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为什么雌雄的数量比例总是均衡的 – 译学馆
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为什么雌雄的数量比例总是均衡的

Why Are There As Many Males As Females?

进化就是繁殖 对吧?
Evolution is all about reproduction, right?
似乎这就是关键
It’s like…the whole point.
那么试着解释这个:雄性海象在求爱上十分不幸
So try to explain this: Male elephant seals are pretty unlucky in love.
多数雄性海象不会有机会繁殖
Most never get a chance to reproduce.
88%的交配成功率下仅有4%的雄性海象能完成繁殖
Just four percent of males can account for eighty-eight percent of mating.
剩下那百分之九十六的雄性仅仅是用了资源
The other 96 % of males just use up resources
对下一代没做出任何贡献
and don’t contribute anything to the next generation.
即使群体有少量雄性和大量雌性
Yet even though it would be a lot more efficient to have just a few males,
将会显著提高效率
and a bunch of females,
可在每个繁殖季还是有几乎等量的雌雄个体出生
roughly equal numbers of each sex are bornevery season.
这种现象普遍存在于地球的每个动物群体里
We see this in almost every animal specieson Earth.
为什么大多数动物
Why do most animals,
从鹰到犰狳 从蓝鲸到我们人类 总是有着
from eagles to armadillos to blue whales to us, make equal numbers of
等数量的雄性和雌性呢?
males and females?
雌雄个体数量平衡在自然界中非常常见
The equal ratio of sexes is so common in nature,
多数人从未惊奇为什么会这样
most people never wonder why it is that way.
我们得暂停一会儿来说明一下
And it’s worth pausing here
这里我们讨论的是生物学方面的性别
for a second to make clear we’re talking about biological sex,
而非简简单单的性别
not gender.
大家可能都知道当雄性的XY染色体和雌性的XX染色体任意组合时
You probably learned that when male (XY) andfemale (XX) chromosomes shuffle and combine,
生出雌性或雄性的可能性为50%
there’s a 50% probability of either sex.
但这只是告诉我们结果是什么
But this just tells us how it IS,
并没有说明为什么是这种方式 而不是其他方式
not why it is this way rather than some other way,
从生物学方面出发 这个才是我们真正想要去了解的
and in biology that’s really what we wantto know.
如果你认为“进化”主要是让你的基因一代一代传下去
If getting your genes from one generation to the next is the whole point of evolution,
那么你可能认为最好的方法应该是
then you’d think the best strategy would be to put a lot
让雌性产生足够多的卵细胞
of eggs into the female basket.
精子很容易形成 所以雄性能“产出”很多的精子
Sperm are really cheap to produce, so males make…
雄性动物一般
a lot of them, and animal males typically
不会承担很多照顾孩子的责任
don’t do most of the childcare.
所以 在这种情况下
So you get a situation
仅仅只需要少数的雄性来保持人口的
in which you only need a few males to keep a population churning
增长
along.
这不仅仅对于海象而言是一个理想的情况
This seems like the ideal situation not just for elephant seals,
对很多其他的动物来说 也是理想的情况
but for many animal species.
然而 海象能繁殖出相同数量雄性和雌性
Yet elephant seals produce both sexes in equal number,
这就导致出现了许多
and the result is a whole lot of mooching
没有伴侣而到处闲逛的雄性海象
mateless males.
随着进化论的发展 没有伴侣的雄性就像一个失败者
As far as evolutionary strategies go, this one seems like a loser.
这不是闲者生存
It’s not survival of the moochiest!
这是适者生存 在生物学里
It’s survival of the fittest. Fittest,
适者生存
in biology,
意味着 可以有自己的幼崽 但是96%的象海豹并不
means makes-the-most-offspring, and 96 % of elephant seal males get a big zero
适合
for fitness!
这真的是太疯狂了!
That’s totally bonkers!
那我们该如何解释这个现象呢?
So how can we explain it?
我们先设想有一群吃人的紫色群体
Let’s start with a hypothetical population of purple people-eaters,
在这些紫色群体里呢
in which out of every
有十个宝宝出生 九个雌性 一个雄性
10 purple-people-eating babies born, 9 are female and 1 is a male.
当他们想要一些可爱的吃人宝宝时
When it comes to making adorable people-eating babies,
每个雄性都将和其他的九个
every male will get to mate about
雌性结成配偶
nine times as often as every female.
如果每一次配偶的结果是产生两个宝宝
If every mating results in two new purple people-eaters,
那么在他们的下一代
then in the next generation,
一个雄性的基因将会出现在18个幼崽的体内
each male’s genes are carried on by 18 offspring,
但是平均下来每个雌性的基因
but each female–on average–gets their
只能出现在两只幼崽体内
genes into just 2.
我们来计算一下“适合”的分数
When we count up the “ fitness ” score,
在这一群体中 雄性的“适合”分数比雌性多九倍
every male in this population has 9 times
如果
the fitness of every female. If,
通过进化
through the random chance of evolution,
改变原先的配偶方式
one muncher manages to change in a way that
九个雄性对应一个雌性
it produces say, 9 male kids for every female kid,
这种配偶方式下生出的幼崽有一个大的优势
those offspring would have a huge advantage.
随着这种基因的改变
WIth this genetic change,
一个新型雄性的孙辈数量将会比没有发生基因变化的
one of these new males would produce roughly four and a half
雄性多四点五倍
times more grandchildren than a male withoutthe change.
这种新型转变发生的非常快
This new mutation will spread like gangbusters and
仅仅几代之间 紫色吃人群体中
in a few generations, you’d have more
雄性的数量就比雌性的多很多
males than females in the whole purple peopleeater population.
但基于这一点
But at this point,
这种能产生90%的雄性后代的转变不再是一种优势
the mutation causing 90 % male offspring would no longer be an advantage.
环境改变 雌性数量变得稀少 现在
The environment has changed, and now femalesare the rarer sex. Now,
任何发生基因变化的父母
any parent with a mutation that results
都会因有雌性后代而会受到优待
in extra FEMALE offspring would be favored.
接下来这种转变会持续发展 直到雌雄的比例又变回到雌性数量更多的状况
Then THAT mutation would spread until the sex ratio moves BACK towards more females.
反复这种过程 我们又将回到很多的雄性的境况中去
Repeat this whole process and we move back towards more males…
然后又是越来越多的雌性
then more females…
越来越多的雄性
then more males…
人口将会
The population would be
像一个跷跷板一样 两端的分别是雌性和雄性 直到最后
in a constant see-saw towards one sex or the other until, finally,
一种雄雌数量平衡的局面产生
a mutation arises that results in an equalratio of offspring.
那神奇的均衡性别比例
That magic 50:50 sex ratio is the only
也就是进化的平衡 而这
one that is evolutionarily stable… and that,
就是为什么
right there, is why we
这一过程会一直循环往复
see it again and again and again…
当然 50比50的比例 并不仅是指后代的数量
Of course, the 50:50 ratio isn’t just aboutnumbers of offspring.
实际上是关于父母投入了多少时间及资源到他们的后代身上
It’s really about how much time and resources parents invest in offspring.
我们能看见50比50的比率是因为无论何时父母们都投入了均衡的时间和精力
We see 50:50 sex ratios whenever parents invest the equally
到他们的雄性和雌性后代身上
in their male and female children.
如果投入是不等的 比如
If investment is unequal, like,
如果雌性需要父母付出比雄性多两倍的时间和精力
if females require twice the parental care of males,
你就会发现最终出生的雄性比雌性多两倍
you’d expect to see twice as many malesborn than females. Sure,
当然 在这种情况下雄性能遗传他们基因的机会
here each male has less chance to pass
会更少 但是他们付出的
on their genes, but they cost half as much
是雌性付出的一半 所以这才能达到均衡
as females, so it evens out.
例如 澳大利亚毛尾负鼠妈妈们
Australian brushtail possum moms, for example,
在她们女儿身上付出了更多的资源
invest more resources in their daughters,
因此 当她们资源有限的时候 她们就会有更多的雄性后代
so when these resources are scarce, they havemore male offspring.
就像在生物学中
Like everything in biology,
有很多的奇奇怪怪的例外 但却令人吃惊
there are lots of weird exceptions to this, but it’s amazing
到底50比50这种规则有多普遍
how universal the 50:50 rule actually is.
你可以观察有些物种里 有些父母
You see it in species where parents care
会照顾他们的后代 有些却不会
for their babies and where they don’t, when
当他们有很多其他的伴侣 或仅仅只有一个时
they have many partners, or just one.
你甚至可以发现
You even see it
在有些物种里面一种性别要比另一种性别死的多
in species where one sex dies a lot more than the other.
这向我们展示了自然选择
It shows us the incredible power
在塑造物种特征时的
of natural selection to shape the characteristics of
神奇力量
a population.
毕竟 如果爱是一个战场
After all, if love is a battlefield,
那么至少两性双方都是势均力敌
at least each side is evenly matched.
常怀好奇之心
Stay curious.

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大自然神奇的一面:雌雄比例总是均衡的

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