Unit 2: The Variety of Living Organisms

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Investigating Variation 

One look around you and it is clear that living things differ.

If one species differs from another this is called interspecific variation.

But members of the same species also differ from each other. This is called intraspecific variation.

Everyone of the billions of organisms on planet Earth is unique. Even identical twins, who are born with the same DNA, vary as a result of different experiences.

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Making Measurements


Sampling involves taking measurements of individuals, selected from the population of organisms which is being investigated.

In theory, if these individuals are representative of the population as a whole, then the measurements can be relied upon.

But, there are reasons why the data collected may not be representative:

Bias: The selection process may be biased. The investigators may be making unrepresentative choices, either on purpose or by accident.

Chance: Even if bias is avoided, the individuals chosen may, by pure chance, not be representative.

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Making Measurements

Random Sampling 

The best way to prevent bias is to eliminate as much human involvement in choosing the samples. This can be done by carrying out random sampling.

1) Divide the study area into a grid of numbered lines.

2) Using randon numbers, create coordinates.

3) Take samples from the intersection of each pair of coordinates. 

We cannot completely remove chance from the sampling process, but we can minimise it's effect.

1) Large sample size - The greater the same size, the more reliable the data will be.

2) Analysis of data collected - Carrying out tests allows you to decide whether the variation is a result of chance or other possible cause.

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Causes of Variation

Genetic and Environmental Causes 

Variation is the result of two main factors, genetic differences and environmental influences. In most cases it is a combination of both.

Genetic Differences: Occur due to different genes that individual organisms posses. They may change from generation to generation. This happens as a result of:

1) Mutations - Sudden changes to the genes and chromosomes, may be passed on to the next generation.

2) Meiosis - This nuclear division forms the gametes. This mixes up the genetic material before it's passed into the gamete, all which differ.

3) Fusion of Gametes - In sexual reproduction the offspring inherit characteristics of each parent. Which gametes fuse together at fertilisation is random, so adds to the variety.

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