114 



CLASSIFICATION, PHYIOGENY, AND EVOLUTION: 



Darwin's ideas on variation were confused by being 

 associated directly with the survival of the fittest. 

 Actually, variation and survival are two distinct 

 things. Variation is primarily due to heredity and 

 environmental influence upon the basic hereditary 

 pattern. Moreover, heredity and changes in heredity 

 {mutalinns) provide the basis for evolution. Heredity 

 and environment together cause the many functional, 

 structural, and behavioral traits that may be bene- 

 ficial, neutral, or harmful to organisms. 



Generally there is a natural selection for the bene- 

 ficial or neutral traits and against the harmful ones. 

 However, selection is not simply a matter of organ- 

 isms with the best characteristics surviving and those 

 with the poorest ones dying. Chance has a part to 

 play. Chance alone might cause either the elimina- 

 tion of the best features or the retention of the worst. 

 On the other hand, there is believed to be a direct 

 relationship between value of a feature and survival. 

 This allows one to modify Darwin's ideas and state 

 that organisms with beneficial traits are more likely 

 to survive than individuals with harmful traits. 



Darwinism in the strictest sense is no longer com- 

 pletely accepted. The prevalent expanded concept of 

 evolution is called neo-Darwinism. Neo-Darwinism 

 stresses the importance of the basic units of heredity, 

 the genes; it considers the principal source of vari- 

 ation to be gene mutations, and the principle force of 

 the direction of evolution to be natural selection 

 (chance is of much less importance). 



Neo-Darwinism, phylogeny, and the principles of 

 evolution all are interrelated. Historically, the inter- 

 relations came from the accumulation of evidence for 

 evolution; the development of the science of genetics, 

 which provided the mechanisms for evolution; and 

 the explanation of various characteristics of evolution. 



EVIDENCE FOR EVOLUTION 



FOSSIL RECORD AND GEOLOGICAL TIME 



The record of past life is far from perfect; hov\ever, 

 it is more extensive than would be necessary for one 

 to consider evolution a fact rather than conjecture 

 (see Table 7.1). There are good fossil series that por- 

 tray gradual changes in individual lines of develop- 

 ment and probable major lines of past development of 

 I he living phyla. The animal record is somewhat 

 better than thai of plants in this respect. The animal 



record shows that the first animals were invertebrates; 

 then came primitive fishes, amphibians, reptiles, and 

 finally birds and mammals. 



The fossil record definitely refutes the old idea of 

 catastrophism, that life was created and abruptly 

 destroyed many times. In spite of gaps in the record, 

 fossils indicate trends of gradual to abrupt decline in 

 some past life and of almost simultaneous diversifica- 

 tion in others. Many times the number of species liv- 

 ing today evolved and later became extinct; however, 

 most of the geological record implies somewhat inde- 

 pendent origin and extinction. Although part of this 

 record might suggest catastrophism, diflferent groups 

 of organisms reacted differently during the infrequent 

 times of widespread extinction; while some formers 

 were declining, others were increasing. 



ANATOMY 



The anatomical evidence for evolution is over- 

 whelming. For example, the subject of comparative 

 anatomy of the vertebrates demonstrates the affinity 

 among vertebrate body systems (Figure 7.7). How- 

 ever, comparative anatomy need not be limited to 

 vertebrates alone. One also can follow the lines of 

 development and like origin of body parts in inverte- 

 brates and in plants. 



COMPARATIVE PHYSIOLOGY 



Like anatomy, body function is important proof of 

 evolution. Physiology is accomplished by chemical 

 processes. In closely related organisms the chemicals 

 and processes are quite similar; the more distantly re- 

 lated the organisms, the greater the differences in 

 physiology. 



Important sources of comparative physiological 

 data are blood chemistry tests in which blood pro- 

 teins are analyzed. The data allow the generaliza- 

 tion that the more closely related the species, the 

 more similar the blood chemistry, the more distantly 

 related, the more dissimilar the proteins. 



EMBRYOLOGY 



The importance of embryology was once over- 

 emphasized by the so-called limgenelic Law. This 

 "law" was that ontogeny recapitulates phylogeny, that is, 

 the developmental history of an organism recreates 

 the evolutionary history of its ancestors. This premise 



