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I would like to understand evolution. Here are a few questions

  1. Why are there different alleles for a gene?
  2. Is the different alleles of a gene are mutated versions of a gene?
  3. Why selection pressure favoured the existence of more than one alleles?
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I would like to understand evolution

The best way to do so is to start an introductory course on evolutionary biology. Consider having a look at evo101 for example.

Why are there different alleles for a gene?

Because mutations bring variation. Note that not all genes are polymorphic. In other words, there might be genes for which, all individuals in the population share the same allele.

The existence of this genetic variance is absolutely central to the whole concept of evolution. In short, mutations increase genetic variance while natural selection and genetic drift reduce it.

Is the different alleles of a gene are mutated versions of a gene?

Yes

Why selection pressure favoured the existence of more than one alleles?

In the exception of cases of balancing selection (e.g. heterozygote advantage, negative frequency-dependent selection), selection does not yield to the existence of genetic variance, rather the opposite in fact. Selection "selects" for a given allele, which in turn reduces the genetic variance. Mutations bring new genetic variance in populations and selection reduces it. Genetic drift (another "driver of evolution") also reduces genetic variance.

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1. Why are there different alleles for a gene?

Variants of a gene are called alleles. In other words, an allele is one of the possible forms of a gene. If there is genetic variation in a population (and in all species, there is variation), then there are multiple alleles.

Evolution reflects changes in DNA sequences and allele frequencies within a species over time. These changes may be due to mutations, which introduce new alleles into a population. New alleles can also, for instance, be introduced in a population by gene flow, when two populations that carry unique alleles breed together.

2. Are different alleles of a gene mutated versions of that gene?

Yes.

3. Why does selection pressure favored the existence of more than one allele?

This question is unclear and/or poorly-stated. Are you asking why there is variation in a population? The answer is that, in spite of the fact that DNA replication (copying) is high-fidelity and quite accurate, it is imperfect. This means that over a long lineage of inheritance, new alleles will emerge, and their frequency in the population will change. Also, DNA accumulates damage and mutations occur due to chemical factors. DNA is not a perfectly stable molecule. Mechanisms of repair of DNA exist but are imperfect. It is impossible for a species to maintain perfect genetic identity, especially over prolonged periods of time, when mutations accumulate.

If you are asking why variation (or more strictly, diversity) is favorable for a species, here is an excerpt from the Wikipedia entry on genetic diversity, which explains it concisely:

Genetic diversity serves as a way for populations to adapt to changing environments. With more variation, it is more likely that some individuals in a population will possess variations of alleles that are suited for the environment. Those individuals are more likely to survive to produce offspring bearing that allele.

There is also an important difference between diversity and variability to consider. Diversity is the total number of genetic characteristics in a population. Genetic variability is the generation or presence of genetic differences between individuals of a population. Variability is introduced through migration events, homologous recombination during meiosis, and other things.

Addendum/protip: asking why is often less fruitful than asking how. The how is mutation. The why makes me want to include thermodynamics and statistical mechanics as an explanation, which is definitely an unnecessarily complicated explanation.

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  • $\begingroup$ The terms genetic diversity (aka. expected heterozygosity) and genetic variance are well defined. I had never seen the terms genetic variantion and genetic variability being stricktly defined. I have always used the term genetic variation is a loosely defined way. The definitions you're providing seem a bit unclear to me. Would you mind indicating your sources? $\endgroup$ – Remi.b Aug 22 '18 at 11:49
  • $\begingroup$ The vision that genetic variation is favorable to a species is neglecting the genetic load (typically mutation load and drift load) it is causing. $\endgroup$ – Remi.b Aug 22 '18 at 11:50
  • $\begingroup$ Sorry, I meant diversity and variability. Thanks for the fix. $\endgroup$ – S Pr Aug 22 '18 at 11:51
  • $\begingroup$ Oh... so the definition should rather be Genetic diversity is the fraction of individuals that are heterozygote in the population as expected by Hardy Weinberg rule or If 'p' is the frequency of one allele at a biallelic locus, then the genetic diversity is defined as 2p(1-p). When you say "variabiility", did you mean "variance" or does the term "genetic variability" has a commonly accepted definition I am unaware of? $\endgroup$ – Remi.b Aug 22 '18 at 11:59
  • $\begingroup$ See here and here and here for commonly accepted definitions of genetic variability. I disagree with your proposal to edit the definition; it does not clarify anything, or help pedagogically. Diversity is a simple concept in ecology and genetics and does not require rigorous analytical definitions unless the term is defined so in a rigorous, analytical context. As you can see, the question is far from advanced. $\endgroup$ – S Pr Aug 22 '18 at 14:05
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1.why there are different alleles for a gene?

alleles of a gene are mutated versions of a gene

why selection pressure favoured the existence of more than one alleles?

Reason

1) No reason. Allele is both not 'selected for' nor 'selected against'. The allele do not cause detriment or advantage. It is a neutral mutation. An example is ear lob, connected or not connected. It does not affect hearing. It builds up in a population by chance due to the founder effect.

2) It has an advantage but only in heterozygous form. Example Sickle cell anemia. In homozygous form this allele is quite lethal. However in heterozygous form (ie pair with the normal allele), it confers resistance to the malaria parasite. So selection does favour the sickle cell anemia allele but only to a point. Once the allele becomes too common in the population it is selected against, as the probability of homozygous individuals of sickle cell allele occurring becomes more common.

3) Because allele is selected for in some parts of organism's ecological range. Example human skin colour. Human range from poles of the earth to the equator. And thus are exposed to varying light intensity. At the equator, dark, almost black pigmentation is favoured as it protects against sun damage (ie sun burn) and skin cancer. Up in the temperate region, fair almost white skin is an advantage as it allows better absorption of the limited UV light for vitamin D and folic acid production, which is required for bone and limb development. Different light intensity selects for different levels of skin pigmentation. So we have several skin alleles.

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  • $\begingroup$ Does this mean that existence of unaffected alleles are example of silent mutation? $\endgroup$ – Kamal Aug 22 '18 at 11:05
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    $\begingroup$ There is no such thing as an 'unaffected' allele. Alleles are simply variants of genes. Some alleles are favorable, some are unfavorable, some are (un)favorable only in the presence of other alleles. Some have no effect on survival and reproduction, so there exist no active selective pressures. Silent mutations create new alleles which are functionally identical - i.e. the resultant protein is the same. However, even silent mutations may affect the way the DNA or mRNA behaves, or how it is regulated (e.g. how RNA interference may work through microRNAs) and thus its abundance in the cell. $\endgroup$ – S Pr Aug 22 '18 at 11:45

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