The lack of allele variation in the northern elephant seal population is an example of

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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 A) gene flow B) genetic drift C) mutation D) inbreeding

2

In relation to natural selection, evolution is the A) process B) outcome C) mechanism D) purpose

3

Moto Kimura's theory that opposed natural selection was the A) natural theory B) nearly neutral theory C) neutral theory D) adaptive theory

4

Scientists generally agree that heterozygous advantage is A) rare B) frequent C) pervasive D) none of the above

5

The occurrence of large or small beak sizes among seed crackers in the absence of medium-sized beaks is an example of A) directional selection B) stabilizing selection C) disruptive selection D) none of the above

6

Even though sickle-cell anemia is usually fatal to homozygous individuals, the disease persists because: A) gene therapy has alleviated the condition B) the disease is carried on a dominant allele C) individuals with one allele for sickle-cell anemia are resistant to malaria D) a combination of all of the above E) none of the above

7

Sickle-cell trait in humans is a classic example of ____________________. A) how mutations can lead only to tragic outcomes B) why outbreeding is important C) the superior fitness seen in heterozygotes D) how every organism is an integrated gene complex

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. A) True B) False

9

For a woman living in the United States, which genotype would be the most advantageous to have? A) homozygous for the sickle cell allele B) heterozygous for the sickle cell allele C) homozygous for the normal hemoglobin allele D) it doesn't matter; all are equally advantageous

10

For a woman living in central Africa, which genotype would be the most advantageous to have? A) homozygous for the sickle cell allele B) heterozygous for the sickle cell allele C) homozygous for the normal hemoglobin allele D) it doesn't matter; all are equally advantageous

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? A) 1 B) 5 C) 10 D) hundreds E) thousands

12

Mating with relatives is called A) inbreeding B) outcrossing C) random mating D) clines E) polymorphic

13

The random loss of alleles in a population is called A) mutation B) selection C) genetic drift D) electrophoresis E) gene flow

14

Which of the following factors is most likely to contribute to gene flow between populations? A) random mating B) migration C) mutation D) genetic drift E) inbreeding

15

In the Hardy-Weinberg equation, the term 2pq represents the frequency of the A) dominant homozygotes B) recessive homozygotes C) dominant allele D) recessive allele E) heterozygotes

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? A) 2 cm B) greater than 2 cm or less than 2 cm C) greater than 2 cm and less than 2 cm D) can't tell from the information given

17

Refer to question 16, but this time answer what you would expect after 10 generations of disruptive selection. A) 2 cm B) greater than 2 cm or less than 2 cm C) greater than 2 cm and less than 2 cm D) can't tell from the information given

18

Refer to question 16, but this time answer what you would expect after 10 generations of directional selection. A) 2 cm B) greater than 2 cm or less than 2 cm C) greater than 2 cm and less than 2 cm D) can't tell from the information given

19

How common is genetic polymorphism in natural populations? A) essentially all loci are polymorphic B) essentially no loci are polymorphic C) depending on the species, all loci are polymorphic or none are D) depending on the species, more or less than half the loci are polymorphic

20

What percent of a typical human's loci are heterozygous? A) 2% B) 5% C) 37% D) 55% E) 76%

21

Why is genetic polymorphism important to evolution? A) individual variability provides the raw material for natural selection to act on B) genes cannot mutate unless they are polymorphic C) only heterozygous individuals are selected in natural populations D) the Hardy-Weinberg equilibrium is less likely to be disturbed in polymorphic populations E) none of the above; genetic polymorphism is not 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? A) 0.8 B) 0.4 C) 0.6 D) 0.2 E) 0.1

23

Referring to question 22, what is the frequency of homozygous red flower plants in the population? A) 0.04 B) 0.16 C) 0.32 D) 0.48 E) 0.64

24

Referring to question 22, what is the frequency of homozygous white flower plants in the population? A) 0.04 B) 0.16 C) 0.32 D) 0.48 E) 0.64

25

Referring to question 22, what is the frequency of plants in the population that are heterozygous for flower color? A) 0.04 B) 0.16 C) 0.32 D) 0.48 E) 0.64

26

What is the ultimate source of genetic variability? A) mutation B) migration C) genetic drift D) nonrandom mating E) selection

27

The movement of new genes into a population as a result of migration or hybridization is called A) founder principle B) selection C) gene flow D) bottleneck effect E) adaptation

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? A) founder effect B) random mating C) genetic drift D) bottleneck effect

29

Inbreeding A) increases the rate of mutation B) increases the proportion of homozygous individuals in a population C) never occurs in plants D) all of the above E) none of the above

30

Evolution by natural selection works best on a population having no variation. A) True B) False

31

According to Darwin's theory of evolution, evolution occurs through natural selection operating on populations in ecosystems. A) True B) False

32

The theory of population genetics and how evolution occurs includes all but which one of the following A) Mating must be random. B) The size of the population is small. C) There is no influx of genes from other populations. D) No genotype has selective advantage over another.

33

Using the Hardy-Weinberg Principle, which expression represents the frequency of the homozygous recessive genotype? A) p2 B) 2pq C) q2 D) q

34

Which statement most accurately reflects what population geneticists refer to as "fitness"? A) Fitness is the measure of an organism's adaptability to various habitats. B) Fitness reflects the number of mates each individual of the population selects. C) Fitness refers to the relative health of each individual in the population. D) Fitness is a measure of the contribution of a genotype to the gene pool of the next generation.

35

Mutation is a relatively unimportant source of variation and is not the foundation for evolution. A) True B) False

36

The fact that the majority of human newborns weigh around 7 pounds is reflective of ________________. A) directional selection B) stabilizing selection C) disruptive selection D) None of these is correct.

37

Organisms that are least likely to experience extinction over the long term are most likely to be found in _______________. A) areas inhabited by humans B) very stable habitats C) desert D) savanna

38

Which one of the following would cause the Hardy-Weinberg principle to be inaccurate? A) The size of the population is very large. B) Individuals mate with one another at random. C) Natural selection is present. D) There is no source of new copies of alleles from outside the population. E) None of the answers is correct.

39

The lack of allele variation in the nothern elephant seal population is an example of: A) mutations B) founder effect C) artificial selection D) bottleneck effect E) outcrossing

40

Which one of the following populations would most quickly lead to two groups with few shared traits? A) a population with disruptive selection B) a population with directional selection C) a population with stabilizing selection D) a population with no selection E) none of the above

41

Mutations tend to have little effect on the allele frequency in a population. A) True B) False

42

The effects of genetic drift are most apparent in small populations. A) True B) False

43

Inbreeding increases the proportion of homozygous individuals in a population. A) True B) False

What caused the bottleneck of the northern elephant seal population?

What is the bottleneck effect example?

What is an example of the founder effect?

What is genetic drift example?