MODELOS EVOLUTIVOS GRADUALISMO Y EQUILIBRIO PUNTUADO Gradualism or

MODELOS EVOLUTIVOS: GRADUALISMO Y EQUILIBRIO PUNTUADO

“Gradualism", or traditional Darwinian evolution. Remember, there are THREE MAIN criteria necessary for evolution to occur via natural selection

GRADUALISMO Descendencia Variación Herencia Selección

Stephen Jay Gould Niles Elderigde EQUILIBRIO PUNTUADO

Natural Selection + additional factors, such as the environment this would best be exemplified by Stephen Jay Gould's and Niles Elderidge's PUNCTUATED EQUILIBRIUM theory, which holds that natural selection only works in short spurts amongst long relatively stable periods of stasis, lasting hundreds of millions of years, in which species don't change (? ).

Evidence for Punctuated Equilibrium: Again, there abundant FACTS within the fossil record that evolution in a manner as proposed by Gould and Elderidge. Actually, theory of punctuated equilibrium was first proposed on the basis of paleontological findings.

Briozos: Metrarabdotus Cheetham (1986) examinó 1000 fósiles del género. Usando 46 caracteres morfológicos, Cheetham dibujo un arbol monofilético conectandolos por medio de los caracteres:

Briozos: Metrarabdotus

Pequeñoa cambios relativos ocurrieron dentro de las MORFOESPECIES, mientras que grandes cambios fueron observados entre las MORFOESPECIES NO se encontraron ejemplares “intermedios” entre las morfoespecies. [Jackson y Cheetham (1990, 1994) examinaron 7 especies vivas de Bryozos del género y confirmaron que las morfoespecies identificadas difieren significativamente entre ellas en el número de loci para aloenzimas. ]

CAMBIO EVOLUTIVO GRADUAL Sheldon (19) estudió 3458 ejemplares de 8 linajes de trilobites. Los linajes mostraron un cambio gradual convincentemente pronunciado: Al principio y al fin de cada linaje podrían ser clasificados como diferentes especies (y solo en un caso como en diferente género).

Ambos procesos, Puntualismo y el Gradualismo pueden ocurrir. Al revisar 58 estudios semejantes , Erwin and Anstey (1995) concluyeron: "Paleontological evidence overwhelmingly supports a view that speciation is sometimes gradual and sometimes punctuated, and that no one mode characterizes this very complicated process. " Eldredge y Gould argumentaron que “estasis es un dato", el cual debe jugar un papel más prominente en las explicaciones evolutivas.

Richard Dawkin Evidence AGAINST Punctuated Equilibrium: As with Dawkin's point of view, punctuated equilibrium does not fully explain the reason why we see the sudden appearance of complexity, nor does it explain the apparent "directionality" towards complexity.

(2) What explains punctuated evolution? Why might morphological evolution be rapid around speciation events? Why might morphological evolution be relatively static during other periods of time? Eldredge and Gould's (1972) explanation (following Mayr): Peripatric speciation of a small isolated population might lead to rapid changes in a daughter population (drift), whereas large parental populations remain relatively unchanged. Gould and Eldredge's (1993) explanation (following Futuyma): Populations are constantly changing, but genetic mixture across populations prevents sustained differences from accumulating. Speciation "locks up" the changes that a population has undergone. Alternative explanations? ?