Statements About Evolution

Kylie

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With those 5, I agree as you state them here. But in past posts, you drew conclusions from those 5, for example: while in the paragraph above you say "if an animal is able to reproduce slightly more than the average because it has genes which result in it having some advantage in reproduction..." while in past posts, what came across was (my paraphrase) "thus they are able to reproduce more", assuming facts not in evidence. I hope in your paragraph above you meant only "if".

What I mean is that the average number of offspring produced by individuals with this trait will be slightly above the average for the whole population.

For example, a zebra can produce an offspring every two or three years. The exact time will depend on many factors, but an important one will be the zebra's ability to prepare its body. After all, producing offspring is a rather intensive process and takes a lot of energy and resources.

If a zebra has genes that mean she is better able to store energy reserves, for example, she may find that her body is better able to prepare for producing an offspring. As such, she might be able to reproduce every two years. A different zebra without this gene will be closer to the average, say, 2.5 years. Since Zebras have a breeding season, this would work out to a gap of two years, then a gap of three years, and alternating in this way (remember, this is on average). But a zebra that has genes that leave her poorly prepared may find that it takes her three years every single time.

So, let's follow the herd of zebra over the course of a decade, say 2000 to 2010. The first zebra (with the genes which help her reproduce) is able to fall pregnant 6 times, in 2000, then in 2002, 2004, 2006, 2008 and 2010. The average zebra will fall pregnant 5 times, in 2000, 2002, 2005, 2007 and 2010. And the below average zebra will fall pregnant 4 times, in 2000, 2003, 2006 and 2009. If we assume all other things are equal (the chances of any foal succumbing to disease or predators, for example), then the zebras with the genes for the improved ability to store reserves have produced more offspring in the same time, and the zebras with the worse ability have produced fewer offspring.

Oh, and an example of better genetics resulting in more offspring isn't going to do the job. Your generalization isn't going to rest on one (or even a few) example(s). Mules are better in some ways than horses and donkeys, including (from what I remember hearing), hardiness. So by your reasoning (yes, I know you don't mean to be talking about mules here) they should have more opportunities to breed. But they don't breed. (Ha! Yes I know, the defeat of a generalization isn't going to rest on just one example either.)

This fits in perfectly well with evolution, actually. Yes, mules have many benefits over horses and donkeys. But the fact that they can't produce offspring of their own prevents the "mule" combination of genes from spreading. Let's say that there was a herd composed of horses and donkeys. And let's also say that (for the purposes of this example) mules could produce offspring. Surely, you would expect to find that mules start off as quite rare (after all, the herd is mostly horses and donkeys at the start). But while there will be cases of horses breeding with horses and donkeys breeding with donkeys, there will also be cases where horses and donkeys interbreed.

If we assume that the chances of a male of one species breeding with a female of the other species, there are four different pairings available:

Male horse breeds with female horse
Male horse breeds with female donkey
Male donkey breeds with female horse
Male donkey breeds with female donkey

If each of those pairings is random, then fully half the pairings will be between a horse and a donkey, producing mules. And if mules are on average better than either horses or donkeys (specifically referring to how well they are able to produce offspring of their own, whether it be by being better able to survive in general, or by being better able to gain access to a mate), then we would expect to see that over multiple generations, the mules quickly increase within the population until they dominate it. If we came back to check the heard after many generations, we'd find most members of this herd would be mules.

But as you said, mules are sterile and can't produce any offspring of their own. So in our hypothetical herd, fully half the pairings would lead to an evolutionary dead end. Evolution is changes accumulating over generations, and getting those generations requires reproduction. Since the mules can't breed, they can't pass on any genes, and so their line ends with them.

And I'm still not sure if your generalization is meant to be within one species or not.

Do you mean cases of two individuals from different species interbreeding? As you pointed out in your mule example, that is impossible for all practical purposes. If you are talking about a population of one species evolving into a population of a different species, that's what I'm leading to. But I'm just laying the framework at the moment, since I'd like you to have an understanding of the process rather than just making the claim and saying you have to accept it without understanding it.
 
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Kylie

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No reservations there. It is obvious.

Glad to hear it.

Now, statement 2. The genes we have are typically the main cause that we have many of the traits that we do. Things like eye colour, predisposition to certain diseases, that kind of thing.

(Note that I am not saying that genetics is the ONLY cause for things like that, nor am I saying that environmental factors can't be the main cause for some traits. My point here is that on average the traits we have a predominately controlled by genes.)

This site has more information.What Is a Gene? (for Kids) - Nemours KidsHealth
 
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Gottservant

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Glad to hear it.

Now, statement 2. The genes we have are [...]

You keep saying "Good happens eventually" but you never say "for who most".

If God creates a way for creatures to be the greatest they can be, surely God deserves praise?
 
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Estrid

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What I mean is that the average number of offspring produced by individuals with this trait will be slightly above the average for the whole population.

For example, a zebra can produce an offspring every two or three years. The exact time will depend on many factors, but an important one will be the zebra's ability to prepare its body. After all, producing offspring is a rather intensive process and takes a lot of energy and resources.

If a zebra has genes that mean she is better able to store energy reserves, for example, she may find that her body is better able to prepare for producing an offspring. As such, she might be able to reproduce every two years. A different zebra without this gene will be closer to the average, say, 2.5 years. Since Zebras have a breeding season, this would work out to a gap of two years, then a gap of three years, and alternating in this way (remember, this is on average). But a zebra that has genes that leave her poorly prepared may find that it takes her three years every single time.

So, let's follow the herd of zebra over the course of a decade, say 2000 to 2010. The first zebra (with the genes which help her reproduce) is able to fall pregnant 6 times, in 2000, then in 2002, 2004, 2006, 2008 and 2010. The average zebra will fall pregnant 5 times, in 2000, 2002, 2005, 2007 and 2010. And the below average zebra will fall pregnant 4 times, in 2000, 2003, 2006 and 2009. If we assume all other things are equal (the chances of any foal succumbing to disease or predators, for example), then the zebras with the genes for the improved ability to store reserves have produced more offspring in the same time, and the zebras with the worse ability have produced fewer offspring.



This fits in perfectly well with evolution, actually. Yes, mules have many benefits over horses and donkeys. But the fact that they can't produce offspring of their own prevents the "mule" combination of genes from spreading. Let's say that there was a herd composed of horses and donkeys. And let's also say that (for the purposes of this example) mules could produce offspring. Surely, you would expect to find that mules start off as quite rare (after all, the herd is mostly horses and donkeys at the start). But while there will be cases of horses breeding with horses and donkeys breeding with donkeys, there will also be cases where horses and donkeys interbreed.

If we assume that the chances of a male of one species breeding with a female of the other species, there are four different pairings available:

Male horse breeds with female horse
Male horse breeds with female donkey
Male donkey breeds with female horse
Male donkey breeds with female donkey

If each of those pairings is random, then fully half the pairings will be between a horse and a donkey, producing mules. And if mules are on average better than either horses or donkeys (specifically referring to how well they are able to produce offspring of their own, whether it be by being better able to survive in general, or by being better able to gain access to a mate), then we would expect to see that over multiple generations, the mules quickly increase within the population until they dominate it. If we came back to check the heard after many generations, we'd find most members of this herd would be mules.

But as you said, mules are sterile and can't produce any offspring of their own. So in our hypothetical herd, fully half the pairings would lead to an evolutionary dead end. Evolution is changes accumulating over generations, and getting those generations requires reproduction. Since the mules can't breed, they can't pass on any genes, and so their line ends with them.



Do you mean cases of two individuals from different species interbreeding? As you pointed out in your mule example, that is impossible for all practical purposes. If you are talking about a population of one species evolving into a population of a different species, that's what I'm leading to. But I'm just laying the framework at the moment, since I'd like you to have an understanding of the process rather than just making the claim and saying you have to accept it without understanding it.

If Il may?

Considering how fast most organisms
reproduce, a 0.001% enhanced silurvival trait
should soon yeild a lot of individuals with the gene.
 
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Mark Quayle

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Glad to hear it.

Now, statement 2. The genes we have are typically the main cause that we have many of the traits that we do. Things like eye colour, predisposition to certain diseases, that kind of thing.

(Note that I am not saying that genetics is the ONLY cause for things like that, nor am I saying that environmental factors can't be the main cause for some traits. My point here is that on average the traits we have a predominately controlled by genes.)

This site has more information.What Is a Gene? (for Kids) - Nemours KidsHealth
Yes, of course.
 
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Mark Quayle

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If Il may?

Considering how fast most organisms
reproduce, a 0.001% enhanced silurvival trait
should soon yeild a lot of individuals with the gene.

Of the fast-reproducing organisms, and all other things being equal, yes, or, if "survival trait" means, among other things, reproducibility (i.e. non-sterile).

What I mean is that the average number of offspring produced by individuals with this trait will be slightly above the average for the whole population.

"Would tend to be" is not the same as "will be". Or maybe if you said, "All other things being equal..."

For example, a zebra can produce an offspring every two or three years. The exact time will depend on many factors, but an important one will be the zebra's ability to prepare its body. After all, producing offspring is a rather intensive process and takes a lot of energy and resources.

If a zebra has genes that mean she is better able to store energy reserves, for example, she may find that her body is better able to prepare for producing an offspring. As such, she might be able to reproduce every two years. A different zebra without this gene will be closer to the average, say, 2.5 years. Since Zebras have a breeding season, this would work out to a gap of two years, then a gap of three years, and alternating in this way (remember, this is on average). But a zebra that has genes that leave her poorly prepared may find that it takes her three years every single time.

So, let's follow the herd of zebra over the course of a decade, say 2000 to 2010. The first zebra (with the genes which help her reproduce) is able to fall pregnant 6 times, in 2000, then in 2002, 2004, 2006, 2008 and 2010. The average zebra will fall pregnant 5 times, in 2000, 2002, 2005, 2007 and 2010. And the below average zebra will fall pregnant 4 times, in 2000, 2003, 2006 and 2009. If we assume all other things are equal (the chances of any foal succumbing to disease or predators, for example), then the zebras with the genes for the improved ability to store reserves have produced more offspring in the same time, and the zebras with the worse ability have produced fewer offspring.

This fits in perfectly well with evolution, actually. Yes, mules have many benefits over horses and donkeys. But the fact that they can't produce offspring of their own prevents the "mule" combination of genes from spreading. Let's say that there was a herd composed of horses and donkeys. And let's also say that (for the purposes of this example) mules could produce offspring. Surely, you would expect to find that mules start off as quite rare (after all, the herd is mostly horses and donkeys at the start). But while there will be cases of horses breeding with horses and donkeys breeding with donkeys, there will also be cases where horses and donkeys interbreed.

If we assume that the chances of a male of one species breeding with a female of the other species, there are four different pairings available:

Male horse breeds with female horse
Male horse breeds with female donkey
Male donkey breeds with female horse
Male donkey breeds with female donkey

If each of those pairings is random, then fully half the pairings will be between a horse and a donkey, producing mules. And if mules are on average better than either horses or donkeys (specifically referring to how well they are able to produce offspring of their own, whether it be by being better able to survive in general, or by being better able to gain access to a mate), then we would expect to see that over multiple generations, the mules quickly increase within the population until they dominate it. If we came back to check the heard after many generations, we'd find most members of this herd would be mules.

But as you said, mules are sterile and can't produce any offspring of their own. So in our hypothetical herd, fully half the pairings would lead to an evolutionary dead end. Evolution is changes accumulating over generations, and getting those generations requires reproduction. Since the mules can't breed, they can't pass on any genes, and so their line ends with them.

That's obvious, and I'm sorry to cause you to belabor the point. My reservations have little to do with understanding the general principle, but with agreeing to the ready conclusions drawn from it. You demonstrate the general principle, but then conclude something along the lines of, "thus those genetically enabled to survive longer will produce more offspring with the trait", which evolves into, "thus the genetic trait for survivability will eventually take over the herd."

Do you mean cases of two individuals from different species interbreeding? As you pointed out in your mule example, that is impossible for all practical purposes. If you are talking about a population of one species evolving into a population of a different species, that's what I'm leading to. But I'm just laying the framework at the moment, since I'd like you to have an understanding of the process rather than just making the claim and saying you have to accept it without understanding it.

But the process depends on certain conclusions besides the statements. It depends on whether the statements are general tendencies, or whether they are actual (and reproducible) in most cases. I expect that you will follow your statements, sooner or later, with, "So, you see, evolution theory is valid", without proving that all the statements are valid, or applicable in most cases.

If I may? It's not that these are axiomatic—they are not a priori—so much as they are generalizations to be accepted "for the sake of argument." They are a posteriori, that is, knowledge dependent on empirical evidence. That can be explored, of course, but she is trying to lay some groundwork first, to find out where the discussion needs to go.

I would like to think you are right. But even when I trust the motives behind what a person is trying to do, 'judging by myself' or according to what I sometimes find myself doing, I am suspicious of the tendency to jump logical steps. Once the framework is laid, is she then going to produce her conclusion as if she had proven it, or is she going to back up to show that 'tendency' is 'actuality'? In other words, is she going to say her conclusion is logical, in an if/then statement (if the statements are true, then the conclusion is logical), and thus that TOE is possible, or is she going to say that 'since' "you agreed that the statements are true" thus I have no reason to disagree with TOE.
 
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Kylie

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Yes, of course.

Glad we are agreed.

Statement 2 said that the genes for these traits can (note I said CAN, not WILL) be passed from parent to child when the parent reproduces.

For example. I have a daughter. She's got a full set of genes, and half of those genes she got from me, and the other half of her genes she got from her dad. If I have a gene that gives me some advantage, then there's a 50% chance that this advantage-giving gene was passed to her, since she got half of her genes from me.

Any reservations with this?
 
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Kylie

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Of the fast-reproducing organisms, and all other things being equal, yes, or, if "survival trait" means, among other things, reproducibility (i.e. non-sterile).

I know you weren't replying to me here, but I'll take this opportunity to say that the word "survivability" should really be taken to mean the gene's survival.

If a gene causes the individual that has it to have a slightly greater chance to reproduce, then that gene has a slightly great chance of being passed into any offspring.

"Would tend to be" is not the same as "will be". Or maybe if you said, "All other things being equal..."

I know it doesn't mean "will be", it's not a guarantee. But I did provide a specific example of how that works immediately afterwards.

That's obvious, and I'm sorry to cause you to belabor the point. My reservations have little to do with understanding the general principle, but with agreeing to the ready conclusions drawn from it. You demonstrate the general principle, but then conclude something along the lines of, "thus those genetically enabled to survive longer will produce more offspring with the trait", which evolves into, "thus the genetic trait for survivability will eventually take over the herd."

I think my example demonstrates the process by which the gene for the particular trait would tend to become more common within the population.

But the process depends on certain conclusions besides the statements. It depends on whether the statements are general tendencies, or whether they are actual (and reproducible) in most cases. I expect that you will follow your statements, sooner or later, with, "So, you see, evolution theory is valid", without proving that all the statements are valid, or applicable in most cases.

How about a real-world example?

Elephants have large front teeth called tusks which they use for many things. Digging for water, stripping bark from trees for food, competing with other elephants for mates and social status. Over countless generations, natural selection has favoured those individuals with larger tusks - the individuals with larger tusks have had advantages hat help the reproduce. They find it easier to dig for water. They find it easier to strip bark. When they compete against other elephants, their large tusks make it more likely they will win.

But hunters have added a very strong selection pressure that selects AGAINST the genes that produce large tusks. They target the elephants with the largest tusks and kill them for the ivory. Since they target the animals with the largest tusks, these individuals are more likely to die young, and so have fewer chances to pass on the "large tusk gene" to their offspring.

As a result of this, the elephant populations are starting to evolve to have smaller tusks. This is a direct result of the selective pressure that the hunters are putting on the elephants. The gene that produces large tusks carries with it a much higher risk, while genes for smaller tusks carry a benefit - it makes the elephant with the "smaller tusk gene" less likely to be killed, and thus the smaller tusked elephants are more likely to produce more offspring simply by virtue of the fact that they are living longer than their large-tusked herdmates.

Elephants have evolved to be tuskless because of ivory poaching, a study finds
 
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Mark Quayle

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Glad we are agreed.

Statement 2 said that the genes for these traits can (note I said CAN, not WILL) be passed from parent to child when the parent reproduces.

For example. I have a daughter. She's got a full set of genes, and half of those genes she got from me, and the other half of her genes she got from her dad. If I have a gene that gives me some advantage, then there's a 50% chance that this advantage-giving gene was passed to her, since she got half of her genes from me.

Any reservations with this?
I have no idea. If I went according to my intuition, I'd say it was a 100% chance she got it from you, but only 50% chance she has that trait. From what I understand, the advantage-giving gene is present in both halves of a pair of chromosomes; half of each pair is passed to the offspring in the egg, and half in the sperm. The halves join to become a new pair, neither half of which is the sperm's nor the egg's, but a combination of both. But I don't know.

Obviously, if she has the trait, she inherited it from you if not from both you and her father. Also, if she has the gene, whether it expresses as a trait or not, she also passes that gene down to her offspring, so that your grandchild may have that trait.

Also, there is the terminology in there somewhere of 'dominant' and 'recessive', which has something to do with whether a trait is found in the offspring. As I said, I am ignorant in these things.
 
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Mark Quayle

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I know you weren't replying to me here, but I'll take this opportunity to say that the word "survivability" should really be taken to mean the gene's survival.

If a gene causes the individual that has it to have a slightly greater chance to reproduce, then that gene has a slightly great chance of being passed into any offspring.



I know it doesn't mean "will be", it's not a guarantee. But I did provide a specific example of how that works immediately afterwards.



I think my example demonstrates the process by which the gene for the particular trait would tend to become more common within the population.



How about a real-world example?

Elephants have large front teeth called tusks which they use for many things. Digging for water, stripping bark from trees for food, competing with other elephants for mates and social status. Over countless generations, natural selection has favoured those individuals with larger tusks - the individuals with larger tusks have had advantages hat help the reproduce. They find it easier to dig for water. They find it easier to strip bark. When they compete against other elephants, their large tusks make it more likely they will win.

But hunters have added a very strong selection pressure that selects AGAINST the genes that produce large tusks. They target the elephants with the largest tusks and kill them for the ivory. Since they target the animals with the largest tusks, these individuals are more likely to die young, and so have fewer chances to pass on the "large tusk gene" to their offspring.

As a result of this, the elephant populations are starting to evolve to have smaller tusks. This is a direct result of the selective pressure that the hunters are putting on the elephants. The gene that produces large tusks carries with it a much higher risk, while genes for smaller tusks carry a benefit - it makes the elephant with the "smaller tusk gene" less likely to be killed, and thus the smaller tusked elephants are more likely to produce more offspring simply by virtue of the fact that they are living longer than their large-tusked herdmates.

Elephants have evolved to be tuskless because of ivory poaching, a study finds
This might be 'moving the goalposts' but I mean it as an aside: Will the tuskless variety be considered a 'new species'? Where do obvious trends of change within a species, such as has been shown with certain flies and other species, become 'evolution' and not just reversible change? For that matter, if change appears to be irreversible, does that mean a new species?
 
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This might be 'moving the goalposts' but I mean it as an aside: Will the tuskless variety be considered a 'new species'? Where do obvious trends of change within a species, such as has been shown with certain flies and other species, become 'evolution' and not just reversible change? For that matter, if change appears to be irreversible, does that mean a new species?
It wouldn't be a new species, but the process is certainly still evolution, just on a smaller scale in scope and time.
 
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Mark Quayle

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It wouldn't be a new species, but the process is certainly still evolution, just on a smaller scale in scope and time.
So if it is change at all, to proponents of TOE all the small trends so far within species, are part of the origin of new species?
 
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So if it is change at all, to proponents of TOE all the small trends so far within species, are part of the origin of new species?

Such "small trends" as elephants that are now tuskless constitutes "evolution" but not necessarily "the origin of a new species."
 
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Mark Quayle

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Such "small trends" as elephants that are now tuskless constitutes "evolution" but not necessarily "the origin of a new species."
Yes, I understand that. I mean as they migrate from tuskless to longer toes and leaner bodies etc etc. When are dogs no longer dogs?
 
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DialecticSkeptic

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Yes, I understand that. I mean as they migrate from tuskless to longer toes and leaner bodies etc etc. When are dogs no longer dogs?

Ah, the Sorites paradox. Does a single grain of sand constitute a pile? No. What about two grains of sand? No. And yet at some point we do suddenly have a pile of sand. So, what number of grains was the threshold? There is a similar problem in evolutionary biology (image). We can have populations of a species forming a biogeographic ring—populations A, B, C, and D—where the two ends are different species (red and blue) and yet the linked populations interbreed (reddish blue and bluish red). Somewhere along this line of evolution was the origin of a new species. Where? I don't have an answer, but I do love the problem.

Edited to add: What would make it more obvious is genetic drift coming along and eliminating populations B and C. Now all we have are A (red) and D (blue) and it's just obvious that they're different species. No Sorites paradox. It seems to me that this is a bit like what we have in the fossil record. Populations A, B, C, and D all lived, died, and decomposed, but fossil remains from A and D were preserved (through some random natural disaster, let's say). Obviously different species, and we can infer their relatedness.
 
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Populations A, B, C, and D all lived, died, and decomposed, but fossil remains from A and D were preserved (through some random natural disaster, let's say). Obviously different species, and we can infer their relatedness.
How do you know B and C even existed, if they decomposed?
 
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Kylie

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I have no idea. If I went according to my intuition, I'd say it was a 100% chance she got it from you, but only 50% chance she has that trait. From what I understand, the advantage-giving gene is present in both halves of a pair of chromosomes; half of each pair is passed to the offspring in the egg, and half in the sperm. The halves join to become a new pair, neither half of which is the sperm's nor the egg's, but a combination of both. But I don't know.

Obviously, if she has the trait, she inherited it from you if not from both you and her father. Also, if she has the gene, whether it expresses as a trait or not, she also passes that gene down to her offspring, so that your grandchild may have that trait.

Also, there is the terminology in there somewhere of 'dominant' and 'recessive', which has something to do with whether a trait is found in the offspring. As I said, I am ignorant in these things.

Let me phrase it another way.

Let's say I have a gene for x-ray vision (we'll call it the X-gene), and let's say that my husband doesn't. When I fell pregnant with my daughter, she would have a 50% chance of getting the X-gene. We can't be sure until we either do a genetic test or we just wait and see if she can see through walls.

The dominant and recessive stuff is closely related to this topic. Eye colour is a good example. Genes can come in different versions, called "alleles." You have genes for eye colour, and two of the alleles are a brown eye allele, and a blue eye allele.

I have blue eyes, and my mother did too. My father has brown eyes. Brown eyes are a dominant trait. Each person has two copies of the gene for eye colour, one from their mother and one from their father. In the diagram below, a gene for brown eyes is denoted with a "B", while the gene for blue eyes is a "b".

With a dominant gene, if you have one copy, you will have the trait. With eye colour, brown eyes are the dominant trait. So, if your eye colour gene combination is Bb, then the B (being dominant), will override the b, and the person will have brown eyes.

In the first chart, both parents have one of each, so they both have blue eyes. Let's say the mother is along the top, and the father is down the side. You can see that both the mother and the father have Bb genes, so they each have brown eyes. They each have a copy of the blue eye allele, but since they also each have the dominant brown eye allele, that overrides the blue, and they each have brown eyes.

When they procreate, half of their genes each go to their offspring (the different ways the genes can combine are represented by the squares in the middle. In the top left square, we see what would happen if the mother (on the top) provides her B allele and the father provides his B allele. The offspring will have the gene combination BB, which is two genes for brown eyes. So their child in this case will have brown eyes.

But when we look at the top right square in the first example, we see that the mother provides a b allele, the recessive blue eye gene. But the father still provides his dominant B allele. With a gene combination of Bb, the offspring has a dominant brown eye gene which overrides the blue eyed allele, and so the offspring will again have brown eyes. However, in this case they still carry an allele for blue eyes which might get passed along when they have children.

Pretty much the same thing happens in the lower left square, although this time the father is providing the b gene and the mother is providing the B gene.

In the lower left square, both parents provide the recessive b allele, and since this offspring only gets the b alleles, they will have blue eyes.

The second example is what happened with my parents (which is why I proposed that the mother is represented by the top). My mother had blue eyes, so she could not have had a B allele (since if she did, it would have given her brown eyes, since it is the dominant version). So she must have had the bb combination. I can also figure out that my father had a Bb combination. After all, if his combination was BB, then I MUST have received a B allele from him, and since that is the dominant allele, I would have had brown eyes. So I must have got a b allele from him, plus one of my mother's two b alleles, thus giving me bb as well.

punnett_square_eyes_yourgenome.png
 
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Kylie

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This might be 'moving the goalposts' but I mean it as an aside: Will the tuskless variety be considered a 'new species'? Where do obvious trends of change within a species, such as has been shown with certain flies and other species, become 'evolution' and not just reversible change? For that matter, if change appears to be irreversible, does that mean a new species?

The definition of "species" is a rather difficult one. It's typically defined as a group of individuals who can produce fertile offspring. So all the different kinds of dogs are considered the same species, since you can breed a Dalmatian and a German Shephard and the puppies will be fertile themselves. But the example you used a while back with horses and donkeys producing mules, well, since mules themselves are sterile, then horses and donkeys are considered different species.

But it all depends on how closely related two groups are. Let's say you have a population, and they are divided for some reason. Let's say that a river changes course during a flood, and now you have two groups, one on each side of the river, and they can't interbreed any more. Each population will now evolve in their own way, and since conditions on the different sides may be different, the two different populations will evolve to suit the unique pressures they face, and thus they will gradually become more and more different.

If you come back ten years after the separation and take a female from the north side and a male from the south side, they'd probably still be able to interbreed. That wouldn't be enough time for them to evolve apart enough to make them different species. But if you come back a thousand years later, you could find that there's more trouble getting them to breed successfully. And if you came back a million years after, you'd probably find they wouldn't be able to interbreed at all.

But there's no one point where it stops being Species A and becomes Species B. Even if we look at a single population and consider individuals who are isolated not by space, but by time, we'd see the same thing. You could follow a group of animals and watch as they evolve over a million generations. G1 would be able to breed with G2 (and several generations down the line too). G15 would be able to interbreed with both G14 and G16. G165,287 would be able to interbreed with G165,286 and G165,288. But you would likely find that G165,287 could NOT breed with G1. Even though each generation changes only slightly, those slight changes all add up. Over a small number of generations, those changes are not enough to prevent successful breeding, but over many generations, those changes can be enough to prevent it.

So, to get back to your question, the tuskless variant wouldn't be considered a separate species, For that to happen, the tuskless elephants would need to be different enough from the tusked elephants that interbreeding could not produce fertile offspring.
 
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DialecticSkeptic

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How do you know B and C even existed, if they decomposed?

It was a hypothetical scenario to illustrate the point I was trying to make. I'm explaining a concept, not making a specific knowledge claim.
 
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It was a hypothetical scenario to illustrate the point I was trying to make. I'm explaining a concept, not making a specific knowledge claim.
Only on paper, I take it?

Please note: Just in case you're wondering where I'm coming from with this, here's an excellent example:

maxresdefault.jpg


Notice the blue lines on that chart?

Those blue lines gloss over more missing links than Carter has liver pills.

It makes macroevolution look nice and neat and flowing smoothly.

But truth be told, evolution is nothing more than a game of connect-the-dots that looks good only on paper.
 
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