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N.Wells



Posts: 834
Joined: Oct. 2005

(Permalink) Posted: Jan. 26 2007,21:11   

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(From Cornelius Hunter) It is strange that evolutionists never get around to addressing the scientific issue. Wesley Elsberry appears to be denying convergence, but that can't be true. If he has an explanation for convergence then let's hear it. If not, then admit it. Here is the question for evolutionists: How is it that similarities such as the pentadactyl pattern are such powerful evidence for evolution, in light of equal and greater levels of similarity in distant species, such as displayed in the marsupial and placental mouse?


Let’s unpack the mistakes here.

1) Biologists have spent a lot of time over the last 170 years dealing with issues relating to similarity and convergence and their implications for evolutionary theory.  Do a Pubmed search on homology, homoplasy, or analogy, for example.  The charge that evolutionists “don’t get around to this” is completely false, and can only be indicative of profound ignorance of the field, or mendacity.  

2) Neither Wesley nor any biologist is in the position of denying convergence.  Biologists find it a fascinating subject, and spend a lot of time on it because it can tell a lot about evolutionary processes.

3) Biologists absolutely do have an explanation for convergence.  Organisms that start different may, if they take up similar life styles, become more similar over time if the requirements of their niche cause adaptation toward the same morphological/functional solution.  This is convergent evolution by natural selection.  A shining example is the different lineages of animals that have taken up a mole-like existence.  Burrowing requires specific adaptations: a strong forehead, short & strong arms and legs with spade-like hands, and eyes are useless (and may even be a liability due to the chances of injury and infection).  A giraffe would make a terrible burrower.  This has lead to impressive similarities between marsupial “moles”, golden moles (chrysochlorid insectivores), N. American / Eurasian moles (talpid insectivores), and, to a lesser degree, naked Somali mole rats.  (The marsupial ‘moles’ and the golden moles are especially similar).  


Let’s concentrate on the specific question: “How is it that similarities such as the pentadactyl pattern are such powerful evidence for evolution, in light of equal and greater levels of similarity in distant species, such as displayed in the marsupial and placental mouse?”

First, this question is misphrased.  The important thing about the forelimbs of birds, bats, dogs, pterosaurs, pigs, moles, anteaters, dolphins, and so forth is that their differences overwhelm their similarities, but their similarities are deeper and are the result of common inheritance.  In contrast, their similarities are in many ways far less than the similarities between golden moles and marsupial moles or between ‘flying’ squirrels and ‘flying’ phalangers, but the latter similarities are superficial and are appear not to have resulted from shared inheritance.   Both sets of comparisons and contrasts provide powerful evidence for evolution.

First, some definitions:
Homology: underlying similarity, due to shared inheritance, despite divergence
Analogy: superficial similarity, despite lack of common ancestry, due to convergent evolution.
Homoplasy: similarity for any reason other than common ancestry, including drift.

Evolutionary biology interprets bird wings and bat wings as being analogous with respect to flight, but homologous with respect to being limbs constructed of a scapula, a humerus, a radius, an ulna, and several carpals, metacarpals, and phalanges.  (The limbs are homologous, but their flight is orthologous.)   Dolphins and ichthyosaurs are similarly claimed to be analogous with respect to body form, live birth, and precocious offspring.  Insect wings and vertebrate wings are considered to be analogous, while being only in the remotest sense homologous.

If we see the same embryological tissues contribute to two features, the same genes activated during its construction, utilization of the same developmental pathways, and the same bones ending up in much the same places in the same basic relationships to adjacent bones, nerves, blood vessels, and so forth, then we can make a reasonably secure claim of homology.  If we additionally have a fossil record that shows similar structures or a gradation of change in probable intermediates then the claim is that much stronger. For example, we have many very different vertebrate forelimbs in terms of shape and function, but they are all constructed out of a scapula, a humerus, a radius, an ulna, some carpals, some metacarpals, and a basic pattern of five phalanges (although the carpals, metacarpals, and phalanges can evidently fairly easily experience fusion, reduction, or loss).  

However, proving claims of homology can get complicated, as we have instances of morphologic and functional similarity being retained despite loss and substitution of the underlying genes, and there is no reason why convergence, parallelism, reversal, and stasis can’t all contribute to a single complex evolutionary history. Also, there is a good deal of confusion over the terms, both in the professional literature and elsewhere.

A key point that should not be lost sight of here is that the evolution is considered by biologists to be a vastly superior explanation for apparent homology than intelligent design or special creation.  Notwithstanding claims of an ineffable designer, it is hard to see why a designer would want to construct all vertebrate forelimbs, which fairly efficiently serve such a great variety of functions, from such a limited menu of underlying components, when they could so easily be improved by additional components and modifications.  It is far easier to see them as the result of a highly contingent history constrained by evolutionary processes and  ancestry.


With respect to the supposedly amazing similarity of the thylacine and the wolf, please forgive my repeating myself from the main UD thread: the thylacine and the wolf have got dramatically different reproductive systems and numerous skeletal differences (e.g., epipubic bones and shorter legs not adapted for wolf-like fast running on the thylacine).  The teeth are very different: the wolf has three large upper incisors per maxilla, whereas the thylacine has four tiny ones.  The thylacine has three upper premolars and four upper molars, whereas the wolf has four upper premolars and two molars.  The wolf, like all canids, has an amazing large, bladed, shearing P4, which the thylacine completely lacks.  In the lower jaw we again see 3 premolars and four molars in the thylacine, while the wolf has four premolars and three lower molars, although in this case it is the M1 that is huge and occludes with the upper P4.  (For details, see http://www.naturalworlds.org/thylacine/skull/dentition_comparison.htm )   These features that are different in the thylacine are shared with other marsupials, whereas the differences in the wolf are shared with other canids and other placental mammals, including fossils.  They are thus indicative of evolutionary relationships, and a great evolutionary distance between thylacines and wolves.

  
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