Diversity of organisms
International Baccalaureate · IB Diploma · Biology · HL Topic 6 3:27 English narration · English + 中文 subtitles burned in
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Two species look similar but their DNA comparison places them in different branches.
两个物种外观相似，但DNA比对显示它们分属不同分支。
Classification uses several kinds of evidence rather than appearance alone.
分类依据多种证据，而非仅凭外观。
A species is a biological grouping.
物种是一种生物分类单元。
A taxon is a named group in a classification.
分类群是分类系统中一个有名称的类群。
A cladogram represents a hypothesis of relationships based on shared derived characteristics or molecular evidence.
系统发育树表示基于共有衍征或分子证据提出的亲缘关系假说。
A branch point represents a common ancestor, not an individual alive today.
分支点代表共同祖先，而不是今天存活的某个个体。
A clade is a common ancestor and all its descendants.
演化支是一个共同祖先及其全部后代。
Read relationships from the branching order.
从分支顺序中读取关系。
Rotating branches at a node does not change ancestry.
在节点处旋转分支不会改变祖先关系。
A branch length represents time or change only when the diagram gives a scale.
只有当图表提供比例尺时，分支长度才代表时间或变化。
In the lab: Record character states in a matrix before drawing a tree.
在实验室中，绘制系统发育树之前，先在矩阵中记录性状状态。
Define which state is ancestral using an appropriate comparison.
使用适当的比较方法确定哪个状态是祖先状态。
Test whether a new molecular dataset supports the same grouping.
检验新的分子数据集是否支持相同的分类群分组。
Known: an alignment contains 40 sites, with 6 differences.
已知：该比对包含 40 个位点，其中存在 6 处差异。
Difference proportion equals 6 divided by 40, which is 0.15, or 15 percent.
差异比例为 6/40，即 0.15，或 15%。
This is an observed comparison, not a calibrated molecular clock, unless rate assumptions and corrections are supplied.
这是一个观察到的比较结果，除非提供了速率假设和校正，否则不能视为已校准的分子钟。
Your turn: There are 8 differences in 50 aligned sites.
轮到你了。 在 50 个比对位点中有 8 处差异。
Calculate percentage difference.
请计算百分比差异。
Pause the video and work it out.
请暂停视频，自己算一算。
Check: 8 divided by 50 is a 16 percent difference.
核对：8 ÷ 50，差异为 16%。
A modern species is not automatically the ancestor of another modern species.
现代物种并不自动成为另一现代物种的祖先。
Similarity caused by convergent evolution can mislead a classification based on one trait.
趋同进化导致的相似性可能会误导基于单一性状的分类。
Not true: Every cladogram branch length is a measurement of elapsed time.
不正确。 并非所有支序图分支长度都代表经过的时间。
Branch lengths mean time only in trees explicitly drawn with a time or change scale.
只有在明确绘制了时间或变化刻度的树中，分支长度才表示时间。
Many cladograms encode relationships only, with arbitrary lengths.
许多支序图仅编码亲缘关系，其分支长度为任意设定。
A species is a biological grouping.
物种是一个生物分类群。
A taxon is a named classification group.
分类单元是一个命名的分类组。
Read relationships from the branching order.
从分支顺序中读取关系。
Watch for this: a modern species is not automatically the ancestor of another modern species.
注意：现代物种并不自动是另一个现代物种的祖先。
An alignment has 7 differences in 25 sites.
该比对在 25 个位点中存在 7 处差异。
Find the percentage difference.
请计算差异百分比。
Say what rotating a branch does.
说明旋转分支的作用。
The result is a difference of 28 percent.
结果是差异为28%。
Record characters, compare sequences and infer the branching.
记录性状，比较序列，推断分支。
Then test the tree against another dataset before you trust it.
在相信这棵树之前，用另一组数据检验它。
Alignment A has 6 differences in 60 sites; B has 9 in 30.
比对A在60个位点中有6处差异；比对B在30个位点中有9处差异。
Compare proportions.
请比较两者的比例。
Explain why these two values alone cannot date a divergence.
解释为何仅凭这两个数值无法确定分歧时间。
A is 10 percent and B is 30 percent.
A 为 10%，B 为 30%。
Site choice, substitution-rate assumptions and repeated substitutions affect interpretation; percentage difference alone supplies no calibrated time.
位点的选择、替换速率的假设以及重复替换都会影响解释；仅凭百分比差异无法给出经过校准的时间。
Pause and check yourself. Explain each point on screen in your own words, then change one number in the last example and say which assumption must still hold.
暂停一下，自我检查：用自己的话解释屏幕上的每个要点，再改动最后一个例子中的一个数字，说明哪个假设仍然必须成立。