Founder And Bottleneck Effects

As shown in the previous section, genetic drift causes its most dramatic and rapid changes in small populations. However, even a population that is large most of the time but has an occasional generation of very small size can experience pronounced evolutionary changes due to drift in the generation of small size. If the population size grows rapidly after a generation of small size, the increased population size tends to decrease the force of subsequent drift, thereby freezing in the drift effects that occurred when the population was small. These features are illustrated via computer simulation in Figure 4.5. Figure 4.5a shows four replicate simulations of genetic drift in populations of size 1000, over 100 generations, with an initial allele frequency of 0.5. Figure 4.5b shows parallel simulations, but with just one difference: At generation 20 the population size was reduced to 4 individuals and then immediately restored to 1000 at generation 21. In contrasting Figure 4.5a with 4.5b, the striking difference is the radical change in allele frequency that occurs in each population during the transition from generation 20 to 21, reflecting drift during the generation of small size. However, there is relatively little subsequent change from the allele frequencies that existed at generation 21. Thus, the pronounced evolutionary changes induced by the single generation of small population size are "frozen in" by subsequent population growth and have a profound and continuing impact on the gene pool long after the population has grown large. These computer simulations show that genetic drift can cause major evolutionary change in a population that normally has a large population size as long as either

• the population was derived from a small number of founding individuals drawn from a large ancestral population (founder effect) or

• the population went through one or more generations of very small size followed by subsequent population growth (bottleneck effect).

We will now consider some examples of founder and bottleneck effects.

There are many biological contexts in which a founder event can arise. For example, there is much evidence that Hawaiian Drosophila are on rare occasions blown to a new island on which the species was previously absent (Carson and Templeton 1984). Because this is such a rare event, it would usually involve only a single female. Most Drosophila females typically have had multiple matings and can store sperm for long periods of time. A single female being blown from one island to another would often therefore carry over the genetic material from two or three males. Hence, a founder size of 4 or less is realistic in such cases. (Single males could also be blown to a new island, but no population could be established in such circumstances.) If the inseminated female found herself on an island for which the ecological niche for which she was adapted was unoccupied, the population size could easily rebound by one or two orders of magnitude in a single generation, resulting in a situation not unlike that shown in Figure 4.5b.

Founder events are also common in humans. The village of Salinas, located in a remote mountainous area in the Dominican Republic, had a population of about 4300 people in 1974 (Imperato-McGinley et al. 1974). Seven generations prior to that time, the village was much smaller. One of the founders at that time was a man by the name of Altagracia Carrasco who had several children by at least four different women. Because the population size was small at that time, the alleles carried by Altagracia constituted a sizable portion of the total gene pool. Indeed, this is the general impact of a founder event: The alleles carried

Figure 4.5. Computer simulation of genetic drift in four replicate populations starting with initial allele frequency of 0.5 over period of 100 generations. (a) Population size is kept constant at 1000 individuals every generation. (b) Same replicates are repeated until generation 20, at which point the population size is reduced to 4 individuals. The population size then rebounds to 1000 individuals at generation 21 and remains at 1000 for the remainder of the simulation to simulate a bottleneck effect.

Figure 4.5. Computer simulation of genetic drift in four replicate populations starting with initial allele frequency of 0.5 over period of 100 generations. (a) Population size is kept constant at 1000 individuals every generation. (b) Same replicates are repeated until generation 20, at which point the population size is reduced to 4 individuals. The population size then rebounds to 1000 individuals at generation 21 and remains at 1000 for the remainder of the simulation to simulate a bottleneck effect.

by the founding individuals automatically become frequent in the founding population and subsequent generations. In the case of the Salinas human population, even an allele carried exclusively by this one man would by necessity come to be in relatively high frequency due to the founder effect. It turns out that Altagracia did indeed have a unique allele; he was a heterozygote for a single base substitution of thymine for cytosine in exon 5 of the autosomal 5-alpha-reductase-2 gene, causing a tryptophan (TGG) replacement of arginine (CGG) at amino acid 246 of the enzyme (Cai et al. 1996). This amino acid change in turn results in an enzyme with low catalytic activity, such that individuals homozygous for this allele have a severe deficiency in the activity of the enzyme 5-alpha-reductase-2. Although autosomal, being deficient for this enzyme has its most dramatic effects in XY individuals because this enzyme normally catalyzes the conversion of the male hormone testosterone to dihydrotestosterone (DHT). During development of an XY individual, a testosterone signal causes the developing gonads to differentiate into testes rather than ovaries. However, DHT is the hormone required for full masculinization of the external genitalia. As a consequence, XY individuals who are homozygous for this allele develop testes, but most of their remaining sexual differentiation occurs along the female pathway, the default route of development in mammals. As a consequence of the high frequency of this allele in the village of Salinas, many XY babies are born that have testes internally but externally appear to be female and are subsequently raised as girls. However, these homozygotes only have an enzyme deficient in activity, not a complete absence. When these "girls" reach puberty, their testes start producing such high levels of testosterone that the low amount of active 5-alpha-reductase-2 that they have can at long last produce sufficient DHT to trigger the development of the external male genitalia. Hence, shortly after these girls enter puberty, they are transformed into males. Although this is an unusual situation in most human populations, it is so common in the village of Salinas because of the founder effect that the townspeople even have a word for these girls who turn into men: guevedoces ("penis at 12" years of age).

Our second example is one of a founder effect followed by bottleneck effects (Roberts 1967, 1968). Tristan da Cunha is an isolated island in the Atlantic Ocean. With the exile of Napoleon on the remote island of St. Helena, the British decided to establish a military garrison in 1816 on the neighboring though still distant island of Tristan da Cunha. In 1817 the British Admiralty decided that Tristan was of no importance to Napoleon's security, so the garrison was withdrawn. A Scots corporal, William Glass, asked and received permission to remain on the island with his wife, infant son, and newborn daughter. A few others decided to remain and were joined later by additional men and women, some by choice and some due to shipwrecks. Altogether, there were 20 initial founders. The population size grew to 270 by 1961, mostly due to reproduction but with a few additional immigrants. The growth of this population from 1816 to 1960 is shown in Figure 4.6.

Because there is complete pedigree information over the entire colony history, the gene pool can be reconstructed at any time as the percentage of genes in the total population derived from a particular founding individual (Figure 4.7). This method of portraying the gene pool can be related to our standard method of characterizing the gene pool through allele frequencies by regarding each founder as homozygous for a unique allele at a hypothetical locus. Then, the proportion of the genes derived from a particular founder represents the allele frequency at the hypothetical locus of that founder's unique allele in the total gene pool.

The top histogram in Figure 4.7 shows the gene pool composition in 1855 and 1857. Note from the population size graph in Figure 4.6 that a large drop in population size occurred between these years. This was caused by the death in 1853 of William Glass, the original founder. Following his death, 25 of his descendants left for America in 1856. This bottleneck was also accentuated by the arrival of a missionary minister in 1851. This minister soon disliked the island, preaching that its only fit inhabitants were "the wild birds of the ocean." Under his influence, 45 other islanders left with him, thereby reducing the population size from 103 at the end of 1855 to 33 in March 1857. Note that in going from 1855 to 1857 the gene pool composition changes substantially; the relative contributions of some individuals show sharp decreases (founders 1 and 2) whereas others show sharp increases (founders 3, 4, 9, 10, 11, and 17). Moreover, the genetic

Population Bottleneck
Figure 4.6. Population size of Tristan da Cunha on December 31 of each year from 1816 to 1960. Adapted from M. D. F. Roberts Nature 220: 1084-1088 (1968). Copyright ©1968, by Macmillan Publishers.

contributions of many individuals are completely lost during this bottleneck (founders 6, 7, 12-16, 19, and 20). Thus, the gene pool is quite different and less diverse after the first bottleneck.

Figure 4.6 reveals that the population grew steadily between 1857 and 1884. With the exception of a few new immigrant individuals (founders 21-26), the basic shape of the gene pool histograms changes very little in those 27 years (the second histogram from the top in Figure 4.7). In particular, note that there is much less change in these 27 years than in the 2 years between 1855 and 1857. Hence, the changes induced by the first bottleneck were "frozen in" by subsequent population growth.

Figure 4.6 shows that a second, less drastic bottleneck occurred between 1884 and 1891. The island has no natural harbor, so the islanders had to row out in small boats to trade with passing vessels. In 1884, a boat manned by 15 adult males sank beneath the waves with the resulting death of everyone on board, making Tristan the "Island of Widows." Only 4 adult men were left on the island, 2 very aged, leading many of the widows and their offspring to leave the island. This reduced the population size from 106 in 1884 to 59 in 1891. The third histogram in Figure 4.7 shows the impact of this second bottleneck on the island's gene pool. As with the first bottleneck, some individual contributions went up substantially (founders 3,4, and 22), others went down (founders 9 and 10), and many were lost altogether (founders 21 and 24-26).

After this second bottleneck there was another phase of steady population growth (Figure 4.6). The shapes of the gene pool histograms change little from 1891 to 1961 during this phase of increased population growth (the bottom histogram in Figure 4.7, which excludes the impact of a few additional immigrants). Once again, this shows how subsequent population growth freezes in the changes induced by drift during the bottleneck.

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