Essay: Population bottleneck
A population bottleneck is a drastic reduction in a population’s effective size (number of individuals that effectively reproduce) (Nei & Chakraborty, 1975). Disruptive events, such as overhunting (e.g., Gautschi et al., 2003; Weber et al., 1999) and habitat destruction (e.g., Bellinger et al., 2003), can lead to a population bottleneck. Only a small and random assortment of individuals survives the bottleneck event (Figure 1A), randomly reconfiguring the frequencies of alleles (genetic variants) in the population (Chakraborty & Kimmel, 2001). In such a scenario, some alleles may decrease in frequency or even entirely disappear, significantly reducing genetic variation (e.g., Weber et al., 2000).
Following a bottleneck event, the reduced effective population size may lead to an even greater loss of genetic variation. As long as the effective population size remains small, the relative strength of genetic drift (random changes in frequencies of alleles over generations) increases, while that of natural selection (changes in frequencies of alleles based on the differential fitness of individuals) decreases (Figure 1B) (Pray, 2008). Genetic drift can further reduce genetic variation by causing the random loss of even more alleles. Also, a reduced effective population size increases homozygosity because of inbreeding (higher chance of mating between close relatives) (Nei & Chakraborty, 1975). If close relatives share deleterious alleles, these will rapidly spread in the population and cause a reduction in its overall fitness (inbreeding depression) (Layman & Busch, 2018).
Low genetic variation and its consequences can drive the extinction of a population that has faced a bottleneck. Even if the population recovers its original effective size, low genetic variation often remains. An example of this long-term effect is the overhunting of the northern elephant seals (Mirounga angustirostris) in the 19th century (pictured in Figure 1A). During this period, humans extensively hunted the seals to the point that only as few as 20 individuals survived. Since then, the population has recovered to an estimated size of 225,000 individuals. However, genetic variation in the northern elephant seals is still extremely low (Bonnell & Selander, 1974; Hoelzel et al., 1993; Weber et al., 2000; Sanvito et al., 2013).
References
Bellinger, M. R., Johnson, J. A., Toepfer, J., & Dunn, P. (2003). Loss of genetic variation in greater prairie chickens following a population bottleneck in Wisconsin, USA. Conservation Biology, 17(3), 717-724.
Bonnell, M. L., & Selander, R. K. (1974). Elephant seals: genetic variation and near extinction. Science, 184(4139), 908-909.
Chakraborty, R., & Kimmel, M. (2001). Bottleneck Effect. Encyclopedia of Genetics, 233–235.
Gautschi, B., Müller, J. P., Schmid, B., & Shykoff, J. A. (2003). Effective number of breeders and maintenance of genetic diversity in the captive bearded vulture population. Heredity, 91(1), 9-16.
Hoelzel, A. R., Halley, J., O'Brien, S. J., Campagna, C., Arnborm, T., Le Boeuf, B., Ralls, K. & Dover, G. A. (1993). Elephant seal genetic variation and the use of simulation models to investigate historical population bottlenecks. Journal of Heredity, 84(6), 443-449.
Layman, N. C., & Busch, J. W. (2018). Bottlenecks and inbreeding depression in autotetraploids. Evolution, 72(10), 2025-2037.
Nei, M., Maruyama, T., & Chakraborty, R. (1975). The bottleneck effect and genetic variability in populations. Evolution, 1-10.
Pray, L. (2008) Genetic drift: bottleneck effect and the case of the bearded vulture. Nature Education 1(1),61
Sanvito, S., Meza, A. D., Schramm, Y., Hernández, P. C., Garrigos, Y. E., & Galimberti, F. (2013). Isolation and cross-species amplification of novel microsatellite loci in a charismatic marine mammal species, the northern elephant seal (Mirounga angustirostris). Conservation Genetics Resources, 5(1), 93-96.
Weber, D., Stewart, B. S., Garza, J. C., & Lehman, N. (2000). An empirical genetic assessment of the severity of the northern elephant seal population bottleneck. Current Biology, 10(20), 1287-1290.
Development
Day 1
Ideias de tema:
1º - Exaptação ***
2º - Efeito gargalo
3º - Evolução do DNA mitocondrial **
Exaptação: Outline (linhas gerais)
Parágrafo 1: Adaptação x Exaptação; O que é exaptação; contextualização
Parágrafo 2: Explicação detalhada e foco exemplos
Parágrafo 3: Como identificar exaptação a partir da reconstrução histórica (Figura 1*)
*Árvore filogenética com exemplo de exaptação
** Muito extenso. Pode ser voltado para uma questão específica. Como origem do DNA mitocondrial.
***Não é contemplado pela teoria evolutiva. É a definição de um processo histórico. Uma alternativa é adaptação.
Day 2
Tema selecionado: Efeito gargalo
Outline geral
Parágrafo 1: Introdução e contextualização - O que é efeito gargalo?; contextualização: quando ocorre?
Parágrafo 2: Desenvolvimento -
Parágrafo 3: Consequências - quais são as consequências na frequência alélica da população?
Outline detalhado
Parágrafo 1:
\\sentença tópico\\: Efeito gargalo é caracterizado pela redução significativa da variabilidade genética populacional (Nei et al. 1975).
\\sentença\\:O efeito gargalo ocorre subsequentemente a eventos drásticos naturais () ou antrópicos () que resultam na redução do tamanho populacional efetivo (i.e., número de indivíduos que efetivamente se reproduzem).
\\sentença\\: Essa redução modifica aleatoriamente as frequências alélicas das populações originais (Figura 1*).
*Figura representando a amostragem aleatória de indivíduos
Parágrafo 2:
\\sentença\\: A redução do tamanho populacional torna a população menos suscetível aos efeitos da seleção natural, tornando os efeitos da deriva genética mais pronunciados.
\\sentença\\: como a deriva se torna mais pronunciada?
\\sentença\\: como esses fatores afetam a frequência alélica?
Parágrafo 3:
\\sentença\\: Implicações evolutivas e para conservação
\\sentença\\: desenvolvimento
Ensaio:
Parágrafo 1
A population bottleneck is a drastic reduction in a population’s effective size (number of individuals that effectively reproduce).
The bottleneck may be caused by events such as environmental disasters (e.g., earthquakes, floods and fires), overhunting, and habitat destruction due to human activities (e.g., logging and mining).
A bottleneck event can decimate a population, leaving behind only a small assortment of individuals that survive at random (Figure 1).
In such a scenario, some alleles (gene variants) may decrease in frequency or even entirely disappear, significantly reducing genetic variability.
For as long as the effective population size remains small, genetic drift (random changes in frequencies of alleles over generations) can further reduce genetic variability by causing the loss of even more alleles (Figure 1).
Parágrafo 2
For as long as the effective population size remains small, genetic drift (random changes in frequencies of alleles over generations) can further reduce genetic variability by causing the loss of even more alleles.
The reduced effective population size also leads to an increased likelihood of breeding between relatives (inbreeding).
This means that the frequency of homozygotes will necessarily increase through time. If relatives share deleterious alleles, these will rapidly spread in the population, causing a reduction in the overall fitness (inbreeding depression).
- endogamia reforça a perda da variabilidade
The dramatic loss of genetic variability means that there is less variation for natural selection to act upon.
Thus, populations that have undergone a bottleneck event are more vulnerable because they might not be able to respond to environmental changes.
However, the allele will quickly decline in frequency if homozygotes are comparably much less fit or unviable.
Thus, even though inbreeding depression can lead to extinction, population bottlenecks may have a purifying effect by purging populations of deleterious alleles when their negative effects are strong enough.
Parágrafo 3
Due to the profound effects that a bottleneck event has on populations, the bottleneck effect is an important concept to conservation and evolutionary biology.
The dramatic loss of genetic variability means that there is less variation for natural selection to act upon.
Thus, populations that have undergone a bottleneck event are more vulnerable because they might not be able to respond to environmental changes.
//For example, human actions can create severe bottlenecks for other species.
The habitat loss of a population of greater prairie chickens (Tympanuchus cupido) during the 19th and 20th centuries in Illinois led to a drastic reduction in population size.
Surviving birds had low levels of genetic variation and less than 50% hatched eggs compared with much higher hatching rates of the larger populations.
These data suggest that genetic drift led to a loss of genetic variability and an increase in the frequency of harmful alleles.//
Outra versão:
Efeito gargalo se refere a um evento evolutivo caracterizado pela redução extrema da diversidade genética de uma dada população biológica (i.e. grupo de organismos de uma mesma espécie) como resultado de uma perda drástica de grande parte de seus integrantes (REFERENCE). Tal diminuição drástica no número de indivíduos pode ocorrer em função de eventos de origem natural (como terremotos e enchentes) ou mesmo causados pelo homem (como caça) (REFERENCE). A redução rápida da população reconfigura as frequências dos genes transmitidos na população posterior ao evento de maneira aleatória (REFERENCE) (Figura 1*).
Após sofrer o efeito gargalo, a população pode tanto se recuperar mantendo uma baixa variabilidade genética quanto ser extinta (REFERENCE). Tais resultados são gerados pelas influências antagônicas dos dois principais mecanismos evolutivos que geram mudanças nas frequências genéticas das populações: a seleção natural (i.e., transmissão de variantes genéticos baseada em seus fitness diferenciados) e a deriva genética (i.e. transmissão aleatória de variantes genéticos) (REFERENCE). Seguindo a redução do tamanho populacional, a perda da variação genética implica que a população sobrevivente se torna menos sujeita aos efeitos da seleção natural (REFERENCE), pois […] as variantes genéticas podem possuir menor fitness?. Em contraste, a magnitude dos efeitos da deriva genética sobre a população sobrevivente é ampliada, e eventos puramente estocásticos podem rapidamente determinar a manutenção ou a extinção das variantes genéticas remanescentes (REFERENCE).
Investigar as assinaturas históricas e atuais dos efeitos fundadores nas populações auxilia grandemente a nossa compreensão sobre a origem, manutenção e extinção dos organismos ao longo do tempo (REFERENCE). Ainda, investigar as causas e consequências dos efeitos fundadores permite um maior entendimento dos níveis dos impactos humanos sobre as populações naturais, auxiliando até mesmo em estratégias mais precisas de conservação das espécies (REFERENCE).
Day 3
A population bottleneck is a drastic reduction in a population’s effective size (number of individuals that effectively reproduce) (Nei & Chakraborty, 1975).
Disruptive events, such as overhunting (e.g., Gautschi et al., 2003; Weber et al., 1999) and habitat destruction (e.g., Bellinger et al., 2003), can lead to a population bottleneck.
A bottleneck event decimates a population, leaving behind only a small assortment of individuals that survive at random (Figure 1).
Only a small and random assortment of individuals survives the bottleneck event (Figure 1), which randomly reconfigures the frequencies of alleles (genetic variants) in the population.
In such a scenario, some alleles (gene variants) may decrease in frequency or even entirely disappear, significantly reducing genetic variation.
For as long as the effective population size remains small, genetic drift (random changes in frequencies of alleles over generations) can further reduce genetic variation by causing the random loss of even more alleles.
The consequences of population bottlenecks are many.
The bottleneck event is followed by a rapid variation in the relative strength of genetic drift (random changes in frequencies of alleles over generations) and natural selection (changes in frequencies of alleles based on the differential fitness of individuals).
One of the most direct impacts is an even greater loss of genetic variability.
Following a bottlenek event
As long as the effective population size remains small, genetic drift can further reduce genetic variation by causing the random loss of even more alleles.
Also, a reduced effective population size increases homozygosity because of inbreeding (higher chance of mating between close relatives).
If close relatives share deleterious alleles, these will rapidly spread in the population and cause reduction in the overall fitness (inbreeding depression).
A dramatic loss of genetic variation means that there is less raw material for natural selection to act upon.
Thus, populations that have undergone a bottleneck event are more vulnerable because they might not be able to respond to environmental changes.
Low genetic variation and inbreeding depression can drive the extinction of a population that have undergone a bottleneck.
Even if the population recover its original effective size, low genetic variation often remains.
The example of northern elephant seals clearly illustrates this long-term effect.
In the 19th century, humans extensively hunted northern elephant seals to the point that only as few as 20 individuals survived.
Since then, the population have increased its size to around 30,000 individuals.
However, genetic variation in the population is still extremely low compared to southern elephant seals, which didn't undergo a population bottleneck (Weber et al., 2000).