It is hard to speak of evolution without inadvertently attributing to it intent. I might say, ‘beetles evolved to fly’, which sounds like the beetles had a choice in the matter. Of course they didn’t. Or I might say, ‘evolution guides us’, or ‘evolution wants us’. No, evolution can’t guide us or want us to do anything; it’s not a sentient entity, but a process.
We use these expressions because they are convenient shortcuts to refer to the process of natural selection (evolution).
Here are two quick (and basic) examples to explain how evolution works: A few million years ago, during a drought in an African savannah, a giraffe is born with a slightly longer neck and it can reach leaves other giraffes cannot. This giraffe is more likely to survive and pass on its genes. Over time all giraffes end up with the gene, and longer necks. Evolution has ‘guided’ a physical change.
A monkey born with the propensity to break nuts and eat the kernels has an abundant food source, and is more likely to survive the drought and pass on its genes. Over time, all its descendants have the propensity to break nuts and eat the kernels. Evolution has ‘guided’ a behavioural change.
Another giraffe might be born with genes giving it an extra leg. This giraffe has trouble running fast and is easily caught by a lion, and doesn’t live long enough to pass on its genes. Therefore, giraffes with five legs haven’t evolved.
So, although gene mutations are random, over generations the mutations beneficial to the species can become normal to the species. That’s evolution. It is generally thought that most, if not all, physical features and behavioural traits of organisms evolved in this way. That’s why, for instance, an octopus can change colour in a second.
‘How do different species come about?’
Let’s say that on one side of a mountain range there are frequent famines and the giraffes evolve long necks, but on the other side there is plenty of rainfall and they have plenty to eat. A giraffe born on that rainy side with a longer neck will have no advantage, so that mutation isn’t favoured. (A giraffe born with a longer neck might have offspring with a longer neck, but that offspring will mate with giraffes with normal necks. As the giraffes with normal necks are faring well, the ‘long neck gene’ provides no advantage. Outnumbered by all the ‘normal neck genes’, in time the giraffe population will revert to normal sized necks.)
Over tens of thousands of years, with hundreds of famines and different geography and conditions, the two groups of giraffes will develop so many differences that if you were to bring them together and mate them, they could not produce fertile offspring. The two types of giraffes would have become separate species.
The giraffe’s closest extant (living) relative is the okapi, which lives in the Congo rainforest.
‘What is a subspecies?’
Imagine a river that becomes full of crocodiles and isolates a thousand giraffes on a large island. These giraffes can’t get back to the mainland to join the others, so the two groups can’t interbreed. Over time, the island giraffes develop their own characteristics (such as different markings or shorter necks).
If these island giraffes were transported to the mainland and were able to interbreed successfully with the mainland giraffes, they would be the same species. However, because of their different characteristics the island giraffes (the minority) would be a subspecies.
‘Lizards are different to giraffes. How can they be related?’
Their common ancestor goes back more than a hundred million years.
‘An eel-like creature from 505 million years ago was a forerunner to all vertebrates, from fish to humans. Fossil evidence confirms that Pikaia gracelens had a rod of elastic tissue running along its back, making it the oldest chordate ever found.’ New Scientist, 10 March 2012.
‘Did humans evolve from gorillas and chimpanzees? Or monkeys?’
No, but we share a common ancestor. There were creatures that over a long time, over large areas and in varying conditions, evolved into different animals, depending on the environmental forces. A simple and speculative example: if rodent-like animals lived in rainforests which offered abundant food in the trees, they would probably stay in the trees, and over millions of years become monkeys or apes. If any of those creatures had been born with the inclination and ability to walk on two legs they would have gained no survival advantage, so that mutation would quickly be bred out. Such a population might eventually evolve into another type of ape, but it wouldn’t evolve to be a bipedal ape.*
Let’s say another population of the same rodent-like animals also lived in savannah plains, and found food in the long grass. The ones born with the ability to stand on two legs and see above the grass might have a significant advantage, and be more likely to survive and produce offspring. Over millions of years they might evolve into land dwelling apes, walking on two legs. That’s how different animals might evolve from one common ancestor.
‘Are there other factors contributing towards evolution?’
One factor is epigenetics, which is about genes being influenced by the environment. For example, a creature living in drought conditions, unable to feed itself properly, might give birth to young that are smaller than normal. When those young grow up they might also give birth to young that are smaller than normal, even though the drought had ended and they (the parents) had eaten well! Had the drought not ended, then giving birth to smaller young might be an advantage for those young – they would need less food to stay alive. Another example: a creature living an abnormally stressful life might give birth to young that grow up more prone to becoming stressed than they otherwise would be, and when they give birth, their young might be born with genes switched on ready to make them stressed in even mild circumstances.
Some viruses can also contribute towards a creature’s evolution by infecting its sperm or egg, thereby changing the creature’s DNA slightly. Those changes would be inherited by its young, and if those changes are beneficial then they will be passed on to future generations by the process of natural selection. For example, primates like chimpanzees, gorillas and us have within us ancient virus DNA that helps our females give birth to healthy young.
* Susannah Thorpe and her colleagues of The University of Birmingham suggest another possibility: that our ancestors evolved to stand on two legs while still in the trees. Balancing on two feet and using their hands to hold branches for balance helped them reach the fruit on small, outlying branches (New Scientist, 9 June 2007). Another theory suggests that we became bipedal from having to wade in water.