题目
When would populations of polar bears drop over the past million years?()A. When polar bears were forced back to the land-based habitats of black bears.B. After the initial divergence between polar and brown bears.C. During the warm periods.D. During a period between 800,000 and 600,000 years ago.
When would populations of polar bears drop over the past million years?()
A. When polar bears were forced back to the land-based habitats of black bears.
B. After the initial divergence between polar and brown bears.
C. During the warm periods.
D. During a period between 800,000 and 600,000 years ago.
题目解答
答案
C. During the warm periods.
解析
本题考查北极熊种群数量变化与环境的关系，核心在于理解气候变化对北极熊栖息地的影响。北极熊依赖海冰捕猎，温暖时期（如间冰期）海冰减少，导致栖息地缩小，食物获取困难，种群数量下降。需排除与黑熊无关、时间点不明确或不符合气候背景的选项。
选项分析
选项A
北极熊与黑熊无直接演化关系，北极熊起源于棕熊。即使栖息地改变，未直接说明种群数量下降的原因，故排除。
选项B
初始分歧后是北极熊适应北极环境的开始，但未明确此时种群数量是否下降，缺乏直接关联，排除。
选项C
温暖时期（如间冰期）海冰减少，北极熊捕猎困难，栖息地缩小，直接导致种群数量下降，符合题意。
选项D
80万至60万年前可能处于冰期，海冰较多，利于北极熊生存，与种群下降无关，排除。