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Author Topic:   What would a transitional fossil look like?
Meddle
Member (Idle past 327 days)
Posts: 179
From: Scotland
Joined: 05-08-2006


(3)
Message 121 of 403 (850718)
04-13-2019 10:55 AM
Reply to: Message 109 by Faith
04-12-2019 12:36 PM


Re: Thought Experiment for Faith
But they don't. The differences are much greater than those between cats and dogs: I described the differences that define their respective genomes. The chimp's extra long muscular arms, muscular torso and short legs with hand-like feet, plus skull shape etc etc etc, amount to greater differences in body structure than those between cats and dogs.

Can I ask how do you think the genome creates the structures of the body? You talk about mutations in terms of overall damaging genes, which suggests you are thinking of genes that code for proteins. But this is only part of the story, because there is also the mechanisms which control when these genes are expressed and how they interact.
An embryo is a population of cells and as they multiply gene expression changes. So cells in a certain region form a limb bud, elongates to form an arm and finally a hand, all while the timing of when different genes are expressed changes. What you describe as differences between humans and chimps is simply differences in timing of when genes are expressed. It may not even need mutations the protein coding genes for these differences to occur.
Another example for humans and chimps is the skull development of the foetus. For both species development is almost identical, with expression of genes at first developing the size of the cranium, then there is a point when this expression is reduced and instead the expression switches more to development of the jaws. In humans this switch occurs later in development than in chimps.

Edited by Meddle, : No reason given.


This message is a reply to:
 Message 109 by Faith, posted 04-12-2019 12:36 PM Faith has responded

Replies to this message:
 Message 122 by Faith, posted 04-13-2019 12:29 PM Meddle has responded

  
Meddle
Member (Idle past 327 days)
Posts: 179
From: Scotland
Joined: 05-08-2006


Message 148 of 403 (850755)
04-13-2019 9:52 PM
Reply to: Message 122 by Faith
04-13-2019 12:29 PM


Re: Thought Experiment for Faith
I was merely observing the fact that the adult body structure is basically the same in each species. I assume this is controlled by some part of the genome and I would like to know more about how the genome produces it but I don't see the relevance of the stages of growth as you are discussing it.

But that is the point, there is nothing in the genome to define what the final body structure will be. What we perceive as a final body structure is the culmination of many generations of embryonic cells multiplying and their gene expression changing in response to the cells that surround them and the signalling proteins they're exposed to.
This also means that changes earlier on in development carry a greater risk of damaging the individual as they can impact how cells interact later on in development. Therefore the innovations to allow species to diversify usually occur towards the end of development, and are constrained by natural selection, which results in your observation that body structures in related species follow a similar pattern. So dogs are dog-like and apes are ape-like.
But that also means the differences between humans and chimps are minor because they occur towards the end of development, so my example of the skull, where the timing of gene expression changes the ratio of cranium to jaw size; or your example of limbs, where gene expression determines how far a limb bud is extended into a leg and eventually the length of toes. After all, this is not much different than what differentiates a gorilla from a gibbon.


This message is a reply to:
 Message 122 by Faith, posted 04-13-2019 12:29 PM Faith has not yet responded

  
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