Scope, Sequence, and Coordination

A Framework for High School Science Education

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Structures that Reveal Common Ancestry

Natural Selection and its Evolutionary Consequences
Natural selection and its evolutionary consequences provide a scientific explanation for the fossil record of ancient life forms, as well as for the striking molecular similarities observed among the diverse species of living organisms.

The millions of different species of plants, animals, and microorganisms that live on Earth today are related by descent from common ancestors.

Further Description:

The fossil record is an accurate but very incomplete account of organisms that flourished on Earth millions of years ago. Through geological forces such as sedimentation and compression, the replaced remains of organisms, often as imprints, have produced fossil-bearing rock strata. This fossil rock record supplies scientific evidence that supports the Darwinian theory of natural selection by showing the continual replacement and displacement of species over geologic time. Darwin proposed the mechanism of "gradualism," whereby a species accumulates small changes over many generations. Improved forms would be selected while less improved forms would become extinct. Darwin hypothesized that the fossil record in rock would eventually uncover these "gradual" changes of species, showing an accurate account of their evolutionary history.

Recent evidence suggests, however, that gradualism is not totally supported by new fossil finds. It seems that fossil gaps are actually not flaws in the fossil record but a true picture of how species have originated. "Punctuated equilibrium" has been proposed as an explanation that supports the fossil rock record as an accurate account of evolutionary events. Fossil records indicate many species "maintain equilibrium" or remain unchanged and then "suddenly" new changes happen. These new forms come from preexisting ones and remain intact with little change for periods of long geological time until the next episode, or punctuation, takes place. To best understand these changes, they need to be tied to genetics and how such rapid changes might occur.

The fossil records not only indicate a catalog of species that have become extinct but also reveal the extent to which major groups are related to each other through homologies of common ancestry.

Descent from a common ancestry can be understood by examining avenues of speciation, namely phyletic change and cladogenesis. Phyletic change is based on the gradual accumulation of variations until a species no longer resembles its predecessors. Speciation has occurred because of selection acting upon new traits. These changes have been observed in fossil records, as exhibited in the phylogeny of the modern horse.

Cladogenesis connects ancestral origins through a splintering process. In this process the origin of a new species is the direct result of the isolation of small populations from ancestral stocks. Favorable genetic variations of ancestral populations were acted upon and increased quickly. Because the population was isolated, the variations would not be "diluted" and new species could develop.

Taken collectively and supported by the fossil remains, phyletic change and cladogenesis can result in adaptive radiation or evolution from a common ancestry to a number of different forms, each specialized to fill a different habitat and ecological niche.

Concepts Needed:

Grade 9

Fossils, geological forces, imprints, sedimentation, compression, rock strata, extinction, common descent, ancestral stock, natural selection

Grade 10

Homologous, analogous and vestigial structures, speciation, adaptive radiation, phylogeny

Grade 11

Carbon dating, radioactive decay, adaptive radiation

Grade 12

Speciation, founder effect, gene flow, geologic time table

Empirical Laws or Observed Relationships:

Rates of change in certain key elements (e.g., carbon-14, and the ratio of lead-206 to uranium-238 in rocks or the ratio of trapped helium to radium) give us methods to date fossils and minerals. (The results of such measurements show Earth to be approximately 4.5 billion years old.) DNA and immunological relatedness; the assembly of facts of evolutionary change; nested hierarchies or similarity for morphological and DNA characters, which show that evolution has resulted in a branching tree of diversification.

Theories or Models:

Natural selection, gradualism, punctuated equilibrium, law of superposition, law of cross-cutting relationships, law of evaluded fragments, descent with modification, cladogenesis, phyletic change, divergent and convergent evolution

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Micro-Unit Description:

Structures that Reveal Common Ancestry
Students should explore homologies of animals that reveal common ancestry. They should also consider how structures without a common evolutionary origin can be similar in function but not in structure (give examples, such as the wings of birds and insects).

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