What is ‘evolution by natural selection’? In this science blog we summarize some of the more important issues.
It seems that in the natural world plants and animals are very well ‘adapted’ to go about their daily lives finding food. Humming birds have amazing wings that allow them to hover in exactly the right position so they can maneuver their long beaks to sip nectar from deep inside flowers. They can fly forwards, backwards and sideways to ensure they are always in the correct position for feeding. This amazing agility also means that adult birds can escape predators.
Giraffes, with their long necks, are able to eat those hard to reach leaves at the top of acacia trees that no other herbivores stand a chance of reaching. Their large size means that they are not often attacked by predators.
Naked mole rats live permanently underground. They use their large protruding teeth to dig tunnels and have lips that seal just behind their teeth to prevent soil from filling their mouths as they dig. They have small lungs which help them cope with a lack of oxygen underground. They can run equally fast forwards and backwards! Living permanently underground means they are less easily caught by predators.
Atlantic wolf fish produce a natural antifreeze to keep their blood circulating in the very cold environment in which they live. They do not eat fish but instead use their powerful jaws to feed on marine creatures that have hard shells. Their diet includes sea clams, crabs and sea urchins.
So how did the humming bird develop those amazing wings? How did the giraffe develop its long neck? How did the wolf fish develop anti freeze in its blood? How did the naked mole rat develop the ability to breath underground on limited amounts of oxygen?
It is the ‘science of evolution by natural selection’ which can explain all these things (and much, much more).
We shall only be dipping into the subject however, since there are thought to be more than 8 million species of living organisms on Earth, many species of which are still yet to be discovered! These (discovered and undiscovered) species include animals, fish, insects, plants, fungi, algae and ‘protozoa’.
‘Protozoa’ are single celled organisms that mostly feed on bacteria. In this picture you can see an oval shaped ‘protozoa’ feeding on much smaller specks of bacteria in sandy soil. The larger angular shapes you can see are grains of sand.
It often takes millions of years for organisms to evolve. The scale of change can be quite breathtaking and difficult to understand. Life on earth started evolving 3.5 billion years ago which was only yesterday in ‘geological’ time!
Look closely at this clock; if the 4.6 billion years of time since the formation of planet Earth could be squeezed into one hour, this is what you would see!
The point to remember that an amazing amount of evolution and change can take place in such a vast period of time. And to think that the first modern man first evolved in only the last tenth of a second before nine o’clock!
The examples of change in the mammals we discuss below took place very ‘recently’- between 65 and 15 million years ago. Can you identify the descendants of these ancestral mammals, including their great-great great-great-great-great-great great-great-great-great-great (quite a few million ‘greats’!) grand children? (answers at bottom of page)
All organisms need to reproduce if their species is to survive.’Reproduction by natural selection’ explains how organisms have evolved to allow individuals of that species to have the best chance of finding a mate to maximize their chances of reproducing.
Organisms are born with an instinct about how they should behave. Organisms can sometimes be taught how to survive by their parents, but nevertheless the fact remains that most organisms have an inbuilt instinct about what to do in different situations.
Where do organisms get these ‘instincts’ from?
The bongo fawn has a storage system in its body called a ‘genetic database’. This ‘genetic database’ takes the form of unique sequences of DNA molecules which provide the bongo fawn with a set of instructions about what it should do in different situations.
A complete set of genetic instructions ( all the chapters in the ‘book’) is called a ‘genome’.
The ‘genome’ is based on the collective experiences of great-great-great…etc grandfather (and grandmother) bongos long deceased. Information in the ‘genetic database’ is passed down from generation to generation and is regularly updated.
The instruction to ‘run for your life’ would have been passed down the generations by those ‘great-great-great’ grandfather (and grandmother) bongos which had survived attacks by lionesses and other predators.
The lioness’s ‘instruction manual’ for hunting bongo is also stored in DNA molecules in its very own ‘genetic database.’ This very different ‘instruction manual’ is based on the experiences of previous generations of lionesses which have successfully hunted bongo.
DNA molecules are found in the center (or nucleus) of all cells in the body of the impala. (apart from red blood cells which have no DNA) A total of 46 DNA molecules can be found in each cell. There are billions of cells in the body of an impala, so there are a huge number of DNA molecules! This is a picture of a group of cells magnified under a microscope. These cells can be found on the surface of skin inside the retina of the eye. The blue dots are the nucleii.
DNA is very important in the study of evolution. The differences in DNA provide proof of how similar (or different) two species are – and thus how closely or distantly related they are.
Life on earth started more than 3.5 billion years ago with the emergence of a single species. This first species is thought to be a single molecule called ‘ribonucleic acid’ (RNA) that was able to make exact copies of itself when submerged in water. The RNA self copying (or self replicating) molecule started the whole process whereby over time many millions of new and diverse species were created.
These species included all living things included single celled organisms, plants, fish, reptiles and mammals.
The species that were able to adapt, survive and reproduce in changing and often hostile habitats survived. The species that were not able to adapt to changing conditions and habitats, became extinct.
Most of the millions of the species that have ever lived in our world have become extinct. Of the species that have survived all have had to adapt and change in order to survive and reproduce. Many species have changed beyond all recognition when compared to ancient (and sometimes more recent) ancestors.
The diagram below is a version of our ‘evolutionary family tree’ giving an overview of different ‘families’ of organisms living today. Every ‘family’ alive today evolved from those first self copying (or ‘self-replicating’) RNA molecules 3.5 billion years ago.
To keep matters simple, plants and trees have been excluded from this ‘evolutionary family tree’.
As you can see from the branches of this ‘family tree’ we are all related to other organisms, including worms and insects!
In fact we share the DNA of all other forms of life, including bananas and pumpkins! However we are more closely related to pumkins than bananas- we share 75% of the same DNA molecules as pumpins and 55% of the same DNA molecules as bananas!
Protungulatum donnae is the earliest known common ancestor of all placental mammals including humans, lions and rats! That means that this animal is also a common ancestor of all other mammals mentioned here including the indohyus and the eohippus.
Phosphaterium escuillei is one of the earliest known ancestors of the elephant.
Eohippus lived 50 million years ago and is thought to be a common ancestor of both horses and rhinos.
Indohyus is one of the earliest known ancestors of the whale and was the size of a domestic cat.