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The problem I've always had with evolution is the actual lack of variation between animals. More specifically, the lack of observable gradual change between species.

Take for example the hammerhead shark. It obviously has a strong connection with a normal shark, but where's the "half-hammerhead" shark? Where's the species that only has a SLIGHTLY wider head?

I really doubt that a single mutation on a normal shark resulted in a head shaped like that, it had to happen gradually. Right? As far as I understand, evolution works by random mutation in a child being somehow slightly beneficial to that animal so it get's to live longer and reproduce more, eventually resulting in it's genetic material in dominating the inferior ones before it. But if it was for whatever reason beneficial to have a slightly wider head, why don't all the sharks have slightly wider heads now? On the other hand, if it wasn't beneficial, how come the hammerhead still exists?

Furthermore, I feel like some characteristics of a certain animal can't really have gradual benefits. There's a lizard that shoots blood out of it's eyes. The original ancestor certainly didn't, and I'm sure that if it's child only bled slightly from it's eyes it wouldn't really help if much in life. So for it to actually become beneficial, it seems to me that there had to be a ton of lizards that had a disadvantage of bleeding slightly. In fact, there had to be so many that they evolved even further in the same direction and kept doing that until they bled so much it's an actual benefit (sorry for this turning so gross). So why did the inbetweeners survive long enough to evolve even more, and why did they THEN die out?

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closed as off-topic by Remi.b, daniel, Bez, The Last Word, Mad Scientist Jul 18 at 20:12

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The fossil record in general contains transitional forms for some animals and not for others. Since geology and other factors enter into this, I think one has to choose a specific animal and ask why the fossil record might be deficient. Where would we find a transitional hammerhead shark, for example? –  daniel Jul 18 at 16:30
    
My question isn't about fossil evidence. It's about why those species aren't here TODAY. Or if they aren't because they're extinct due to their inferiority, then how did they live long enough for the superior species to evolve from them? –  Darwin Jul 18 at 16:32
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Have you studied the variety of hammerhead sharks? Do they all have precisely the same proportions? Even checking the Wiki entry, it indicates the winghead shark has the most pronounced cephalofoil. That would mean others are much less so. So there are gradations. It may not be a perfect spectrum but it may be that intermediates were poorly adapted. –  daniel Jul 18 at 16:34
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Remi.b asked my next question for me. If there were only two grades of hammerhead would you insist on the two intermediates? And if there were four would you insist on eight, one between each gradation? –  daniel Jul 18 at 16:42
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I'm closing this according to our policy on creationist questions. If you're interesting in understanding evolution you're welcome to ask here. Questions that simply state the common creationist talking points will be closed. –  Mad Scientist Jul 18 at 20:12

3 Answers 3

More specifically, the lack of observable gradual change between species.

Most significant phenotype differences occur over several thousand generations, which means several thousand years on up. While we certainly can create experiments where a controlled form of evolution occurs within a very small time-frame, I'm going to assume that you're not interested in closed laboratory experiments given the nature of your question.

It obviously has a strong connection with a normal shark, but where's the "half-hammerhead" shark? Where's the species that only has a SLIGHTLY wider head?

The data below does answer you question, but in reverse. Hammerheads are much older than most of their relatives; so their cousins have actually grown smaller heads... but there are certainly intermediates.

From Andrew Martin (1993 - mtDNA data) via Elasmo-research.org:
*The circles represent the head-width vs. body-length ratio.

Hammerhead taxa

Martin's conclusion: "...the hammer appeared rather suddenly and fully formed, and was later optimized along different lines to suit various selection pressures."

I really doubt that a single mutation on a normal shark resulted in a head shaped like that, it had to happen gradually. Right?

Not necessarily. Single Nucleotide Polymorphisms (a.k.a. - "SNPs" - a change in one nucleotide) can have pretty wide-ranging effects.

The vast majority of humans, for instance, are lactose intolerant in adulthood. However, lactose tolerance evolved twice from two different SNPs in two different human populations: One European and one African. One gene. One regulatory mechanism change. Massive benefits (essentially opening up a whole new resource in adulthood).

Significant differences in phenotypes (the physical appearance of something) can require more gene mutations, or not. It depends on where the mutation is in the genome and what the gene does.

But if it was for whatever reason beneficial to have a slightly wider head, why don't all the sharks have slightly wider heads now?

The simple answer is because either there wasn't enough selective pressure that the rest of the shark phenotypes succumbed to that the hammerhead avoided, or that the hammerhead design doesn't give enough of an advantage to completely dominate the gene pool. An alternate, but more abstract answer, is that there simply hasn't been enough time for the hammerhead design to dominate the shark phenotype. Remember, evolution works by generations - not years. It would take unknown thousands of generations for a weakly-advantageous design to dominate an entire niche - if it ever would.

Also, it is worth it to point out that evolution doesn't only select for advantageous mutations. Mutations that aren't very detrimental can still exist within a population as long as the detriment doesn't prevent resource exploitation and offspring production. Think Huntington's Disease which severely restricts a person's survivability, but only after their mid-30's (in general) - plenty of time for the person to have sex and raise a kid into semi-adulthood. It is a genetic defect which sticks around only because it allows humans to reproduce before it seriously affects them.

The original ancestor certainly didn't, and I'm sure that if it's child only bled slightly from it's eyes it wouldn't really help if much in life.

Maybe, maybe not. Maybe it shot something else before blood, and then a mutation caused it to shoot blood instead - but the overall effect (deterring predators) was the same so it doesn't really matter.

You're correct that some traits might appear very awkward if they had evolved gradually, but the question then becomes: Did they evolve gradually? Sometimes we simply do not know.

So why did the inbetweeners survive long enough to evolve even more, and why did they THEN die out?

Like I said above, just because a trait is detrimental doesn't necessarily mean it will be eliminated. As long as the being can survive long enough to reproduce successfully with the trait, the trait might not be eliminated for a very, very long time- if ever.

As for in-betweeners dying out - it's difficult to think of it like that. You're talking bout LCAs ("Last Common Ancestors") of extant (currently-living) species. While some LCAs just die out for some reason or another, it's more common that the LCA simply continued to evolve and become a genetic relative due to some sort of isolation.

Think of it like the in-betweeners coming to a fork in the road. Some choose right, some choose left. There isn't anybody left behind - merely two populations that are changing over time that were once one group until they don't really mate between each other anymore (or can't). Heck, even Biologists have trouble deciding at what point a population is considered a different "species" because of discoveries in the last 50 years - so right now a firm answer like "It takes 15% of the genome to change for a new species to emerge" isn't possible as it doesn't really work like that.

We'll get to it eventually, but for the time-being, it might be easier to say that everything currently living on Earth is an in-betweener of something else in the future. Everything in its current iteration will eventually die off, but not before something else emerges from our offspring which may or may not be significantly different from us due to however many genetic mutations our offspring have from us. It could be one that opens up a whole new realm of possibilities (like lactose tolerance) or an accumulation due to selective pressures (like Phoemelanin production and pale skin in humans [Redheads] to produce more Vitamin D3 at higher latitudes).

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Your shark head diagram sort of says it all. –  daniel Jul 18 at 21:58

Could not fit in a comment...

This sounds like a very basic question in evolutionary biology that often ask for a very long answer. But I think that you may get the answer you're looking for just if we ask you back how many inbetweeners would you expect to exist between the hammerhead shark and whatever is the closest currently living species of the hammerhead shark? If you had one more inbetweener would you still be surprised to not have more inbetweeners?

Your question might actually be "Why are there species rather than individuals that show a long continuum of various traits?". If this is your question, then the answer would indeed need to be fairly long and explain about the process of speciation.

Edit

Studying a bit further the question of the hammerhead sharks, I discovered that there are actually 10 species of hammerhead sharks! You can find them here on the french wikipedia page

Let us know whether this comment/answer helped.

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This is a great question (in the first para.). I don't usually upvote comments but +1. –  daniel Jul 18 at 16:39
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Actually, yes. I would ask the question. Why isn't there a continuum of species? Why are there species at all? If the answer is long I'd appreciate if you could point me to some useful links so I could read more about it. –  Darwin Jul 18 at 16:44
    
But I'm still not sure if this answers my original question. How did the blood shooting lizard get to be if for it to exists there had to be lizards that just bled from there eyes slightly? Any intuition tells me that those species would die out in a generation, let alone thousands of generations required for evolution to make the bleeding even stronger. –  Darwin Jul 18 at 16:46
    
So, basically your post can be decomposed into 2 questions. 1) Why are there species rather than a continuum? 2) How can a trait evolved while the intermediate trait seems to be deleterious (deleterious=negative effect on the reproductive success and survival of the carrier). Did I understand your post correctly? If yes, I'd suggest you to split your question into two new posts (vote to close the current post in parallel). Note: The second question being fairly common, there might already has a post on this site that answers it. –  Remi.b Jul 18 at 16:51
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Yeah. That's exactly right. Thank you for organizing my thoughts. –  Darwin Jul 18 at 17:07

If an inbetweener offers a slight advantage to survival, and if the full-fledged trait even more so, then once the full-fledged population becomes sufficiently large, they will have the advantage and the inbetweeners will die out.

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