A classic example of natural selection is the evolution of flowers. Over time, flowers have evolved different shapes, sizes, and colors. Some have even evolved to be fragrant.
Flowers have evolved in this manner because of the advantages it gives the plant. Pollinators are more likely to visit and transfer pollen from plant to plant if the plant is attractive.
The presence of colorful flowers also helps protect the plant from being eaten by herbivores. This is because animals are less likely to eat something that appears distasteful or harmful.
When looking at the genetic basis for flower color, researchers found three genes that influence pigment production. Two of these genes affect whether the flower is painted with red or white, while the third affects how bright or pale the color is.
Natural selection can act on these genes to change their frequency in a population. Consider this hypothetical population of wildflowers in which the frequency of the red allele Cr is P = 0.7.
What is the frequency of the cr allele?
The frequency of an allele refers to how often that allele appears in a population. Alleles can come in several forms, including dominant and recessive.
Dominant alleles are genes that, when present in a gene pair, automatically cause a trait or phenotype to appear. For example, the CC allele is dominant over the cr allele, so plants with the CC allele will have purple flowers regardless of the cr allele.
Recessive alleles are genes that require two copies to be present for a trait or phenotype to appear. For example, if a plant has two cr alleles, then it will have white flowers because the cr allele is recessive to the cc allele.
The frequency of an Aa offspring is 50% Aa and 50% AA. Therefore, if we know P = 0.7 and we choose an Aa individual at random from this population, then there is a 70% chance they carry one Cr-carrier gene and a 30% chance they carry two Cr-carrier genes.
What is the frequency of Cr heterozygotes?
In a diploid population, the frequency of carriers for red flowers is defined as the percentage of individuals that carry one Cr allele and one c allele.
Carriers for red flowers are individuals that have one Cr allele and one c allele. Since there are two alleles for the gene Cr, this population can be divided into three groups:
those with the Cr allele (Cr+), those with the c allele (c+) and those with both alleles (Crcci).
Since we know the frequency of the Cr allele is P = 0.7, then we can determine the frequency of carriers for red flowers by setting P = 1-0.7=0.3. This means that 0.3 or 30% of this population is carriers for red flowers.
What is the frequency of CC homozygotes?
In a population with mixed colors of flowers, the frequency of red alleles combined with white alleles is 0.7 × 0.7, or 0.49.
The frequency of CC homozygotes is then 0.49 / 2 = 0.24. There is a 75% chance that a wildflower with the Cr allele for red flowers will be white due to one copy of the C allele.
Now, consider a population in which all the flowers are red. The frequency of the Cr allele is P = 1, and the frequency of the c allele is P = 0. Therefore, there is a 100% chance that any given flower is red due to having only one copy of the Cr allele.
Therefore, there is a 100% chance that any given flower is red due to having only one copy of the Cr allele.
How does inbreeding affect this population?
Inbreeding is the process that occurs when offspring are produced from parents that are related. This can be through siblings mating, or offspring mating with a sibling or parent.
As mentioned before, plants inherit genes from their parents. When plants breed with a plant that shares similar genes, this is called inbreeding.
The problem with inbreeding is that it increases the frequency of alleles that cause negative traits. When siblings breed, they have a high chance of sharing the C R allele for red flower color.
As the frequency of this allele increases, so does the likelihood of plants having no red flowers. This would then shift the population to all white flowers, which is not very beautiful!
In order to prevent this from happening, some species have what is called social selection. This means that only the most attractive individuals are allowed to breed and pass on their genes.
What are some possible explanations for this pattern?
Some possible explanations include that red-flowered plants are more fit, or survive and reproduce better, than white- or pink-flowered plants.
If this were the case, then we would expect that plants with the red allele would have higher reproductive success due to more flowers per plant and/or more total offspring produced.
Another possibility is that the frequency of red alleles in the next generation is higher due to some non-genetic factors such as pollen contamination or differential seed germination rates.
If this were the case, then we would expect that there would be more red offspring in the next generation, even if all other conditions were equal. Both of these explanations point to external factors affecting genetic patterns.
What alleles would you observe in F1 offspring?
In the first generation offspring, or F1, you would observe only the red allele. All of the offspring would have the CR+ allele.
This is because in order to be CPCR+, an individual must have at least one C allele and at least one R allele.
Since all of the offspring have at least one C or R allele, they are all CPCR+. The P = 0.7 for the color red is maintained in this generation.
Notice that we did not say that all of the offspring were purple. Some of them could be white, but only those with at least one C allele would be classified as such. All of the ones with only R alleles would be classified as red.
The frequency of CPCR+ does not change in this generation.
What would be the genotype and phenotype frequencies for F2 offspring?
In the next generation, offspring are produced through cross-pollination. As described above, parents must contribute one allele for a gene to be passed to the next generation.
So, in this case, parents must contribute either the C allele or the Cr allele to produce F2 offspring. Parents that carry the C allele are called wildtype because they produce offspring that are not red; they are white.
Therefore, in the F2 population, there will be 70% wildtype and 30% red flowers. Once again, all of these plants are genetically identical except for one gene. This makes it easy to do genetic studies with this population!
Again, only 50% of offspring will be carriers of the Cr allele. The other 50% will be CC or CR and thus not carry the Cr allele.
Could you tell whether individuals are related just by looking at their phenotype?
No, you could not. Even if individuals appear very similar, you could not determine whether they are related or not.
The idea that we can tell whether individuals are related just by looking at their phenotype is called phenotypic assortative mating and it is a common assumption made in most models of sexual selection.
But this assumption is not true and we need to understand why or our models of sexual selection will be flawed.
In the case of red flowers in wildflowers, for example, it is entirely possible that two very similar-looking individuals could be completely unrelated. This would lead to what we call mixed mating pools–that is, there would be individuals from different families mating with each other. This would decrease the average relatedness between mates and increase the frequency of the rare allele Cr.
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