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Abstract
A bottleneck in population size of a species is often correlated with a sharp reduction in genetic variation. The northern elephant seal [Mirounga angustirostris] has undergone at least one extreme bottleneck, having rebounded from 20–100 individuals a century ago to over 175,000 individuals today. The relative lack of molecular-genetic variation in contemporary populations has been documented, but the extent of variation before the late 19th century remains unknown. We have determined the nucleotide sequence of a 179base-pair segment of the mitochondrial DNA [mtDNA] control region from seals that lived before, during and after a bottleneck low in 1892. A ‘primerless’ PCR was used to improve the recovery of information from older samples. Only two mtDNA genotypes were present in all 150+ seals from the 1892 bottleneck on, but we discovered four genotypes in five pre-bottleneck seals. This suggests a much greater amount of mtDNA genotypic variation before this bottleneck, and that the persistence of two genotypes today is a consequence of random lineage sampling. We cannot correlate the loss of mtDNA genotypes with a lowered mean fitness of individuals in the species today. However, we show that the species historically possessed additional genotypes to those present now, and that sampling of ancient DNA could elucidate the genetic consequences of severe reductions in population size.
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Copyright © 2000 Elsevier Science Ltd. All rights reserved.
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Abstract
This paper reviews some of the important factors related to the impact of population bottlenecks, using the northern elephant seal [Mirounga angustirostrus] as a case study for illustration. The northern elephant seal was hunted extensively in the 19th century and forced through a bottleneck of approximately 10–20 seals. All measures of molecular genetic variation show current levels for the northern elephant seal to be low. Levels of genetic variation were compared with expectations based on a simulation model that recapitulates demographic growth, based on age-specific data on reproduction and mortality. Predictions from the simulation model are then presented to illustrate the importance of differences in life-history strategy and skewed reproductive success. Either high reproductive skew [e.g. polygyny] or a low growth rate in a population can increase the impact of a bottleneck on molecular variation. Severe population bottlenecks can also disrupt aspects of developmental stability and thereby increase the fluctuating asymmetry and variability of quantitative traits. A comparison of skulls collected before and after the bottleneck showed this to have occurred for some elephant seal quantitative characters.
Keywords
genetic diversity – simulation models – marine mammals.
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Copyright © 1999 The Linnean Society of London. Published by Elsevier Ltd. All rights reserved.
1
The effects of natural selection may be countered by
2
In relation to natural selection, evolution is the
3
Moto Kimura's theory that opposed natural selection was the
4
Scientists generally agree that heterozygous advantage is
5
The occurrence of large or small beak sizes among seed crackers in the absence of medium-sized beaks is an example of
6
Even though sickle-cell anemia is usually fatal to homozygous individuals, the disease persists because:
7
Sickle-cell trait in humans is a classic example of ____________________.
8
A person with sickle cell trait, having one S allele and one normal, will be resistant to malaria and eventually develop sickle cell anemia.
9
For a woman living in the United States, which genotype would be the most advantageous to have?
10
For a woman living in central Africa, which genotype would be the most advantageous to have?
11
Sickle cell anemia is caused by a change in the amino acid sequence of the two beta chains in the hemoglobin molecule. How many amino acids have been changed in each beta chain, compared to normal hemoglobin?
12
Mating with relatives is called
13
The random loss of alleles in a population is called
14
Which of the following factors is most likely to contribute to gene flow between populations?
15
In the Hardy-Weinberg equation, the term 2pq represents the frequency of the
16
A scientist measures the circumference of acorns in a population of oak trees and discovers that the most common circumference is 2 cm. What would you expect the most common circumference[s] to be after 10 generations of stabilizing selection?
17
Refer to question 16, but this time answer what you would expect after 10 generations of disruptive selection.
18
Refer to question 16, but this time answer what you would expect after 10 generations of directional selection.
19
How common is genetic polymorphism in natural populations?
20
What percent of a typical human's loci are heterozygous?
21
Why is genetic polymorphism important to evolution?
22
In a population of wildflowers, the frequency of the allele for red flowers was 0.8. What was the frequency of the white allele, the only other allele for flower color?
23
Referring to question 22, what is the frequency of homozygous red flower plants in the population?
24
Referring to question 22, what is the frequency of homozygous white flower plants in the population?
25
Referring to question 22, what is the frequency of plants in the population that are heterozygous for flower color?
26
What is the ultimate source of genetic variability?
27
The movement of new genes into a population as a result of migration or hybridization is called
28
A virus killed most of the seals in the North Sea [e.g., dropped the population from 8000 to 800]. In an effort to help preserve the species, scientists caught 20 seals and used them to start a new population in the northwest Pacific Ocean. Which of the following factors would most likely have the least impact in this new population?
29
Inbreeding
30
Evolution by natural selection works best on a population having no variation.
31
According to Darwin's theory of evolution, evolution occurs through natural selection operating on populations in ecosystems.
32
The theory of population genetics and how evolution occurs includes all but which one of the following
33
Using the Hardy-Weinberg Principle, which expression represents the frequency of the homozygous recessive genotype?
34
Which statement most accurately reflects what population geneticists refer to as "fitness"?
35
Mutation is a relatively unimportant source of variation and is not the foundation for evolution.
36
The fact that the majority of human newborns weigh around 7 pounds is reflective of ________________.
37
Organisms that are least likely to experience extinction over the long term are most likely to be found in _______________.
38
Which one of the following would cause the Hardy-Weinberg principle to be inaccurate?
39
The lack of allele variation in the nothern elephant seal population is an example of:
40
Which one of the following populations would most quickly lead to two groups with few shared traits?
41
Mutations tend to have little effect on the allele frequency in a population.
42
The effects of genetic drift are most apparent in small populations.
43
Inbreeding increases the proportion of homozygous individuals in a population.