生物系列小短文（一）The diversity of life
Biodiversity（生物多样性） is the totality of genes（基因）,species（物种）, and ecosystems （生态系统）in a region.
Thewealth of life on earth today is the product of hundreds of millions of yearsof evolutionary（进化的） history. Over the courof time, human cultures have emerged and adapted to the local environment,discovering, using, and altering local biotic resources（生物资源）.
Manyareas that now em "natural" bear the marks of millennia of humanhabitation, crop cultivation（耕作）,and resource harvesting（收获）.The domestication（驯养）and breeding（饲养） of local varieties ofcrops and livestock have further shaped biodiversity.
Biodiversitycan be divided into hierarchical categories that describe quite differentaspects o
f living systems that scientists measure in different ways.
∙ Geic
diversity（遗传多样性）
新年谜语
Geic diversity refers to the variation ofgenes within species.
This covers distinct populations of the same species (such as thethousands of traditional rice varieties in India) or geic variation within apopulation (which is very high among Indian rhinos（犀牛）, for example, and very low among cheetahs（印度豹）).
Until recently, measurements of geic diversity were applied mainly todomesticated species and populations held in zoos or botanic gardens, butincreasingly the techniques are being applied to wild species.
"Each species is the repository of an immen amount of geicinformation," writes E. O.Wilson in Biodiversity方特一日游 (1988. Washington, DC: National AcademyPress). "The number of genes range from about 1,000 in bacteria and 10,000in some fungi to 700,000 or more
in many flowering plants and a few animals. Atypical mammal such as the hou mou has about 100,000 genes. ...If stretchedout fully, the DNA would be roughly one meter long. But this molecule isinvisible to the naked eye. ...Th
e full information contained therein, iftranslated into ordinary-size letter of printed text, would just about fill all15 editions of the Encyclopedia Britannica published since 1768."
Species diversity（物种多样性）
Species diversity refers to the variety ofspecies within a region.
Such diversity can be measured in many ways, and scientists have notttled on a single best method. The number of species in a region -- its"species richness"-- is one often-ud measure, but a more preci measurement, "taxonomicdiversity," also considers the relationship of species to each other.
For example, an island with two species of birds and one species of lizardhas greater tax
onomic diversity than an island with three species of birds butno lizards.
Thus, even though there may be more species of beetles on earth than allother species combined, they do not account for the greater part of speciesdiversity becau they are so cloly related.
西沙群岛资料
Similarly, many more species live on land than in the a, but terrestrialspecies are more cloly related to each other than ocean species are, sodiversity is higher in marine ecosystems than a strict count of species wouldsuggest.
Estimates of global species diversity havevaried from 2 million to 100 million species, with a best estimate of somewherenear 10 million, and only 1.4 million have actually been named.
New species are still beingdiscovered--even new birds and mammals. On average, about three new species ofbirds are found each year, and as recently as 1990, a new species of monkey wasdiscovered. Other vertebrate groups are still far from being completelydes
cribed: an estimated 40 percent of freshwater fishes in South America havenot yet been classified.
∙ Ecosystem
diversity（生态系统多样性）
藏经阁黄色Ecosystem diversity is harder to measure than species or geic diversity bec
ceo的职责
au the"boundaries" of communities（群落） -- associations of species -- and ecosystems areelusive.
Nevertheless, as long as a consistent t of criteria isud to define communities and ecosystems, their number and distribution can bemeasured.
Until now, such schemes have been applied mainly atnational and sub-national levels, though some coar global classificationshave been made.
Besides ecosystem diversity, many other expressions ofbiodiversity can be important. The include:
o the relative abundance of蒜茄子的做法
species,
o the age structure of populations,
o the pattern of communities in a
region,
o changes in community composition
and structure over time, and
空姐英语
舍弃的近义词o ecological process as
predation, parasitism, and mutualism.
