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Causes of variation between individuals

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Variation is central to life itself. Variation is what makes every single individual... an individual. Without variation, the evolution of different species would not be possible. What is the cause of variation?


Genes

Genes dictate what proteins our body makes, when, where and how, and are ultimately responsible for the way we look and function. It's not just what genes we have though, it's also about whether they are switched on or off, when they are switched on or off, and in which particular cells they are switched on or off. Us and chimps share a great deal of our genotype (genetic makeup), more than 98%, yet we can agree that there are significant differences between us that make us distinct species.

Features such as height, predisposition to illness, height and weight, or even behaviours such as risk-taking have been linked exclusively or partly to certain genes.


Environment

The environment is not just what most people associate with the word. It's not just the temperature of the room you live in - it's everything outside our bodies which exert their influence on it. Which is pretty much everything from family to friends, to the internet and to diet, exercise, culture and beyond. The environment in the womb where we grew had an impact on our bodies. Even the environment of genes themselves has an impact on their activity.

Genes won't result in the variation seen in a professional body building contest. Those physiques were achieved by the manipulation of one's environment by doing certain activities.


A complex interplay between both

Genetic and environmental factors combined often account for a lot of variation. Twin studies are a popular method of isolating genetic factors from environmental factors and vice versa. Monozygotic twins share the same genes, while dizygotic twins do not. When an incidence of a certain condition, e.g. schizophrenia, is higher between monozygotic twins compared to the general population, it can be deduced that a genetic component is accountable for the condition.

Genes may be responsible for one's weight, yet environmental factors like lifestyle, diet, exercise or surgery can change someone's weight and either bring it closer to the mean, or further away towards the extremes.


Variation terms

Variation between members of the same species is called intraspecific variation. Members of different species are easily distinguishable as very different (interspecific variation), but variation is a key concept within the same species too. So if you were to collect samples of clovers in order to analyse their height, how would you go about making sure you don't happen to pick up all the tall ones or all the short ones? Before you've even analysed them, how would you know what the average height would be?

Chance plays an important role in sampling. Chance cannot be eliminated, but the probability of variation being due to chance can be decreased by collecting a large sample, and random sampling. In this case, the height of clovers would be considered continuous variation, as opposed to discontinuous. Continuous means that the clovers can have any height between certain values e.g. 2.1cm, 2.2cm or 2.25. Could one clover have a height of 2.24cm? Yes, that's perfectly possible.

An example of discontinuous variation is the number of toes. It's either 10, or more rarely a couple more or a couple less. So it could be 8, 10 or 12. Could someone have 12.35 toes? No.

Back to the clovers. You've got the data, and if you were to plot it, then what you'd get is a normal distribution curve. This means that most clovers will have about the same height, with a few deviating from the mean. The extent to which deviation from the mean occurs is called the standard deviation. This shows the spread of data around the mean.

The middle value, which is the mean, will have the highest frequency, whereas the extreme values i.e. midget clovers and gigantoclovers will have the lowest frequency.


The first graph would show a sample of clovers where the vast majority have a similar height, and only a few deviate from the mean. The second graph would represent the heights of a clover sample where most have a similar height, and a significant number deviate from the mean.


Competition, breeding success and survival

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.

Predation

Suppose you start off with equal populations of wolves and rabbits, and all the wolves rely on the rabbits for food. As the wolves start predating the rabbits, the rabbit population will decrease, while the wolf population will be sustained. Now there are fewer rabbits, so some wolves won't have any food left. These wolves will die, so the wolf population will decrease. What will happen to the rabbit population now? Well, there are fewer wolves so they are predated less. The rabbit population will increase, followed by an increase in the wolf population, and so on.

The predator-prey relationship is very intricate, so the two affect each other and hence their population sizes rise and fall accordingly.

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