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  • General

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  • 自然选择自然选择

    What you will learn
    ::你会学到什么

    • What is natural selection and how does it affect allele frequencies
      ::什么是自然选择,它是如何影响所有频率的?
    • The three ways that natural selection affects the phenotype of
      ::自然选择的三种方式影响

    Two women performing fitness exercises in a bright, spacious environment.

    What is fitness?
    ::什么是健身?

    Does this type of fitness have anything to do with natural selection? Usually not. There are countless ways in which an organism can be more "fit," or better adapted to its habitat . And we probably do not know about most of these adaptations .
    ::这种类型的健康是否与自然选择有关?通常不会。生物体可以有无数种方法更适合或更适应其生境。我们可能不知道这些适应措施的大部分。

    Natural Selection
    ::自然选择自然选择

    Natural selection occurs when there are differences in fitness among members of a . As a result, some individuals pass more genes to the next generation. This causes allele frequencies to change.
    ::自然选择发生于某个成员在是否适合性方面存在差异。因此,一些人将更多的基因传给下一代,这导致所有频率发生变化。

    Sickle Cell and Natural Selection
    ::镰状细胞和自然选择

    The example of sickle-cell anemia is described in the Figure   and the Table  . It shows how natural selection can keep a harmful in a gene pool . 
    ::图和表格中描述了镰状细胞贫血的例子。它显示了自然选择如何在基因库中保持有害。

    Comparison of normal red blood cells and sickle-shaped cells illustrating sickle cell anemia.
    Sickle Cell and Natural Selection. Notice the normal-shaped red blood cell and the sickle-shaped cell.
    Genotype Phenotype Fitness
    AA 100% normal hemoglobin Somewhat reduced fitness because of no resistance to malaria
    AS Enough normal hemoglobin to prevent sickle-cell anemia Highest fitness because of resistance to malaria
    SS 100% abnormal hemoglobin, causing sickle-cell anemia Greatly reduced fitness because of sickle-cell anemia

    Here’s how natural selection can keep a harmful allele in a gene pool:
    ::自然选择可以将有害的异域保存在基因库中:

    • The allele ( S ) for sickle-cell anemia is a harmful autosomal recessive . It is caused by a in the normal allele ( A ) for hemoglobin (a on red cells).  This results in sickle-shaped red blood cells that clog tiny , causing pain and damaging organs and . 
      ::镰状细胞贫血的止血球(S)是一种有害的自动止血细胞。它是由血红蛋白(A)正常的止血球(A)导致的。这导致镰状红血细胞凝结细小,造成疼痛和损伤器官以及。
    • Malaria is a deadly disease caused by a parasite , Plasmodium . The parasites are spread by a mosquito vector . Parasites enter a host’s blood through the bite of an infected mosquito. The parasites infect the host’s red blood cells, causing symptoms such as fever, joint pain, anemia, and fatigue.
      ::疟疾是寄生虫Plasmodium引起的致命疾病。寄生虫是由蚊子媒介传播的。 寄生虫通过被感染的蚊子的咬入宿主的血液。寄生虫感染宿主的红血细胞,导致发烧、关节疼痛、贫血和疲劳等症状。
    • Heterozygotes ( AS ) with the sickle- allele are resistant to malaria. Therefore, they are more likely to survive and reproduce. This keeps the S allele in the gene pool.
      ::使用镰刀的异性螺旋藻(AS)抗疟疾。 因此,它们更有可能存活和繁殖。 这使得Selele在基因库中。

     

     

    The sickle-cell example shows that fitness depends on phenotypes. It also shows that fitness may depend on the environment. What do you think might happen if malaria was eliminated in an African population with a relatively high frequency of the S allele? How might the fitness of the different genotypes change? How might this affect the frequency of the S allele?
    ::镰状细胞的例子表明,是否健康取决于苯型。它也表明,是否健康取决于环境。如果非洲人口中的疟疾在Sallele频率相对高的非洲人口中被消灭,你认为会发生什么情况?不同的基因型的适合性会如何改变?这如何影响Salle的频率?

    Natural Selection and Polygenic Traits
    ::自然选择和多源拖累

    Sickle-cell trait is controlled by a single gene. Natural selection for polygenic traits is more complex, unless you just look at phenotypes. Three ways that natural selection can affect the phenotype of polygenic traits  includes stabilizing selection , directional selection , and disruptive selection . Use the video and interactive to learn more!
    ::镰状细胞特性受单一基因控制。 选择多种致病特性的自然比较复杂, 除非您只看pheno类型。 自然选择可以影响多致性特性的苯型的三种方式包括稳定选择、 定向选择和干扰选择。 使用视频和互动来学习更多内容 !

     

     

    1. Stabilizing selection occurs when phenotypes at both extremes of the phenotypic distribution are selected against. This narrows the range of variation. An example is human birth weight. Babies that are very large or very small at birth are less likely to survive. This keeps birth weight within a relatively narrow range.
      ::当选择对视双峰的胎儿分布,两种极端的雌性动物类型就会稳定选择。这缩小了差异范围。一个例子是人的出生体重。出生时非常大或非常小的婴儿不太可能存活下来。这把出生体重限制在相对狭窄的范围内。
    2. Directional selection occurs when one of two extreme phenotypes is selected for. This shifts the distribution toward that extreme. This is the type of natural selection that the Grants observed in the beak size of Galápagos finches.
      ::方向选择发生在选择两个极端苯型类型之一时。 这会将分布转向极端。 这是赠款在加拉帕戈斯角的嘴上观察到的自然选择类型 。
    3. Disruptive selection occurs when phenotypes in the middle of the range are selected against. This results in two overlapping phenotypes, one at each end of the distribution. An example is sexual dimorphism . This refers to differences between the phenotypes of males and females of the same . In humans, for example, males and females have different heights and body shapes.
      ::当选择范围中间的苯型时,就会出现干扰性选择。这导致两种重叠的苯型,在分布的每个端各出现一种。一个例子是性畸形。这是指相同性别的男性和女性的苯型之间的差异。例如,在人类中,男性和女性的高度和身体形状不同。

    Check your understanding!
    ::检查你的理解!

     

     

    Summary
    ::摘要

    • Natural selection occurs when there are differences in fitness among members of a population.
      ::自然选择发生于人口成员健康程度不同时。
    • S ickle-cell anemia shows  how natural selection can keep a harmful allele in a gene pool.
      ::镰状细胞贫血表明,自然选择可以将有害的异象保存在基因库中。
    • Natural selection for a polygenic trait changes the distribution of phenotypes. It may have a stabilizing, directional, or disruptive effect on the phenotype distribution.
      ::多源特性的自然选择会改变苯型的分布。它可能对苯型分布产生稳定、定向或干扰影响。

    Review
    ::回顾

    1. What is natural selection and what are its effects on allele frequencies?
      ::自然选择是什么? 自然选择对Alele频率有何影响?
    2. Describe three types of natural selection for a polygenic trait.
      ::描述三种类型的多种致病特性的自然选择。
    3. How does the recessive sickle-cell allele stay in the gene pool?
      ::休眠镰状细胞Allele怎么留在基因库里?