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Natural selection

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Natural selection

In the wild, each species may exist as one population or multiple populations. Different populations correspond to defined areas - habitats.

The sum of all present alleles for a given gene in a given population is known as the gene pool.

This is essentially a way of thinking about all the individuals in a population contributing their alleles towards the overall allele frequency. The extent of different alleles present gives the genetic diversity of a population.

The allele frequency in a population's gene pool can change as a result of selection. The effectors of selection can be varied, yet the outcome is similar: advantageous or preferred alleles and the traits associated with them increase in frequency, while detrimental or disfavoured alleles and the traits associated with them decrease in frequency.

Here is an all-time classic example. The most frequent initial moth colour in a population landing on tree trunks was dark, to match that of the tree trunks. Few moths could get away with being light-coloured. Once the tree trunks were painted white, the former moths became very apparent to predators, and so the light-coloured moths evaded predation much better and survived to reproduce. Essentially, the tables had turned!

This resulted in the allele for light colour to spread and become the most frequent compared to that for dark colour. The latter sharply dropped in frequency and became the minority.

This is an example of directional selection. It tends towards an extreme, either the light-coloured or the dark-coloured, depending on scenario. 

Selection can also tend towards a "happy medium" and avoid either extreme. This is stabilising selection. If really small lions don't survive long, but really large lions can't supply themselves enough food, then the average lions are selected for and achieve the highest frequency.

Directional selection also takes place when antibiotics are used against bacteria. The adaptive pressure favours bacteria that have the antibiotic resistance gene and can survive the hostile environment.

On the other hand, a scenario such as human birth weight showcases stabilising selection. The average weight is large enough to keep the newborn healthy and increasingly able to survive independently, but small enough to enable the actual birth.

Natural selection therefore results in species increasingly and consistently adapted to their environment via anatomical, physiological or behavioural changes.

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.

Types of selection

We looked at stabilising and directional selection previously.

There is a third type called disruptive selection. Instead of shifting the traits towards an end, or towards a middle ground, disruptive selection splits the pool down the middle, where both extremes of a trait are favourable, but not a middle value.

An example of this is an original population of purple individuals which stand out quite a lot amongst red and blue flowers in a field. They will end up shifting towards either red or blue, but not staying purple as this attracts predators.
Little devil bats. Birds? Anyway.

Niches and adaptations

What is a population? A population is all the individual organisms found in a given habitat, of one species. So you could talk about a population of wolves in the woods. If you want to talk about the wolves and rabbits in the woods, then you'd be referring to a community. A community is made up of the various populations in a habitat. So the summation of all the living things in a given area is called a community. What then is an ecosystem?

An ecosystem comprises the community of living organisms in a habitat, together with all the non-living components such as water, soil, temperature, etc. called abiotic factors.

Why are different organisms of different species able to coexist in the same habitat? How come they don't directly compete with one another and drive others out? Have a watch...

So that's the last and loveliest new term: niche. It rhymes with quiche. A niche is the interaction, or way of life, of a species, population or individual in relation to all others within an ecosystem. It's how it behaves, what it eats, how it reproduces, where it sleeps, etc.; a species' niche is determined by both biotic factors (such as competition and predation) and abiotic factors.

Different things may determine the population sizes within an ecosystem.

Abiotic Factors

Non-living factors such as light intensitytemperature and humidity determine the number of organisms that a habitat can sustain. All species have a varying degree of ability to withstand harsh or fluctuating conditions, called resilience. If an abiotic factor changes dramatically in favour of a population - for example, plenty more light in a field - then the population will increase provided no other factors are limiting. The opposite is true if an abiotic factor changes against the resilience limit of a population - it will decrease.

Biotic Factors

"Living factors" refer to all interactions between organisms, be it a bunny rabbit being predated, or two shrubs competing for sunlight. All individual actions between organisms form a web which impacts on all populations in an ecosystem, therefore determining their sizes.

Interspecific competition refers to competition between members of different species for the same resources (food, light, water. etc.). Often when a new species is introduced in a habitat, say the American ladybird to the UK, if the invader species is better adapted, then the host population decreases in size. This may lead to extinction in some cases of the host species.

[Can't remember the difference between interspecific and intraspecific? Interspecific is like the internet - different things come together.]

Intraspecific competition refers to competition between members of the same species. If a population of apple trees all compete for a source of light, then each apple tree is taking up some light that has now become unavailable to a different apple tree. There are only so many apple trees which that habitat can sustain. The maximum population size sustainable indefinitely in a habitat is called the carrying capacity.

The niche a particular organism occupies depends on their physiological, behavioural, and anatomical adaptations.


What is at the heart of new species formation? It all starts with a single population of a species which for whatever reason (off-spec: 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.

Speciation due to an established barrier such as geographical separation is termed allopatric

Speciation can also occur in absence of a barrier. The individuals of a starting species can share the same physical space and be able to come into contact with each other, yet for other reasons subspecies can still separate within that population in what is termed sympatric speciation.

Sympatric speciation may occur as a result of different members of the former species occupying different niches within the same habitat. Perhaps they start feeding on different sources, behaving differently, having different mating signals, etc.

Evolutionary races in medicine

The use of antibiotics is a common example of how evolutionary arms races are critical in the development and deployment of medicines that target organisms. As long as some individuals in a targeted population are able to survive the antibiotic, or in time can develop resistance, under the selection pressure of antibiotic use an ever increasing resistant population will emerge.

This process can happen many times, as organisms are extremely versatile. Bacteria have already been subjected to many natural antibiotic attacks from other organisms (the original penicillin is produced by the fungus Penicillium) so they already have certain resistance genes or pathways they can develop when required.

The key is to understand the adaptation cycle of different organisms and use antibiotics effectively.

1. Not use antibiotics inappropriately, such as to treat colds (caused by viruses not bacteria)

2. Complete prescribed antibiotics treatments so bacteria are effectively killed and there is little to no chance of remaining bacteria coming back stronger

3. Avoid overuse of the same antibiotic in the same setting such as in hospitals where patients are susceptible to infection and spread can be rapid

4. Keep many different antibiotics archived, especially the strongest ones, so that they can be used against multi-resistant strains if they develop, to avoid a situation where no antibiotics are available that bacteria are susceptible to

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