POLAR bears have patrolled the planet’s icy regions for millions of years longer than previously thought – riding out several episodes of global warming in that time. While this suggests their future might not be so bleak, it is no guarantee they will survive the melting occurring in the polar regions today.Charlotte Lindqvist of the University at Buffalo, New York, and an international team of researchers have just completed the most comprehensive analysis yet of the polar bear genome (基因组). The team looked at DNA from 23 living polar bears and a 110,000-year-old polar bear jawbone. Aided by comparisons with the genomes of brown and black bears, they found that polar bears first emerged as a separate species between four and five million years ago. Previous studies had suggested the species didn’t diverge (分化) from brown bears until much later – perhaps just 600,000 years ago.The analysis also showed that polar and brown bears have sometimes interbred (杂交) since their initial divergence, and that populations of polar bears have been changing over the past million years in accordance with the climate. Polar bear numbers dropped during warm periods but bounced back with prolonged ice ages – most noticeably during a period of cooling between 800,000 and 600,000 years ago. The population looks set to fall again as melting pack ice forces polar bears back to the land-based habitats (栖息地) of brown bears, where interbreeding has recently been observed. Should extensive interbreeding occur, both polar and brown bear populations could decline with the emergence of a new hybrid (杂交的) species.Lindqvist says that the analysis reveals that polar bear DNA has lost diversity as population has decreased, drifted apart and become genetically isolated, suggesting today’s bears have less adaptability to the environmental change, habitat loss, pollution and diseases they now face.Earlier this year, Frank Hailer of the Biodiversity and Climate Research Centre in Frankfurt, Germany, and colleagues estimated that polar bears diverged from brown bears 600,000 years ago – a result that itself pushed back the evolutionary record of polar bears by about 450,000 years. However, Hailer says that the new results broadly are consistent with his team’s findings, which also suggested that polar bears may have evolved much earlier than the 600,000 year mark.1. What is the closest meaning of the phrase “riding out” (Para.1)?A) Skyrocketing.B) Seesawing.C) Surviving. D) Substituting.2. What was found in the research of polar bear genome according to the passage?A) Polar bears appeared as a separate species much earlier than we had thought. B) The DNA of living polar bears is very similar to that of a 110,000-year-old polar bear.C) Polar bears have survived the icy ages and will also survive the melting today.D) Polar bears diverged from brown bears perhaps just 600,000 years ago.3. When would populations of polar bears drop over the past million years according to the passage?A) When the climate became colder.B) After the initial divergence between polar and brown bears.C) During a period between 800,000 and 600,000 years ago.D) When polar bears were forced back to the land-based habitats. 4. Why are the polar bears today less able to adapt themselves to the environment according to the passage?A) Because polar bears are isolated and cannot find new habitats.B) Because polar bears are facing more difficulties nowadays.C) Because the DNA of polar bears has lost diversity. D) Because the interbreeding with brown bears has produced a new hybrid species.5. What can be inferred from this passage?A) Brown bear populations may increase, while polar bear populations decline.B) We are not sure that polar bears will survive the melting of polar regions.C) A new hybrid species of polar bears will have more adaptability to the environmental changes. D) The polar bear populations would increase when they began to interbreed with brown bears.