How could we keep track of the frequency of each allele for a given trait when we have a dominant-recessive interaction? More specifically, how could we account for the visible dominant traits as homozygous or heterozygous, since both look the same?
This is where the Hardy-Weinberg principle comes in. Firstly, there are criteria for when this principle may be applied to a population:
1. Random mating must take place.
2. No migration must occur either inwards or outwards of the population.
3. No mutations must arise in the population.
4. No natural selection must take place due to one trait being better or worse adapted to the environment.
It’s apparent that this is simply rarely, if ever, the case in a real wild population. However, the Hardy-Weinberg principle is useful at predicting allele frequencies in a reliable mathematical model.
The frequency of the dominant allele is noted p while that of the recessive allele is noted q. Both must necessarily account for the whole population, therefore:
p + q = 1
The values are frequencies, so they are noted as percentages. 1 is 100% while 0.5 is 50% and 0.05 is 5%, etc.
If we know that the frequency of the allele for dark fur in a population of koala bears is 0.2, and this allele is dominant over the one for light fur, work out the frequency of the allele for light fur in the population.
p = 0.2
p + q = 1
Therefore, 0.2 + q = 1 so q = 1 – 0.2
q = 0.8 or 80%.
Now the allele frequency has been worked out, how could we work out the actual phenotype of the koala bears in the population. How many are actually dark-furred? How many of the dark-furred ones are homozygous?
For this we use the same equation as before, but squared: (p + q)2
This is equivalent to p2 + 2pq + q2 = 1
Where 2pq is the frequency of heterozygotes, and p2 and q2 the frequencies of homozygous dominant and homozygous recessive respectively.
We want to know how many koala bears have dark fur. We know that the allele frequency for dark fur is 0.2, so 0.22 is the percentage of homozygous dark fur individuals; = 0.04 (4%).
This trait being dominant, the heterozygotes must also have dark fur. The frequency of heterozygous dark fur is 2pq = 2*0.2*0.8 = 0.32 (32%).
So overall, there are (0.4 + 0.32) 0.36 or 36% dark-furred koala bears in the population.
This leaves the remaining 64% with light fur. Note the contrast between the light phenotype only being 64% while the allele frequency for light fur is 80%. If the allele were dominant over dark fur, the frequency would be higher rather than lower.The founder effect
Suppose a boat travelled from one island to another. In the process, several lizards were transferred from the first island to the other. The lizards breed and settle down to form a new lizard population on their new island. This is called the founder effect. The small number of founding lizards formed the genetic base on which the whole population was built. This genetic base is significantly smaller than that of the original lizard population on the first island.
Therefore, the genetic diversity of the new population is lower than that of the original population.
The only difference between the founder effect and genetic bottlenecks is the way in which the new genetic pool is formed. In the founder effect the new pool is formed when a few individuals from a population become geographically isolated, while in genetic bottlenecks the new gene pool is formed when only a few individuals from a population survive a mass disaster, or are the only ones to breed.
The effect is the same: the genetic variation of the new population is decreased compared to the original population.
Separation presents itself over time in terms of geography as well as behaviour, morphology and even reproduction. In the event of two already-separated populations that have diverged over time, coming back together in the form of two respective individuals attempting mating, the ability for them to have offspring, or viable offspring, may be compromised.
They may be successful in having offspring, such as ligers or mules, but they themselves aren’t fertile. This can be due to a huge variety of changes affecting different levels of development: genetic makeup of gametes, ability to fertilise, embryonic development, adult development, and finally viability of adult in terms of being fertile to have their own offspring. This comes full circle, and shows at how many levels this compatibility can diverge out, if two ancestrally connected populations are separated for a certain period of time.
This is an example of divergence at the genetic (chromosomal) level. DNA can be condensed or decondensed, and DNA-binding proteins can fail to work as intended as a result.
The train of thought leading to natural selection includes these key points:
1. Individuals within a population exhibit variety of phenotypical traits caused by both their alleles and the environment.
Primarily the source of this variation is mutation. Secondarily it is meiosis and the random fertilisation of gametes in the case of sexual reproduction.
2. The balance of survival and reproduction is affected by factors including predation, disease and competition. Some appearances and behaviour can attract more predators while others such as camouflage can avert them.
Disease can impede survival and reproduction, while competition enables hidden traits that might have gone unnoticed or been “neutral” before to come in handy when unforeseen selection pressures arise. If the positive outcome of such competition, such as resources needed for survival, are limited relative to the population seeking them, then competition acts further to select certain traits.
3. Any favourable traits controlled by alelles will end up in more offspring, thereby shifting the alelle frequency and over time, the entire gene pool of a population or species.
What is at the heart of new species formation? It all starts with a single population of a species which for whatever reason (genetic bottlenecks, founder effect, etc.) ends up being split geographically to the point where no interbreeding occurs for a certain length of time.
Given that the two habitats are different, the individuals in each population will adapt differently to counteract different selection pressures. Say for example the ants in the forest experience a warmer and more nutrient-rich surrounding compared to the emigrated ants on a nearby, although disconnected, beach.
The adaptations acquired by both populations over a long time will get increasingly disparate. When these pass a threshold, the two populations can no longer interbreed, even if the opportunity were given (due to excessive genetic difference). They have now become separate species! This process is called speciation.
This process alongside natural selection brings together Darwin’s idea about new species emerging via selection over time from ancestral species.