of many years or irregular intervals of many years apart. Gene flow is typically can be estimated from allele frequency data and presented in terms of the number of successful establishment migrants per generation in the new

population. In theory, one migrant per generation between two populations will ensure the two populations remain genetically homogeneous and related, as well as reduce inbreeding depression. In the future, overcoming genetically depauperate populations. Whereas lack of gene flow allows interpopulation differentiation. Hence, understanding historical patterns and rates of gene flow in a conserved population are crucial. Particularly if previously continuous populations become fragmented, the patterns of historical dispersal and gene flow may be disrupted with potentially serious consequences for population viability. For example, if young female orangutans can no longer migrate and confine from their natal social group due to habitat destruction in the surrounding area, their isolated natal populations will experience significantly increased inbreeding. On the other hand, if previously fragmented populations with each population have the unique genetic basis for adapting to local conditions become interacted, gene flow can erode the genetic differences between populations. Consequently, the two populations become one and some unique genes/alleles may be lost (see genetic drift).

In nature, widespread interspecific gene flow may occur between members of two different but related species (i.e., semispecies) or between very distantly related conspecific individuals in hybrid zones and produce hybrids. Hybrids are commonly sterile, or partial sterile in one sex or have high neonate mortality or have genetic disorders, and rarely are fertile. However, if fertile interspecific hybrids (also known as introgressive hybridization) exist, it causes a dilemma in conservation management. Because their occurrence reduces the value of the taxon. But at the same time, it is interesting because they show that the evolution of many groups of species involves both lineage splitting and lineage anastomosis. Hybridization is more common observe in plants than in animals; therefore, not surprisingly in plants, there are many examples of rare species being hybridized with the more common sympatric congeners (genetic assimilation) and become extinct (e.g., [16]).

#### **4.4 Genetic drift**

Genetic drift is referring to the loss of alleles from a population by chance due to a sudden reduction in Ne. This results in loss of fitness unless there is a rapid and continuous recovery. Often in nature, genetic drift happens almost clocklike regularly [5, 7] and followed by a rapid population recovery is referred to as a demographic bottleneck. They can have an immediate impact on variability at molecular genetic loci as genetic drift snatch the innate variation in a population. The evidence of a demographic bottleneck may persist for hundreds of thousands to millions of generations in low levels of variation in the loci of allozyme and molecular genetic markers. On the other hand, a demographic bottleneck can result in a short-term increase in population variation because epistatic variation (due to interactions among genes controlling a trait) is transformed into additive variation. However, whether it is beneficial or harmful to population viability is unknown.

The rate at which alleles are lost from a population by genetic drift can be statistically estimated. Sewall Wright theoretical model showed analytically how the rate of allele loss varies with population size, and concluded that census population size (N) is not important but rather the Ne. Ne is almost always less than N under some populations. Ne taking into account the fact that closely related individuals will share alleles with the same lineage, unequal numbers of males and females, increased variances in family size, and temporal fluctuations. Ne can be defined and estimated in a variety of ways using temporal ecological data, DNA sequences, and a variety of methods to estimate migration rates. Some estimation methods have theoretical value but little operational utility. Even so, by estimating Ne the effects of different population management strategies can be evaluated.

#### *Conservation Genetics for Managing Biodiversity DOI: http://dx.doi.org/10.5772/intechopen.101872*

In many threatened populations, Ne is only 10–30, and at such levels, genetic variation becomes significant for the viability of the population.

Very low genetic variability has been known in many sexually reproductive species whose currently large populations have recovered from one or recurrent demographic bottleneck or extinction. Meanwhile, in a large continuously distributed population (metapopulation) with frequent extirpation and recolonization of subpopulations, reduce metapopulation Ne orders of magnitude below than N can mimic the genetic effects of a demographic bottleneck. In small isolated populations with the absence of factors driving genetic variation (mutation and gene flow), the impacts of demographic bottlenecks are severe. Whereby demographic bottleneck reduces genetic variation (loss of heterozygosity), leading to increased homozygosity and loss of evolutionary adaptability to change (genetic variability or selectively neutral variation). The genetic variability is expected to be lost ½Ne per generation and mostly lost within 2Ne generations. Ne of 10 is predicted to lose heterozygotes five times faster than Ne of 100. This is because 50% of heterozygosity in Ne = 10 will be lost in approximately 20 generations. Therefore, in theory, small isolated populations have a higher rate of loss of heterozygosity and faster loss of variability by genetic drift than large populations and metapopulations).

#### **4.5 Natural selection**

In nature, differences in the survival and reproduction of some genotypes over others as the major agents of microevolutionary changes are known as natural selection. Natural selection attracts the interest of conservation geneticists for two reasons. First, human activities can radically alter selection coefficients in both natural and control populations. Such evolutionary changes of human influence are referred to as artificial selection whether intentional or not. This can be seen in many commercially exploited wildlife species, whereby has resulted in rapid behavioural and natural history changes and consequently reduced fitness. Examples include reduced body size in the game and commercial fish and the impact of hunting only horned or tusked male mammals on social behaviour.

Second, the major challenges to assist wildlife species adapt to ongoing global climate change. In the past, in the absence of humans, natural selection favoured individuals adapted to change and many species shifted their ranges towards accommodating major changes. The rate of directional selection that a population can control in response to some environmental change is in part, is determined by its inherent variability. Unfortunately, in the 21st century, environmental change and destruction such as those associated with global warming are happening too quickly for many species to respond to it. Hence, effective conservation management is necessary to ensure that many species survive.

### **5. Conclusions**

The importance of incorporating conservation genetics in managing biodiversity is undeniable. This is because the understanding of the relationship between evolutionary factors including mutations, non-random mating, gene flow, genetic drift, and natural selection in population/species survival is very important in the current situations where many natural populations are declining towards species extinctions. Therefore, with the relevant literature review in this chapter, it is hoped to provide brief explanations of the importance of assimilating conservation genetics to manage biodiversity. Especially to those who are less aware of the scope of genetic conservation studies.
