4. Adaptation, Natural Selection & Evolution

What you need to know...

    • A mutation is a random change to genetic material.

      • Mutations may be neutral, confer an advantage or a disadvantage.

      • Mutations are spontaneous and are the only source of new alleles.

      • Environmental factors, such as radiation and chemicals, can increase rate of mutation.

    • Variation within a population makes it possible for a population to evolve over time in response to changing environmental conditions.

    • Natural selection/survival of the fittest occurs when more offspring are produced than the environment can sustain.

      • Only the best adapted individuals survive to reproduce, passing on the genes that confer the selective advantage.

    • Speciation occurs after a population becomes isolated and natural selection follows a different path due to different conditions/selection pressures.

Source: SQA

Notes

What is evolution? That's one of those questions which you're pretty sure you know the answer to until someone asks you to really explain it. It's a pretty fundamental aspect of modern Biology, so it's crucial you understand it. Let's begin with what it's not...have a look at this fantastic video from TED-ED:

So, that helps us dispel some of the myths about evolution and even begins to explain what it is. But we need to outline evolution in a little more detail still. To help us with this, we'll break down this topic into three sections: Variation, Natural Selection and Speciation.

Variation

As you will already have gathered from the video above, variation is a key aspect of evolution. Not only are species different from each other (which we'll return to later) but members of a species are different from each other - as we've previously discussed when learning about sexual reproduction. But where does this variation come from? Sexual reproduction brings about new combinations of alleles, but where do alleles come from? Alleles remember are the term we use describe different versions of the same gene, how do these different versions come about? The answer is mutation. Mutations are changes in the DNA code which occur at random when the DNA is being copied before cell division. Some mutations have no effect whatsoever on the protein which is produced from the gene in question, however some do. In some cases the mutation can result in an altered protein which puts the organism which receives it at a disadvantage whereas others can actually provide an advantage to the organism which receives the mutated form of the gene. As already mentioned, the mutations themselves are random as are the effects they have on the organisms which receive them.

The following video explains variation in more detail:

Mutations in genes can occur spontaneously. In other words, they can and do happen at any time at random and without cause. However, some factors are known to increase the rate of DNA mutations such as radiation and chemicals. This webpage contains a great video on the factors which increase the rate of mutation.

Natural Selection

So, now you know where variety comes from, but what has this got to do with evolution? Well, variation within a population makes it possible for a population to evolve over time in response to changing environmental conditions. The mechanism by which this happens is called Natural Selection. Natural Selection is the way in which species have been changing since life began on Earth and it depends on the fact that most species produce more offspring than the environment can sustain. In other words, because of the large number of offspring produced, some will always die. Some of these deaths will be random, but some will be influenced by the alleles these individuals possessed. Alleles which decrease the likelihood of survival are less likely to be passed onto future generations as the individuals which possess them are less likely to survive to reproduction. However, alleles which make the organism better adapted to the environmental conditions provide a selective advantage to these individuals meaning that they are more likely to survive to reproduce and pass these alleles onto their offspring. Therefore, the environmental conditions a population experiences influences the frequency of alleles present in the population over time. This process is outlined in more detail in the following video.

The diagram below summarises the same process of natural selection and is based on the assumption that having a dark colour provides a selective advantage to the population in question.

Image from WikiMedia Commons

Peppered moths are a great example of natural selection and, as ever, the Crash Course guys have a great video which takes the explanation of natural selection even further.

Speciation

The process of natural selection described above explains how populations change over time, but not how new species arise. A species is a group of genetically similar organisms which are able to reproduce to produce fertile offspring. In other words, in order to describe two organisms as belonging to the same species they need to be able to breed and produce offspring which can themselves breed. The process of creating new species, or speciation, involves natural selection but requires something else also - a barrier. If two populations of the same species become isolated from one another, they will no longer be able exchange alleles. It is also likely if they are isolated that they will be subjected to different changes in environmental factors over time. This isolation can result in natural selection following a different path in each of the two populations. For some time they will remain members of the same species, but can eventually become so genetically different that they would no longer be able to reproduce to produce fertile offspring if brought together - at this point they would be described as separate species and speciation would have occurred.

The following video outlines Californian Salamanders as an example of speciation in action:

The Crash Course team have a video on speciation too.