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Speciation and natural selection

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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.


Genetic bottlenecks

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.





Hybrid sterility

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.


Natural selection

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 predationdisease 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.


Speciation


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.

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