Welcome to Mr Aslanov’s Lessons
QUESTION-TYPE BASED TESTS
FunEnglishwithme +99894 6333230
TEST 5 - Finches on Islands
A.
Today, the quest continues. On Daphne Major —one of the most desolate of the Galápagos
Islands, an uninhabited volcanic cone where cacti and shrubs seldom grow higher than a researcher's knee
Peter and Rosemary Grant have spent more than three decades watching Darwin's finches respond to the
challenges of storms, drought and competition for food. Biologists
at Princeton University, the Grants know
and recognize many of the individual birds on the island and can trace the birds’ lineages back through time.
They have witnessed Darwin's principle in action again and again, over many generations of finches.
B.
The Grants' most dramatic insights have come from watching the evolving bill of the medium
ground finch. The plumage of this sparrow-sized bird ranges from dull brown to jet black. At first glance, it
may not seem particularly striking, but among scientists who study evolutionary biology, the medium
ground finch is a superstar. Its bill is a middling example in the array of shapes and sizes found among
Galapagos finches: heftier than
that of the small ground finch, which specializes in eating small, soft seeds,
but petite compared to that of the large ground finch, an expert at cracking and devouring big, hard seeds.
C.
When the Grants began their study in the 1970s, only two species of finch lived on Daphne Major,
the medium ground finch and the cactus finch. The island is so small that the researchers were able to count
and catalogue every bird. When a severe drought hit in 1977, the birds soon devoured the last of the small,
easily eaten seeds. Smaller members of the medium ground
finch population, lacking the bill strength to
crack large seeds, died out.
D.
Bill and body size are inherited traits, and the next generation had a high proportion of big-billed
Individuals. The Grants had documented natural selection at work the same process that over many
millennia, directed the evolution of the Galápagos' 14 unique finch species, all descended from a common
ancestor that readied the islands a few million years ago.
E.
Eight years later, heavy rains brought by an El Nino transformed the
normally meager vegetation
on Daphne Ma ị or. vines and other plants that in most years struggle for survival suddenly flourished,
choking out the plants that provide large seeds to the finches. Small seeds came to dominate the food supply,
and big birds with big bills died out at a higher rate than smaller ones. 'Natural selection is observable/
Rosemary Grant says. 'It happen when the environment changes. When local conditions reverse themselves,
so does the direction of adaptation.'
Dostları ilə paylaş: