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Darwin believed that individuals that are poorly adapted to their environment are less likely to survive and reproduce. This means that their genes are less likely to be passed to the next generation. Given enough time, a species will gradually evolve.

-individuals in a species show a wide range of variation this variation is because of differences in their genes

-individuals with characteristics most suited to the environment are more likely to survive and reproduce. The genes that allow these individuals to be successful are passed to their offspring.

In genetic terms, evolution is any change in the allele frequency of a population.

A recessive allele can become represented in a significant number of individuals in a population, how?

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A recessive allele is one whose phenotype can be masked by a dominant allele. A recessive allele is not necessarily deleterious, and the dominant allele does not necessarily have higher fitness than the recessive allele. Fitness and dominance are often confused, but they are not the same thing.

In some scenarios, the recessive allele indeed confers a fitness benefit, and when it does, it is predicted to increase in frequency rapidly because homozygous recessive individuals have higher fitness than both homozygous dominant genotypes and heterozygotes.

A recessive allele can also persist in a population if it is deleterious in homozygotes but confers a heterozygote advantage. The most famous example of this phenomenon is in Africa, where individuals heterozygous at the sickle cell locus have a fitness advantage because they are resistant to malaria. A higher prevalence of sickle cell phenotypes occur because selection for heterozygotes maintains a high frequency of recessive alleles in the population. For more, see Templeton (1982, http://www.indiana.edu/~curtweb/L567/readings/Templeton.pdf) and Allison (1954, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2093356/).

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    $\begingroup$ There is also effectively no selection pressure against recessive alleles that are deleterious when they are at a low frequency: selection pressure will only occur when mating two individuals that have the recessive allele. It isn't necessary to invoke heterozygote advantage to explain that part (though in some situations, like you point out, it does contribute). $\endgroup$
    – Bryan Krause ♦
    Aug 4, 2017 at 2:10
  • $\begingroup$ @BryanKrause Perhaps you should put that in an answer since, well, it is the answer. The fitness of a homozygous dominant individual and a heterozygote is identical by definition. $\endgroup$
    – canadianer
    Aug 4, 2017 at 4:52
  • $\begingroup$ @Nathan When it comes to fitness as the phenotypic trait of interest, then the sickle cell locus does not show a simple pattern of dominance. There is overdominance (aka heterozygote advantage). $\endgroup$
    – Remi.b
    Aug 4, 2017 at 17:25
  • $\begingroup$ @canadianer The fitness of a homozygous dominant individual and a heterozygote is identical by definition. Yes, if you are talking about dominance for fitness. It is not uncommon to consider some other phenotypic trait though (coat color for example) $\endgroup$
    – Remi.b
    Aug 4, 2017 at 17:26
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Dominance

Dominance explains the relationship between alleles in determining the phenotype. Consider two alleles A and a. If A is dominant, then individuals AA and individuals Aa present the same dominant phenotype while individual aa present the recessive phenotype.

Selection and dominance

When you ask

A recessive allele can become represented in a significant number of individuals in a population, how?

I am not sure why it seems unlikely to you. No one said that the recessive phenotype was less fit than the dominant genotype.

If the alleles a and A are present at the same frequency in the population, then most individuals will carry the dominant phenotype. But the alleles A and a may well not be at the same frequency. Selection (as well as other evolutionary processes that I won't talk about here) will affect the frequencies of these alleles. If the dominant phenotype has a higher fitness than the recessive phenotype, then the allele A will increase in frequency. If, on the other hand, the recessive phenotype has higher fitness than the dominant phenotype, then the allele a will increase in frequency.

From allele frequency to genotype frequency

Let $p$ and $q$ be equal to the frequencies of the allele a and A, respectively, such that $p = 1-q$ is necessarily true. Under a few assumptions (see here), the frequencies of the genotypes AA, Aaand aa are $p^2$, $2pq$ and $q^2$, respectively. For more information on why this is true, you can have a look at Solving Hardy Weinberg problems.

Let's imagine a scenario where, the recessive phenotype has a higher fitness. The allele a will increase in frequency in the population until eventually reaching a frequency of $p=0.95$. At such allele frequency, $0.95^2 = 0.902$ (or 90.2%) of the population will carry the recessive phenotype.

Note that at very low frequency of the recessive allele, the recessive phenotype is very rare (as it's frequency is the square of the frequency of the allele) and there is almost no selection acting on the locus of interest. But all of that is a story for another time.

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My answer below demonstrates how recessive alleles can fixate and persist in potentially large numbers of the population. https://biology.stackexchange.com/a/61176/9771

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  • $\begingroup$ If a question is a duplicate it should be closed as such. If it is not but the answer can be entirely found in an other answer, it is better IMO to partially copy-paste and re-adapt the answer to the current post than just providing a link. A link only is at best a comment, not an answer. $\endgroup$
    – Remi.b
    Aug 7, 2017 at 15:48
  • $\begingroup$ Ordinarily I'd have partially copied as you say, but as the other thread is figure heavy, I wasnt on my computer at the time, and there are other answers which will probably be accepted, I figured an answer was valid. $\endgroup$
    – Joe Healey
    Aug 7, 2017 at 18:24
  • $\begingroup$ Ok. Note, that figures are included as links on each post and are stored independently of the post. So, if you just copy paste the whole thing (incl. the links) you don't have to bother about figures more than about any other piece of text. There is no need to download, re-upload the figures which would indeed be a pain without a computer. $\endgroup$
    – Remi.b
    Aug 7, 2017 at 18:44
  • $\begingroup$ Ah yeah good point! I did think SE would reformat the link to the title of the question though. I thought it usually happened automatically but i must be missing something. $\endgroup$
    – Joe Healey
    Aug 7, 2017 at 18:47
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Recessive diseases are insidious because most of the time the dominant (healthy) allele masks them. Then every once in a while, someone in the village gets a wide range of chronic, unexplainable symptoms that the local shaman explains as a random curse.

While these are a no-brainer in the developed world, they are a daunting problem for primitive cultures and third-world nations where superstition is much more commonly practiced than science and no one knows what a "genome" is.

With even distribution, a quarter of the population has the disease and half of the population have 1 allele for the disease.

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    $\begingroup$ I only read the first sentence. 1. Dominant does not necessarily mean healthy 2. By definition the heterozygote shows the dominant phenotype. It is always true when you have a pure dominance relationship. It is not only true "most of the time". $\endgroup$
    – Remi.b
    Aug 4, 2017 at 17:21

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