© Colin Frayn, 2008 Homologous structures Homologous structures are those that appear similar in related species Evolution predicts that.

© Colin Frayn, 2008www.frayn.net

Homologous structures• Homologous structures are those that appear similar in

related species

• Evolution predicts that certain organs, over a great deal of time, may change their function as a lineage evolves

• Exaptation : The modification of use of an existing organ for a different function in a direct evolutionary descendant.

• Do similar features merely propose the same designer?– No, because in most cases the best option for a designer would

have been to completely redesign the organ from scratch rather than modify an existing organ to do something that it wasn’t originally ‘meant’ to do

– Also, we have intermediate stages for many of these


Haeckel’s Embryos (1)

Stagesof

development

Differentanimals

A copy (1892) of Ernst Haeckel’s original embryo

sketches


Haeckel’s Embryos (2)• Haeckel faked some aspects of his original

drawings– They are nowadays accepted as fraud

• Some aspects were accurate and relevant– ‘Recapitulation theory’– Many aspects of ontogeny (embryo growth)

really do echo historical evolutionary stages• Whales have hair and leg bones• A structure resembling the tail of primates recedes to

form the human coccyx bone• Swim bladder of fish develops first as a component of

the gut, according to its presumed evolution, and then later detaches from the gut completely


Ontogeny recapitulates phylogeny• The ‘subtitle’ of Haeckel’s

Recapitulation hypothesis

• This theory suggests that embryological stages remind us of the evolutionary history of an organism

• This is somewhat true, but is largely discredited in the sense that Haeckel intended it


Gill slits in embryos• Do human embryos have ‘gill slits’

reminiscent of their evolutionary history?

• No, though they do have features that resemble gill slits called pharyngeal pouches – In humans, these features have nothing

to do with breathing– All vertebrate embryos have these– In fish, these do develop into gills– Common features are strong evidence

for common ancestry


Vestigial organsVestigial organs are supposed ‘remnants’ of

evolutionary history that have not yet fully disappeared

• Tiny leg bones in whales• The human appendix• Eye remnants in blind cave fish

These may have minor functions, but certainly not the ‘obvious’ ones.

They are remnants of a more prominent organ which has become redundant.


The EyeIs the eye too complicated to have evolved?

• The eye could easily have evolved in under a million years– Nilsson & Pelger (1994)

• Many different eye types exist in nature– The eye has evolved many times

• Maybe from a very simple common ancestor organ• Maybe totally independently

• The eye is evidence of poor design– The retinal cells are ‘plugged in’ backwards!– The contrary claims of Michael Denton and others are

not plausible


Eye Anatomy

Blind spot where the optic nerve

crosses the retina


Inverted Retinal CellsThe photoreceptive cells in the retina of the human

eye are plugged in backwards• This means that:

– Less light gets to the photo receptors– We have a blind spot– We are susceptible to many more blinding diseases

• Several creationists have tried to claim that this is a good design after all

• These arguments are flawed– An intelligent designer would have designed the eye differently

• Top-down design would avoid the problems that we see– We have a jerry-rigged organ designed to solve problems that

need not even exist


Darwin and Eye Evolution• Did Darwin write “To suppose that the eye … could have been

formed by natural selection, seems, I freely confess, absurd in the highest possible degree”?

• Yes, and he followed it with the words: “Yet reason tells me, that if numerous gradations from a perfect and complex eye to one very imperfect and simple, each grade being useful to its possessor, can be shown to exist; if further, the eye does vary ever so slightly, and the variations be inherited, which is certainly the case; and if any variation or modification in the organ be ever useful to an animal under changing conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, can hardly be considered real.”

• And then he quoted several pages of suggested evolutionary models.


Irreducible Complexity• Structures that cannot work if any one of their

parts is not present– There are many of these, but…

• Structures may have had alternate uses before evolving to their current form

• Bacterial flagellum– An ‘engine’ for bacteria– A plausible evolutionary model exists– Lots of work has been done in this area

• Immune system cascade– This poses no problem whatsoever– Models to explain the origin of chemical pathways are

abundant


Behe at Dover• In the trial of Kitzmiller vs. Dover Area School District (2005),

Michael Behe testified that several irreducibly complex systems exist.

Quotes from Judge Jones’s ruling:

“As expert testimony revealed, the qualification on what is meant by ‘irreducible complexity’ renders it meaningless as a criticism of evolution. In fact, the theory of evolution proffers exaptation as a well-recognized, well-documented explanation for how systems with multiple parts could have evolved through natural means.”

“We therefore find that Professor Behe’s claim for irreducible complexity has been refuted in peer-reviewed research papers and has been rejected by the scientific community at large.”

“...on cross-examination, Professor Behe was questioned concerning his 1996 claim that science would never find an evolutionary explanation for the immune system. He was presented with fifty-eight peer-reviewed publications, nine books, and several immunology textbook chapters about the evolution of the immune system; however, he simply insisted that this was still not sufficient evidence of evolution, and that it was not ‘good enough.’”

© Colin Frayn, 2008 Homologous structures Homologous structures are those that appear similar in related species Evolution predicts that.

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