Choose from these words to fill the blanks below:

complete, smaller, million, humans, fossil, look, habitats, bounded, paleoanthropologists, reproducing, natural, gradual, accepted, anatomical, reproduction, bonobos, clearly, protecting, relationship, replace, morphologically, anamensis, East, Sex, afarensis, differences, hominim, range, naturalists, biological

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why are species               bounded from each other

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we would expect to find

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              gradual changes in the fossil record

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gradual gradations among living organisms, i.e. a continuous           range of species of animals

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explanation

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as gradual changes occurred, they became more and more adapted to smaller                 habitats

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those gradations gave rise to large populations and               smaller populations

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more populous species would continually               replace the smaller ones

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in Darwin's day, the fossil record was not very                 complete

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today we have extraordinary records of some lineages

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hominins

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from Australopithecus                   anamensis

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to Australopithecus                   afarensis

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two species that lived between 4.2 and 2.9 million years ago

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across much of         East Africa

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we see in them a record of gradual change

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in Darwin's time, species were simply what                       naturalists said they were

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they understood the morphological variation, i.e. study of the size and shape of                     anatomical features

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Darwin's book: The Descent of Man, and Selection in Relation to       Sex

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concerned with the topic of whether different kinds of             humans should be called different species or not

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"It is a hopeless endeavor to decide this point until some definition of the term species is generally                 accepted. We might as well attempt without any definition to decide whether certain number of houses should be called a village, town, or city. We have a practical illustration of the difficulty in the never-ending doubts of whether many closely allied birds, mammals, insects, and plants which represent each other respectively in North America and Europe should be ranked as species or geographical races."

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when                                         paleoanthropologists name species in much the same way today, just in more sophisticated terms

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the evidence that we use to decide whether members of a species have a close                         relationship is not often evidence that is closely at hand

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the                     biological species concept

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a species is a population that includes all of the populations that can actually or potentially reproduce with each other in their               natural habitat

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this definition focuses on                         reproduction

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it doesn't matter in principle how different the members of these species         look

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as long as they are                       reproducing with each other in their natural habitats, in their natural populations

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chimpanzees

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exist in at least four sub-populations across equatorial Africa

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their close relatives the               bonobos separated from the chimpanzees more than a million years ago

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chimpanzee subspecies have existed for more than a half-              million years

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there is not much evidence that early hominim populations were comparable in their                       differences to one another as we see with chimpanzees and bonobos

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but the difference between sub-species and species can be seen as arbitrary and depends on one's intentions

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many conservation biologies who are concerned about                     protecting chimpanzees want to emphasize the biological differences between the different groups and would prefer that different sub-species be called different species

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many biologies who are studying the fossil record who are trying to understand how various fossil forms are                               morphologically different and might be related to each other, are not ready to test how different those morphological differences are consistent with populations who are exchanging significant amounts of genes

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Australopithecus afarensis is a descendant of Australopithecus anamensis

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why don't we just call them one species?

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we don't know that Australopithecus afarensis is the only decendent of Australopithecus anamensis

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we later have more evidence of various               hominim groups adapting to different kinds of ecologies

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but until there is more information, paleontologists prefer to give groups names so they can communicate about them               clearly

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but we often don't have enough information to know when and where major changes might have arisen

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the name gives paleontologists a way to deal with variation and to approach the hypotheses

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when it is argued whether a species is valid or not, it is being argued whether one can trace the             fossil record with the evidence that is available, or whether addition discoveries need to be made to find the common ancestors of these groups