It is estimated that over 99 percent of all species that ever existed have become 6 Q& H( D& ]' }
extinct. What causes extinction? When a species is no longer adapted to a changed [: _( w0 ^( w) ]( `" a1 ]9 c, \
environment, it may perish. The exact causes of a species’ death vary from situation
3 x# i+ M; |; q+ @% aLine to situation. Rapid ecological change may render an environment hostile to a species.
9 |( M+ x' ]$ t" |% |For example, temperatures may change and a species may not be able to adapt. Food, e9 J4 h5 h7 v* ?6 [7 M
Resources may be affected by environmental changes, which will then cause problems- N, D8 v, F4 i: u0 \9 v# }( h
For a species requiring these resources. Other species may become better adapted to an$ e5 z- L" e% E O# U; P j
Environment, resulting in competition and, ultimately, in the death of a species.
* d( `6 j$ ^ K( U2 m The fossil record reveals that extinction has occurred throughout the history of Earth., p o4 n; b+ @6 n; Z4 t
(10) Recent analyses have also revealed that on some occasions many species became extinct
9 S/ a# E$ h7 V9 p5 i at the same time—a mass extinction. One of the best-known examples of mass extinction
7 N" h5 B" {/ m+ g% E" Q7 w+ p occurred 65 million years ago with the demise of dinosaurs and many other forms of life.- Q b7 p3 `# o, E5 ]' c) g
Perhaps the largest mass extinction was the one that occurred 225 million years ago,
3 D3 ~: b, Z7 R N+ C9 k" Q When approximately 95 percent of all species died, Mass extinctions can be caused by2 |: C8 Z* y7 i f
(15) a relatively rapid change in the environment and can be worsened by the close
" b# [5 I6 ~4 N ?0 I interrelationship of many species. If, for example, something were to happen to destroy
, W1 Y- H1 g$ M, N6 H5 e much of the plankton in the oceans, then the oxygen content of Earth would drop,& w. B! p! {- o# N- {, e* Z3 d& i+ U
affection even organisms not living in the oceans. Such a change would probably lead to a mass extinction.$ @1 c0 @+ n( K+ G
One interesting, and controversial, finding is that extinctions during the past 250
# C; @9 f" D& h; J Million years have tended to be more intense every 26 million years. This periodic
$ w# B& w. L- N2 m+ A+ Vextinction might be due to intersection of the Earth’s orbit with a cloud of comets, but
! N) c N9 p5 _. R7 f ]! Tthis theory is purely speculative. Some researchers have also speculated tat extinction1 \2 G' @6 G( ~: e( E9 Z5 p
may often be random. That is, certain species may be eliminated and others may survive( i! H$ S0 C9 E: n# {3 v8 x
(25) for no particular reason. A species’ survival may have nothing to do with its ability or6 `8 F. [9 z! v) C
inability to adapt. If so, some of evolutionary history may reflect a sequence of essentially : m5 O" K, P, f( t! x" H B
random events.
6 h# L @! Z4 k, [
1 R( E: J" w7 \6 w/ ]7 I p( R# e/ Q20. The word “it” in line 3 refers to
' O+ l5 k2 B/ u' @% _" X! ^) H (A) environment4 D: A6 {2 \% w! B' f# f- @" Y
(B) species
6 v- x. C# y7 a6 F (C) extinction
& ?2 ^8 T4 S3 N% k8 l% A3 t/ [ (D) 99 percent
' ?+ Z7 W, d9 i8 U E/ Z4 F21. The word “ultimately” in line 8 is closest in meaning to
2 r% R+ d% L! r4 a9 R (A) exceptionally
4 N* j* p& D8 S2 c (B) dramatically
7 M% p& h. M) j# D (C) eventually
5 O, |" Z; F3 P( ~( h- ]. K8 E (D) unfortunately
. {* g7 b1 G" t" y ?: `2 ]5 x( U* C* G22. What does the author say in paragraph 1 regarding most species in Earth’s history
3 B4 @' \/ E/ B# P. m (A) They have remained basically unchanged from their original forms.
" Q: g; k5 A2 [8 z6 G (B) They have been able to adapt to ecological changes.
& A( x( p; P( l# T' s( u3 N% T(C) They have caused rapid change in the environment.; ?, t2 _1 W( y2 d `9 Z0 \* }
(D) They are no longer in existence. |