It has been claimed by some evolution deniers here that the science of genetics provides no known mechanism for speciation. Here I will refute this claim by providing one out of many many genetic mechanisms by which speciation happens.
For living sexually reproducing species, reproductive barriers that prevent males and females from mating and producing viable offspring is a good measure for distinguishing species. Two groups of organisms retain their distinctive biological traits if hybrid sex has low rates of fertilization and/or hybrids have low fitness and die off.
In oceans many fish and invertebrates mate by external fertilization. That means that during breeding season they converge in certain places and discharge their eggs and seen in the water where they freely mix and fertilize. In coral reefs, many similar kinds of mollusc, sea urchins etc live together and retain their distinctiveness despite the fact that various group's semen and eggs are all floating in the same medium. So the scientific question is, how does this happen.
So here is a general mechanism.
The egg cells have surface that have receptor proteins. The set of proteins that are found in one species is different from those found in similar but different species. The male gamete of the compatible species has detector proteins on their surfaces that selective identify a certain set of egg receptor proteins, and when detected, the male gamete swim towards the egg cells. If the male and female receptors are compatible enough, the two cells are able to bind on the surface. Afterwards, the male sex cell releases a chemical that dissolves a small section of the female egg cell wall so that it's contents can enter. Then fertilization can occur.
It is found that for very similar species of sea urchins or molluscs, it's the receptor proteins that are most different making it more difficult for the male gamete of one species to lock onto the female gamete of another. The rest of the DNA is sufficiently compatible that if such a locking on were to occur, successful fertilization would have occurred (verified in lab). Thus the divergence of receptor proteins, or specifically the divergence of genes coding for these proteins in males and females, is an isolation mechanism for speciation in these groups of marine invertebrates.
Now, let's first look at evidence that the genes that code for these surface protein do indeed diverge by selection. Consider the females of two closely related species of mollusc. Both have genes that code for slightly different variations of receptor proteins on their surface, coming from the same protein family but having somewhat different structure. Because these genes are doing the same thing for these two species and making similar types of proteins being expressed in the same way, these genes are called homologous to one another. Each of these similar proteins are made of chains of amino acids, and since the proteins are similar, many of the amino acids are the same, but a few key ones are different.... and these stretches of different amino acids are caused by DNA letter triplets that differ from each other in the females of the species. Since the genes are few, scientists have looked at these genes and quantified the amount of such divergent codons (a DNA letter triplet) in the females of these related species.
So we know that these proteins diverge and we know that the codons that cause them to diverge. But why would we think that this is caused by natural selection. After all, changes due to mutations that flip DNA letters happen all the time and such changes would naturally accumulate causing these proteins to alter over the generations without any selection. Why would we think that due to the impact of natural environment, some of these variations are being preferentially selected for and other variants selected against as some have greater reproductive success than others?
Well, it turns out that there is an easy way to check this. 4 dna letters in pairs of three make 64 possible combinations. But life has only 24 amino acids to code. Thus many of the amino acids are coded by 2 or more different codons triplets. So those mutations that change a codons to one of its synonyms do not change the amino acid, has no impact on the protein and makes no difference to the cell whatsoever. They are called synonymous mutations. The others that do change the amino acid are called non_synonymous mutations.
Now in a DNA, A can shift to G and C to T and vice versa. Of the codons in a gene, one can therefore calculate what fraction of possible letter changes will be synonymous and what fraction nonsynonymous. Then one can calculate what fraction of the actual observed changes are synonymous or nonsynonymous.
The ratio of the actual number of non_synonymous mutations per nonsynonymous site vs the actual no of synonymous mutations per synonymous site is called the dN/Ds ratio. For homologous genes for two species
If dN/Ds = 1 then redundant mutations have occurred at the same rate as divergent mutations and the gene or its protein are not under selection pressure.
If dN/Ds > 1 then divergent mutations that change protein sequence are being actively selected for. New protein structures are arising that improve fitness and these are being favored by natural selection. This is called directional selection.
If dN/dS <1 then divergent mutations are being actively selected against. The gene and its protein has near maximal fitness and any change reduces fitness and being selected against. This is called purifying selection.
When this ratio is calculated for the genes that code for these surface proteins of closely related species, scientists see that they have dN/dS ratios quite higher than 1. This shows that selection is indeed operating and it is driving these protein sequences further apart by directional selection. The effect of this is the growth of barriers for gene flow, increasing reproductive isolation and speciation.
Thus scientists have demonstrated the mechanism of speciation for these invertebrate groups.
In the next post I will post research links and also the reason why selection is diversifying these protein receptors causing species to split.
Questions.?
For living sexually reproducing species, reproductive barriers that prevent males and females from mating and producing viable offspring is a good measure for distinguishing species. Two groups of organisms retain their distinctive biological traits if hybrid sex has low rates of fertilization and/or hybrids have low fitness and die off.
In oceans many fish and invertebrates mate by external fertilization. That means that during breeding season they converge in certain places and discharge their eggs and seen in the water where they freely mix and fertilize. In coral reefs, many similar kinds of mollusc, sea urchins etc live together and retain their distinctiveness despite the fact that various group's semen and eggs are all floating in the same medium. So the scientific question is, how does this happen.
So here is a general mechanism.
The egg cells have surface that have receptor proteins. The set of proteins that are found in one species is different from those found in similar but different species. The male gamete of the compatible species has detector proteins on their surfaces that selective identify a certain set of egg receptor proteins, and when detected, the male gamete swim towards the egg cells. If the male and female receptors are compatible enough, the two cells are able to bind on the surface. Afterwards, the male sex cell releases a chemical that dissolves a small section of the female egg cell wall so that it's contents can enter. Then fertilization can occur.
It is found that for very similar species of sea urchins or molluscs, it's the receptor proteins that are most different making it more difficult for the male gamete of one species to lock onto the female gamete of another. The rest of the DNA is sufficiently compatible that if such a locking on were to occur, successful fertilization would have occurred (verified in lab). Thus the divergence of receptor proteins, or specifically the divergence of genes coding for these proteins in males and females, is an isolation mechanism for speciation in these groups of marine invertebrates.
Now, let's first look at evidence that the genes that code for these surface protein do indeed diverge by selection. Consider the females of two closely related species of mollusc. Both have genes that code for slightly different variations of receptor proteins on their surface, coming from the same protein family but having somewhat different structure. Because these genes are doing the same thing for these two species and making similar types of proteins being expressed in the same way, these genes are called homologous to one another. Each of these similar proteins are made of chains of amino acids, and since the proteins are similar, many of the amino acids are the same, but a few key ones are different.... and these stretches of different amino acids are caused by DNA letter triplets that differ from each other in the females of the species. Since the genes are few, scientists have looked at these genes and quantified the amount of such divergent codons (a DNA letter triplet) in the females of these related species.
So we know that these proteins diverge and we know that the codons that cause them to diverge. But why would we think that this is caused by natural selection. After all, changes due to mutations that flip DNA letters happen all the time and such changes would naturally accumulate causing these proteins to alter over the generations without any selection. Why would we think that due to the impact of natural environment, some of these variations are being preferentially selected for and other variants selected against as some have greater reproductive success than others?
Well, it turns out that there is an easy way to check this. 4 dna letters in pairs of three make 64 possible combinations. But life has only 24 amino acids to code. Thus many of the amino acids are coded by 2 or more different codons triplets. So those mutations that change a codons to one of its synonyms do not change the amino acid, has no impact on the protein and makes no difference to the cell whatsoever. They are called synonymous mutations. The others that do change the amino acid are called non_synonymous mutations.
Now in a DNA, A can shift to G and C to T and vice versa. Of the codons in a gene, one can therefore calculate what fraction of possible letter changes will be synonymous and what fraction nonsynonymous. Then one can calculate what fraction of the actual observed changes are synonymous or nonsynonymous.
The ratio of the actual number of non_synonymous mutations per nonsynonymous site vs the actual no of synonymous mutations per synonymous site is called the dN/Ds ratio. For homologous genes for two species
If dN/Ds = 1 then redundant mutations have occurred at the same rate as divergent mutations and the gene or its protein are not under selection pressure.
If dN/Ds > 1 then divergent mutations that change protein sequence are being actively selected for. New protein structures are arising that improve fitness and these are being favored by natural selection. This is called directional selection.
If dN/dS <1 then divergent mutations are being actively selected against. The gene and its protein has near maximal fitness and any change reduces fitness and being selected against. This is called purifying selection.
When this ratio is calculated for the genes that code for these surface proteins of closely related species, scientists see that they have dN/dS ratios quite higher than 1. This shows that selection is indeed operating and it is driving these protein sequences further apart by directional selection. The effect of this is the growth of barriers for gene flow, increasing reproductive isolation and speciation.
Thus scientists have demonstrated the mechanism of speciation for these invertebrate groups.
In the next post I will post research links and also the reason why selection is diversifying these protein receptors causing species to split.
Questions.?
