10.35 人口规模和遗传漂流 -- -- 高级
Section outline
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What does it mean to drift?
::漂移意味着什么?To drift means to be carried along by currents of air or . A snowdrift is a deposit of snow sculpted by wind into a mound during a snowstorm. With a genetic drift, allele frequencies can change as they drift just by random chance.
::漂流的方式是随空气潮流或 。 雪流是雪雪堆,在暴风中被风雕塑成一个丘陵。 如果遗传漂移,所有频率都会随着随机漂移而变化。Causes of Microevolution: Population Size and Genetic Drift
::微进原因:人口规模和遗传漂流Recall that the third requirement for Hardy-Weinberg equilibrium is a very large population size . This is because chance variations in allele frequencies are minimal in large . In small populations, random variations in allele frequencies can significantly influence the “survival” of any , regardless of its adaptive value. Random changes in allele frequencies in small populations are known as genetic drift . Many biologists think that genetic drift is a major cause of microevolution .
::回顾Hardy-Weinberg均衡的第三个要求是人口规模非常大,这是因为在大范围内,所有不同频率的概率变化极小。在小的人群中,所有不同频率的随机变化都会对任何人群的“生存”产生重大影响,不管其适应价值如何。小人群中所有频率的随机变化被称为基因漂移。许多生物学家认为遗传漂移是微进的一个主要原因。You see the effects of chance when you flip a coin. If you flipped a penny 4 times, you would not be too surprised if it came up heads 4 times and tails not at all. If you tossed it 100 times, you would be very surprised if the results were 100 heads and no tails. The larger the “population” of coin tosses, the lower the effects of chance, and the closer the results should match the expected 50-50 ratio. The same is true for populations. If we imagine a rabbit population with a very small gene pool of just 2 B alleles and 2 b alleles, it is not difficult to understand that occasionally, chance alone would result in no albino offspring (only genotypes BB or Bb ) – or even no brown offspring (only genotype bb ). However, a gene pool of 100 B alleles and 100 b alleles would be very unlikely to produce a generation of offspring entirely lacking one allele or the other, despite having identical initial allele frequencies of 0.5.
::当你翻硬币时,你会看到机会的效果。如果你翻了一个硬币时,如果翻了一个硬币,你就不会感到太惊讶。如果你翻了一个硬币,如果你翻了一个硬币,你就不会感到太惊讶。如果你翻了4次头和4次不是尾巴。如果你翻了100次,如果结果是100次,你就会感到非常惊讶。硬币的“人口”抛掷的“人口”越大,机会就越低,结果就越接近预期的50-50比率。对于人口来说,同样的情况也是如此。如果我们想象兔子人口拥有一个很小的基因库,只有2个贝勒斯和2个贝勒斯,那么不难理解的是,仅仅机会本身就会导致没有白蚁后代(只有基因型BB或Bb) — — 甚至没有棕色后代(只有基因型bbb) — — 。 然而,一个100个贝勒斯和100个贝勒斯的基因库不太可能产生一代完全缺乏一个或另一个异子的后代,尽管最初的所有基因频率为0.5。Because chance governs and , random variations can influence allele frequencies, especially for small populations. Note that these chance variations can increase the frequency of alleles which have no adaptive advantages or disadvantages – or decrease the frequency of alleles which do have adaptive value. Genetic drift can result in extinction of an allele or an entire population – or rapid evolution ( Figure ). Two sets of circumstances can create small populations for which genetic drift can have major consequences: the bottleneck effect and the founder effect .
::由于机会的支配,随机变化会影响所有频率,特别是小人群的频率。 请注意,这些机会的变化会增加没有适应优势或劣势的异种的频率 — — 或减少具有适应价值的异种的频率。 遗传漂移可能导致异类或整个人口的灭绝 — — 或快速演变(图 ) 。 两组环境可以创造小群,基因漂移可能会对其产生重大影响:瓶颈效应和创建效应。Computer models show that the effect of small population size on allele frequencies is a significant increase in variation due to chance. Each line depicts a different allele. In the small population (above), most of the 20 alleles beginning at frequencies of 0.5 become either “fixed” (frequency = 1.0) or extinct (frequency = 0) within 5 – 25 generations. In the larger population (below), only one pair of alleles shows fixation/extinction – and that occurs only after 45 generations. Note that these variations are independent of natural selection; they do not necessary help fit the organism to its environment.
::计算机模型显示,小人口规模对所有频率的影响是因偶然性而显著增加的变异。 每条线都描绘了不同的变异。 在小人口(以上)中,从0.5频率开始的20个变异物大多在5至25代之内“固定 ” ( 频率=1.0 ) 或绝种( 频率=0 ) 。 在较大人口( 下面)中,只有一对变异物显示固定/扩展 — — 并且只在45代之后发生。 注意这些变异与自然选择无关,它们没有必要帮助有机体适应其环境。The Bottleneck Effect
::瓶颈效应Natural catastrophes such as earthquakes, floods, fires, or droughts can drastically reduce population size – usually without respect to allele frequencies. As a result of the disaster, some alleles may be lost entirely and others may be present in frequencies which differ from those of the original population. The smaller population is then subject to genetic drift, which may further reduce diversity within the population. The loss of diversity resulting from a drastic reduction in population size and subsequent genetic drift is the bottleneck effect ( Figure ). Much of our concern for endangered derives from our understanding of the way in which small population size can reduce diversity by increasing genetic drift. We will look at two examples of the bottleneck effect – one caused by humans and the other probably experienced by our human ancestors .
::地震、洪水、大火或干旱等自然灾难可能大幅缩小人口规模 — — 通常不尊重所有频率。 由于这场灾难,某些异类可能完全丧失,而另一些则可能存在于与原始人口不同的频率中。 然后,人口较少会受到基因漂移的影响,这可能会进一步降低人口的多样性。 人口数量急剧减少以及随后遗传漂移造成的多样性的丧失是瓶颈效应(Figure ) 。 我们对濒危物种的担忧主要来自我们对人口规模小如何通过增加基因漂移减少多样性的理解。 我们将审视两个瓶颈效应的例子 — 一个是人类造成的,另一个可能是人类祖先经历的。A random but major catastrophe which causes a sudden, severe reduction in population size can lead to a bottleneck effect. The reduction in gene pool size - and often diversity - leaves populations subject to genetic drift; chance variations can cause extinction – or accelerated evolution leading to recovery. Even if recovery occurs, genetic diversity remains low.
::随机但大灾难导致人口数量突然严重减少,这可能导致瓶颈效应。 基因库规模的缩小 — — 通常还有多样性 — — 使人口受到基因漂移的影响;机会的变化可能导致灭绝 — — 或加速导致复苏的演变。 即使复苏已经发生,遗传多样性仍然很低。During the 19 th century, overhunting reduced the worldwide population of Northern Elephant Seals ( Figure ) to fewer than 100 individuals. Because an alpha bull typically mates with a “harem” of 30-100 females, it is possible that just a single male fathered all the seals which exist today! After legal protection, their numbers have rebounded to 100,000. However, the effects of the past bottleneck - significant loss of genetic variability – remain. Reduced genetic diversity means that today’s seals are more susceptible to disease and weather . Effects of genetic drift on the gene pool may have contributed to the loss of 80% of pups during the El Nino year of 1997-98.
::19世纪,过度狩猎将全世界北大象海豹(Figure ) ( Northern Elphant Seals)的人口减少到不到100人。 由于一只阿尔法公牛通常配有30-100名女性的“海豹 ” , 有可能只有一位男性生了今天存在的所有海豹!在法律保护之后,它们的数量已经回升到10万。 然而,过去的瓶颈 — — 遗传变异性的巨大损失 — — 的影响依然存在。 基因多样性的减少意味着今天的海豹更容易受到疾病和天气的影响。 基因漂移对基因库的影响可能在1997-98年的厄尔尼诺年导致80%的幼崽死亡。The Northern Elephant Seal population fell to fewer than 100 individuals due to overhunting during the 19 th century. Although their numbers have recovered, the bottleneck effect of reduced genetic variation limits their potential to adapt to future environmental changes.
::北大象海豹人口由于19世纪的过度捕猎而下降到不到100人。 虽然数量已经恢复,但基因变异的减少所带来的瓶颈效应限制了他们适应未来环境变化的潜力。Although the exact cause is unknown, a bottleneck effect for South African Cheetahs ( Figure ) during the last ice age, about 10,000 years ago, has apparently led to extremely low genetic variability. among cheetahs has been compared to that of highly inbred varieties of mice; skin grafts between unrelated individuals are not rejected. These also suffer from low counts. Like many endangered species, cheetahs are threatened not only by habitat loss , but also by reduced genetic diversity, which reduces their potential to adapt to changing environments.
::虽然确切原因不明,但南非Cheetahs(Figure)在大约一万年前的上一次冰河时期的瓶颈效应显然导致了极低的遗传变异性。 在豹类中,与高度植入的老鼠品种相比,其基因变异性非常低;不相干的个人之间的皮肤移植并没有被拒绝,它们也存在低数量的问题。与许多濒危物种一样,猎类不仅受到生境丧失的威胁,而且受到基因多样性减少的威胁,这降低了它们适应变化环境的潜力。Although the cause is unknown, South African Cheetahs apparently experienced a population bottleneck effect 10,000 years ago. Their current genetic uniformity is remarkable; skin grafts between even unrelated individuals do not elicit rejection responses.
::虽然原因不明,但南非的Cheetahs在一万年前显然经历了人口瓶颈效应。 其目前的基因统一性是惊人的;即使是不相干的个人之间的皮肤移植也不会引起拒绝反应。We humans may have experienced a population bottleneck between 70,000 and 75,000 years ago, when supervolcano Mount Toba exploded with category 8 (“megacolossal!”) force in Sumatra. According to anthropologist Stanley Ambrose’s theory , global temperature dropped as much as 5 degrees Celsius for several years, possibly leading to an ice age. Ambrose believes that the environmental effects (“six years of relentless volcanic winter”) reduced the total to less than 10,000. Isolated individual populations would have experienced genetic drift and rapid evolution or extinction.
::我们人类可能经历了70,000到75,000年前的人口瓶颈,当时超级火山峰托巴在苏门答腊(Sumatra)爆发了8类“超大巨石! ” 。 根据人类学家斯坦利·安布罗斯( Stanley Ambrose)的理论,几年来全球气温下降高达5摄氏度,可能导致冰河时代。 安布罗斯(Ambrose)认为环境效应(“6年无情的火山冬天 ” ) 将总数减少到不到10,000人。 孤立的个体人口本会经历基因漂移和快速演变或灭绝。The Founder Effect
::创建者效果Whereas a drastic reduction in population size causes the bottleneck effect, a form of population expansion leads to the founder effect. If a small group of individuals (the founders) breaks off from a larger population to colonize a distant area, they will probably carry with them only a limited amount of the genetic diversity of the original population ( Figure ). For this reason, the new population they establish may differ significantly in genotype and phenotype . Inevitably, it will also be small and therefore subject to genetic drift.
::人口规模的急剧缩小导致了瓶颈效应,而人口扩张的形式则导致了创建效应。 如果一小撮人(创始人)从人口较多的人群中分裂出来,到远方地区殖民,那么他们可能只能携带有限的原始人口的遗传多样性(图 ) 。 因此,他们建立的新人口在基因类型和苯型上可能有很大差异。 不可避免的是,它也会是很小的,因此会受到基因漂移的影响。In this diagram, the red squares and blue dots represent individuals with different alleles. Small groups of individuals (see the right three circles) which leave a larger population (left) to colonize a new area carry with them smaller gene pools, with allele frequencies which may differ significantly from that of the parent population – due to chance. The effects of chance in these new populations become more important; genetic drift may result in extinction or rapid evolution.
::在本图中,红方和蓝点代表了不同异类的个人。 少数群体(右三圈 ) ( 右三圈 ) , 留下较大人口(左三圈 ) 殖民新区域的人携带着较小的基因库, 其异端频率可能与母群有很大差异 — — 偶然性。 机会效应在这些新人群中变得更加重要;基因漂移可能导致灭绝或快速演变。On newly formed islands, such as the Galapagos, Hawaii, and more recently Surtsey, Iceland, founder populations are often the only source of life on the island. Many founder populations probably become extinct, but others evolve rapidly, due to genetic drift. Some may diverge rapidly to occupy many available ecological niches – a process known as adaptive radiation . As we have discussed in past chapters, Galapagos finches and Hawaiian honeycreepers probably each evolved from small populations of a single ancestral finch-like species (see the previous chapter on Evolutionary Theory ).
::在新形成的岛屿上,如加拉帕戈斯岛、夏威夷岛以及最近的冰岛苏尔特西岛,创始人口往往是岛上唯一的生命来源。 许多创始人口可能已经灭绝,但其他人口则由于遗传漂移而迅速演变。 一些人可能迅速分裂以占据许多可用的生态位置 — — 一个被称为适应性辐射的过程。 正如我们在前几章中所讨论的那样,加拉帕戈斯鱼翅和夏威夷蜂巢可能都是从一个类似单一祖先的雀巢物种的小种群中演变而来(见前一章“进化论 ” ) 。Historically and even today, human populations have experienced founder effects. In some cases, and colonization are the cause. Quebec was founded by a group of no more than 2,600 people, ancestors of today’s more than 7 million Quebecois, who show remarkable genetic similarity and a number of heritable diseases, well studied by geneticists.
::历史上甚至今天,人类都经历了创世效应。 在某些情况下,殖民化是其原因。 魁北克是由不超过2,600人组成的群体创立的,他们是当今700多万魁北克人的祖先,他们表现出非凡的遗传相似性和一些遗传性疾病,基因学家对此进行了很好的研究。Cultural isolation, as well as colonization, can result in founder effects. Amish populations in the United States have grown from an initial group of about 200 immigrants, dating back to the mid-1700s. Because they have remained culturally and reproductively isolated from non-Amish Americans, they show considerable uniformity. The Amish today are often studied for their genetic uniformity, as well as certain recessive conditions. Geneticists believe that just one or two of the initial 200 Amish carried a recessive allele for Ellis-van Creveld syndrome (short limbs, extra fingers, and heart anomalies), yet through genetic drift, the isolated Amish population now has the highest incidence of this syndrome in the world ( Figure ).
::美国的阿米什人口从最初的大约200名移民群体中增长到1700年代中期。 因为他们在文化上和生殖上仍然与非美籍美国人隔离开来,他们表现出相当的统一性。 如今,阿米什人常常被研究其遗传上的统一性以及某些休眠条件。 遗传学家认为,最初200名阿米什人中只有一两个携带了埃利斯-范克雷弗尔德综合症(短肢、手指外和心脏异常)的休眠通灵,然而,通过基因漂流,孤立的阿米什人现在的这种综合症发病率是世界上最高的(图 ) 。Polydactyly (extra digits) and short limbs are characteristics of Ellis-van Creveld syndrome, a genetic disease which is rare worldwide but more common in the Amish population. Because the Amish population began with just 200 immigrants and has remained isolated, their high incidence of this syndrome may be a result of the founder effect.
::埃利斯-范·克雷维尔德综合症(Ellis-van Creveld)和短肢具有多种作用(超数字)和短肢特征,这是一种全球罕见的遗传疾病,但在阿米什人中更为常见。 因为阿米什人最初只有200名移民,并且一直与世隔绝,因此他们患上这种综合症的高发率可能是其起源效应的结果。Summary
::摘要-
In small populations, random variations in allele frequencies can significantly influence the “survival” of any allele, regardless of its adaptive value; this phenomenon is genetic drift.
::在人口较少的人群中,不同频率的随机变化会大大影响任何不同频率的“生存”,不管其适应价值如何;这一现象是遗传漂移。 -
Genetic drift can result in the extinction of an allele or an entire population.
::遗传漂移可导致异族或整个人口的灭绝。 -
Alternatively, genetic drift can lead to the rapid evolution of a population.
::或者,基因漂移可导致人口迅速演变。 -
In the bottleneck effect, a catastrophe, disease, or overhunting can dramatically reduce a population’s size and genetic variation, increasing its susceptibility to the effects of genetic drift.
::在瓶颈效应中,灾难、疾病或过度捕猎可大幅降低人口规模和基因变异,增加其受基因漂移影响的可能性。 -
In the founder effect, a small group leaves a larger population to colonize a new area. Again, genetic drift may lead to loss of genetic diversity, extinction, or rapid evolution.
::基因的转移也可能导致基因多样性的丧失、灭绝或迅速的演变。 -
Genetic drift leads to evolution in populations of small size, but results are mostly due to chance.
::遗传漂移导致人口规模小,但结果主要是偶然的。
Review
::回顾-
Describe three possible effects of genetic drift on populations and/or specific alleles.
::描述遗传漂移对人口和(或)特定分布物的三种可能的影响。 -
Why do we consider Northern Elephant seals endangered, even though their population has risen to 100,000 individuals?
::我们为什么认为北大象海豹濒临灭绝,
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In small populations, random variations in allele frequencies can significantly influence the “survival” of any allele, regardless of its adaptive value; this phenomenon is genetic drift.
