“There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.” – Charles Darwin (1859)

Darwin published his book on the theory of evolution in 1859 and this was to become one of the most important works in recent history. However, although Darwin did great work in collecting data in support of the theory of evolution and explaining its elegance towards other scientist, Darwin wasn’t the greatest writer of history. His book “On the origin of species”, is not an easy read and I know more than one biologist who hasn’t succeeded in struggling through this book. The theory of evolution is a very powerful theory which leads to certain predictions and testable hypotheses, there are however also many common misconceptions about evolution. One of them is: “Survival of the fittest”. This means that any organism that is the most suitable for a certain habitat is most likely to survive. So not that the strongest individuals survive, which is something often heard. Here I will try to explain evolution in the simplest way possible by which I hope to take away some misconceptions and give more people the chance to enjoy its elegance.




Evolution – the ingredients

Evolution is not an entity; it is not caused by something or someone. Evolution is a process, the logical consequence of a number of factors. In order for evolution to happen there are some requirements, which I here call ingredients. So what are the ingredients for evolution? Let have a look:

  • Variation in phenotypes
  • Heritability
  • Selection

Variation in phenotypes

The first ingredient I am going to discuss is variation. A phenotype is any measurable aspect of an organism like: length, color, weight, energy consumption, personality and many many many more.

variation
The variation doesn’t have to be large, small differences can be selected for. Variation is often not one or the other, big or small, blue or green etc. A lot of variation is continuous, I for example am 190 cm tall, but people in general can be anywhere between 50 cm and 272 cm tall.

Heritability

Heritability means that differences in phenotypes should be inherited in their offspring. So tall people get tall children, green smilies get green offspring and big mouthed smilies get big mouthed offspring.

Heritability
This inheritance in organisms is possible due to DNA, for more information on DNA see: “DNA, RNA and protein”.

Selection

When we have a population of individuals which have both variation and heritability, the last ingredient necessary for evolution to occur is selection. Selection-pressures can vary widely too. But what is selection? Selection is when certain phenotypes (being taller, green, having a larger mouth and so on) give individuals with that phenotype a better chance of surviving and more importantly, breed!
Anything that causes differences in the amount of offspring for specific phenotypes is called selection. For example, people too small to reach the kitchen cabinet cannot reach food and so die of starvation before they have children. Tall people that can reach the cabinets can eat and have children. Tall people get tall children; dead people don’t get children, so there is a selective pressure to be tall.

Selection
Evolution in practice

So when we have the key ingredients, evolution can happen. But be warned! A single person or organism cannot evolve! Each phenotype occurs in a certain number in a population, that number is a percentage of the total population. For example, if in a population of 10 individuals 5 are green and 5 are blue than each phenotype occurs in 50% of the population. Evolution cannot change green individuals into blue individuals or the other way around. What happens is that due to selective pressures, the percentage of individuals with a certain phenotype changes over generations. If the selective pressure is strong enough than this will remove one of the phenotypes (blue for example) from the population and that phenotype will go extinct after enough generations.

So I am going to give an example of evolution now in the form of a movie. We will describe the population first. We have 10 individuals, 5 of them are blue and 5 of them are green. In each generation, 50% of the population gets eaten by a predatory bird. The surviving 50% will be able to breed and produce 1 offspring per individual. This will keep the total population to 10 individuals. The selective pressure in this case is the predator. The predator prefers to eat blue individuals, so this selective pressure is negative for blue individuals (they have a smaller chance of survival until they breed) and positive for green individuals (they have a larger chance of survival until they breed). So our ingredients for this case are:

  • Variation in phenotype -> Green / Blue Individuals
  • Heritability -> Green individuals get green offspring / Blue individuals get blue offspring
  • Selection -> Predator with a preference for blue individuals

Ok, let’s see what will happen:

So we see, in each successive generation the size of the total population stayed the same, but the percentage of blue individuals became less every generation due to a selective pressure (50% -> 40% -> 20% -> 0%). By identifying the 3 key ingredients of evolution we were able to create a population that evolved from a mixed population of blue and green individuals to a completely green population.

Of course, there are many much more complex situations than this highly simplified version. The population size doesn’t have to stay the same every generation, if blue individuals get twice the amount of children than green once do but are also eaten more often than both phenotypes will stay in the population. Maybe females prefer to mate with a blue individual instead than with a green individual (sexual selection). And we haven’t covered what would happen when a blue and green individual have children together. However, the basic principle stays the same.

I hope that I was able to get the very basics of evolutionary theory across. If anything is unclear or you would like to know more about a certain subject please let me know!

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