Читать книгу Fundamentals of Conservation Biology - Malcolm L. Hunter Jr. - Страница 106

Some populations are quite large: thousands of individuals are loosely connected through a web of interbreeding that ensures gene flow throughout the population. On the other hand, some populations are quite small, perhaps because they are confined to tiny, isolated patches of habitat and have limited dispersal abilities. In this section we are primarily concerned with what can happen to the genetic diversity of small populations, especially among species that typically occur in large populations but have been forced into small numbers. Profound changes occur in reduced populations and thus management of small‐population phenomena is a major focus for conservation biology.

Populations become small for a variety of reasons. Sometimes, large populations experience a catastrophe such as a hurricane and collapse to a few remnant individuals. Sometimes, a few individuals arrive in a new area and establish a new population that is inevitably small at first; this is called a founder event. When a population collapses or a new population is established, the genetic diversity of the original larger population is likely to be reduced because only a sample of the original gene pool will be retained. Returning to the bison example, if you start with a population of 1000 bison with 2000 alleles for MDH‐1 and reduce it to 50 bison, only 100 alleles will remain. Moreover, the remaining sample is not likely to be representative of the original population. This phenomenon is called a genetic bottleneck. Passing through a genetic bottleneck can create two problems: (1) a loss of certain alleles, especially rare alleles; and (2) a reduction in the overall amount of genetic variation. For example, a population that ranged across a continuum from very dark individuals to very light individuals might, after a bottleneck, have only intermediate colored individuals or only dark or only light individuals (Frankel and Soulé 1981), posing problems for future fitness.

The proportion of genetic variation and number of alleles likely to be retained after a bottleneck can be estimated using the formulas presented in Table 5.2. From this table we can see that most of the genetic variation is retained even in a tight bottleneck, 95% with just 10 individuals. The situation is worse, however, for retention of uncommon alleles. In this example, 10 individuals are likely to retain only two of four alleles if three of the alleles were uncommon (2% each of all the alleles). This figure improves to an estimate of 3.63 alleles retained if the alleles are more common, 10% of the total in this example. Genetic data from the whooping crane illustrate this phenomenon: six genotypes were detected in a sample of old museum specimens, but only one of these persists in the modern population after a 1938 bottleneck in which only 14 adults survived (Glenn et al. 1999). A study by Bouzat et al. (1998) of greater prairie chicken microsatellite variation provides another example. Birds in Illinois, which remain only in very small populations, have just two‐thirds as many alleles as those from neighboring states with much larger populations, and many fewer than museum specimens collected before 1960 when the severe population decline began.

Table 5.2 Proportion of genetic variation remaining after a genetic bottleneck.

Based on Frankel and Soulé 1981

		Average number of alleles retained from an original set (m) of 4
Sample size (N) after bottleneck	Proportion of heterozygosity retained	p 1 = 0.70, p2 = p 3 = p 4 = 0.10	p 1 = 0.94, p 2 = p 3 = p 4 = 0.02
1	0.50	1.48	1.12
2	0.75	2.02	1.23
6	0.917	3.15	1.64
10	0.95*	3.63	2.00†
50	0.99	3.99	3.60
∞‡	1.00	4.00	4.00

^* Retention of heterozygosity is approximately equal to 1 − 1/(2N), where N is the population size after the bottleneck. If a population crashed to 10 individuals, about 1 − ½(10) = 1 − 0.05 = 0.95 of the genetic variation of the original population would remain.

^† The formula for estimating how many alleles would remain after a bottleneck is E = m − ∑ _j (1 − p_j) ^2N _, where m is the number of alleles before the bottleneck, p is the frequency of the jth allele, and N is the population size after the bottleneck. From an original set of four alleles the remaining number would be

 4 − ∑ (1 − 0.94)20 + (1 − 0.02)20 + (1 − 0.02)20 + (1 − 0.02)20 =

 4 − ∑ 0.0620 + 0.9820 + 0.9820 + 0.9820 =

 4 − ∑ ~0 + 0.666 + 0.666 + 0.666 = 2

^‡ With a population of infinite size no genetic bottleneck occurs.

A genetic bottleneck is the outcome of a process known as random genetic drift, a process similar in concept to sampling error that you may be familiar with from introductory statistics. Genetic drift is an erratic change in gene frequencies that is likely to occur in small populations because each generation retains just a portion of the gene pool of the previous generation (Frankel and Soulé 1981 ; Hartl and Clark 1997). It is like sampling error because the genetic diversity “sampled” by the few survivors may not be representative of the population as a whole. Table 5.3 presents a formula for estimating the implications of random genetic drift for genetic diversity. The formula is identical to the one for estimating the loss of genetic variation in a bottleneck, with an exponent added to represent the number of generations that a population has continued to remain small. In other words, random genetic drift is the same thing as passing through a genetic bottleneck except that the bottleneck usually lasts multiple generations (compare column 2 in Table 5.3 with column 2 in Table 5.2).

Table 5.3 The proportion of genetic variation retained in small populations of constant size after 1, 5, 10, and 100 generations is approximately [1 − 1/(2N)] ^t , where N is the population size and t is the number of generations. For example, 0.95⁵ = 0.77.

Based on Frankel and Soulé 1981

	Generations
Population size (N)	1	5	10	100
2	0.75	0.24	0.06	<<0.01
6	0.917	0.65	0.42	<<0.01
10	0.95	0.77	0.60	<0.01
20	0.975	0.88	0.78	0.08
50	0.99	0.95	0.90	0.36
100	0.995	0.975	0.95	0.60

We can see that although a population of 10 individuals may retain 95% of its genetic variation after one generation (or after one bottleneck), with random genetic drift for 10 generations only 60% of the variation is likely to be retained, and after 100 generations virtually all the original genetic variation would be lost. A similar pattern exists for the loss of alleles; after many generations of random genetic drift, small populations will usually retain only one allele for a given gene (Table 5.4). In the language of genetics, the gene will have been fixed for that allele. In sum, random genetic drift in a population that remains small for many generations is much more likely to lead to a loss of genetic diversity than is a single bottleneck from which a population recovers quickly.

Table 5.4 Expected number of alleles remaining after t generations for a population of six individuals with 2, 4, or 12 alleles for a gene, assuming equal frequency of each allele.

Based on Frankel and Soulé 1981

	Number of alleles
Generations	m = 2	m = 4	m = 12
0	2.00	4.00	12.00
1	1.99	3.87	7.78
2	1.99	3.55	5.88
8	1.67	2.18	2.64
20	1.24	1.36	1.44
∞	1.00	1.00	1.00

If drift erodes genetic diversity then will mutation simply replenish it? Probably not. The problem is a severe imbalance between the rates at which the two processes operate. A population bottleneck can deplete genetic diversity from a population during just a few generations if the bottleneck is narrow enough. In contrast, it has been estimated that 10⁵–10⁷ generations are required to regenerate allelic diversity for a single gene (Lande and Barrowclough 1987). The genetic machinery of a cell is remarkable at avoiding “copying errors” and thus we cannot rely on mutation to replenish genetic diversity over time scales of conservation concern; preventing bottlenecks and excessive genetic drift by keeping populations of “healthy” size is our best approach.