Human earth shapers/ETHS101/Ecology and Evolution/Concepts of Evolution

Concepts of Biological Evolution

This section is about biological evolution. In fact there are other important types of evolution. Cultural evolution is very important - how did all our attitudes, cultures and technologies come into existence? The answer is that they evolved, though they used a different mechanism from biological evolution. Instead of being carried by genes, culture is transferred from generation to generation by speech, writing and images. Also it is very directly under the control of logical thought (though sometimes that is hard to believe). Geologists talk about geological evolution - how did continental drift, and the burying, folding, faulting and weathering of rocks create the Tasmania we know now? However, this section only explains what we often think of as evolution - biological evolution.

What is basic evolution

There is a popular view that Darwin invented the idea of evolution with his book The Origin of Species, written in 1859. In fact, our understanding of evolution itself has evolved, starting long before Darwin and still continuing. Darwin’s work was important, but the underlying mechanisms only came to be understood in the second half of the 20th century, when genetics came of age as a science.

Evolution can be explained in this way: In its broadest sense, biological evolution is any change in the genetic code of organisms. Evolution of new species just reflects sufficiently large changes in that code. An organism is constructed following the code defined by that organism’s DNA (or in the case of some viruses, a related molecule called RNA).

What is DNA and How Does it Work?

How does the DNA code work?

In summary: DNA is molecule made up of two long, parallel chains of four kinds of components called bases or nucleotides (guanine, adenine, thymine and cytosine – called G, A, T and C for short). The code is defined by the order in which the bases occur. This is called the “sequence”: for example “...AGTCTT...” could be part of a sequence. The codes for species are extremely complex: humans have a code over 3 billion nucleotides long.

In effect, the code is a set of rules that, along with the influence of many features of the environment (such as how much food is available and how good it is), determine what the organism is like. Genes are sections of the code that control certain characteristics of the organism. Typically, genes do this by determining what kind of protein is produced. This matters because the proteins control the structure and function of all organisms. So, one major reason that you look (and function) a bit differently from your sister, quite a lot differently from a chimpanzee and very differently from a cane toad is that your proteins are slightly different from those of your sister, quite different from those of a chimpanzee and very different from those of a cane toad.

The code is carried from generation to generation (it is inherited) by complex processes that allow different organisms to have different codes. For this argument, the first important process is that the DNA is copied each time a new cell is formed, and this process is imperfect – the new copy can a different code. When the new copy is different, it is said to have mutated1. This is critically important to evolution because mutations are the building blocks for change. Sex is an important part of how this inheritance works – the process of sexual reproduction involves random mixing of genes from each parent. This means that the offspring can have different combinations of genes from either of their parents. This in turn means that evolution can operate not only on the individual genes but also on combinations of genes.

We’re all mutants, yes indeed – each of us has very large numbers of mutations. A few are beneficial, a larger number are harmful, but most seem to have no effect.

How does evolution work then?

This is where Darwin’s dangerous idea of evolution by natural selection comes in. So far this discussion shows that chance is an important part of evolution – the changes in genetic code are random (or nearly so). So why isn’t evolution random? Darwin’s idea explains this by showing how the chance events of mutations are exploited. Darwin didn’t know about DNA or genes, but we still invoke his idea as one of the most important explanations for large evolutionary changes, such as the creation of new species and the appearance of important features such as eyes, lungs and brains.

Darwin argued that if children (offspring) inherit characteristics of their parents, and that not all children are successful (i.e. they get to produce children of their own), then, through time, the characteristics of the unsuccessful offspring will become less common and the characteristics of the successful children will become more common. Over enough time, these changes accumulate so that new species and new structures evolve.

Charles Darwin - The Theory Of Natural Selection

However, I’d like to add a few more ideas to those in the video. First, competition is a big part of evolution. Consider the idea that some people have genes that give them long legs and other people have genes that give them short legs. If people with long legs have more children than short-legged people in the same area, then there will usually be more children with genes for long legs. This would mean that people will evolve longer legs. If the reason that people with long legs have more children is because they can run faster to the kitchen table and therefore get more breakfast then we would say that the long-legged folk have outcompeted the short-legged individuals.

One important extra idea is that evolution isn't just about being bigger, faster, stronger, or even being better at getting food. These all matter, but sexual selection is also important. This is where an organism that can attract more mates (or better mates) has more offspring. Whatever characteristics that allow that organism to attract more mates will be favoured by evolution even if they may not be advantageous in terms of getting food, etc.

The final idea of importance is that people have been causing evolution. The breeding of new varieties of plants, animals and bacteria (including genetic modification) are all forms of evolution - the only difference is that humans are the agents performing the selection. People are also unintentionally involved in evolution - the evolution of drug-resistant bacteria is a simple example.

The Evolution of Bacteria on a “Mega-Plate” Petri Dish (Kishony Lab)

The Evolution of Bacteria on a “Mega-Plate” Petri Dish (Kishony Lab)