The theory of evolution is one of the fundamental ideas in biology. It reveals how humans, and all living things are related, in a family tree of life, stretching back over 3 billion years.
The main process driving evolution was first proposed by Charles Darwin (and Alfred Wallace) over 150 years ago. Before then, it was thought that all species were created separately and their forms were fixed and had not changed. Darwin and Wallace showed that life is constantly adapting and changing into new forms.
They explained that life is locked into a constant fight between individuals, species, and the environment. The pressures to adapt was the main mechanism driving evolution through a process called natural selection.
Four basic ideas underpin Darwin’s theory of natural selection:
- Individuals in a species show variation.
- Variation is passed on from parents to offspring.
- Many individuals die before they can breed.
- Individuals with characteristics most suited to their environment are more likely to survive and reproduce. In doing so they pass their successful genes on to the next generation.
In this way, natural selection constantly rejects bad genetic variation and preserves the good characteristics. Over time, successful variation causes populations to diverge, and new species to form.
Life on Earth
Planet Earth is 4.6 billion years old. The first life began in the seas around 3.6 billion years ago. The earliest living things were single-celled creatures such as bacteria and algae. Gradually life became more complex and multicellular life began.
Dinosaurs ruled the Earth until 66 million years ago when a large asteroid hit the planet causing massive climate change, killing all the dinosaurs, and a lot of other creatures too. This cleared the way for a small group of mammals to thrive once conditions returned to normal. They in turn evolved into primates and eventually into humans.
Human beings have only been around for a tiny fraction of the Earth’s history, 200,000 years or so. If the entire history of the Earth was condensed into a 24 hour day we wouldn’t appear until a few seconds before midnight
Inheritance
Reproduction is essential to ensure the continuation of a species. The offspring inherit their characteristics from both parents, many of which which are inherited by passing on genes. Genes can determine characteristics. Eye colour, hair colour and earlobe shapes are examples of inherited characteristics.
In any family there is genetic variation. Brothers and sisters will inherit a different mix of characteristics from their parents (with the exception of identical twins). Where there are lots of offspring, for example in a litter of puppies, there will be differences, some of which may give one an advantage over the others.
Other characteristics are caused by the environment in which the organism lives, and are said to be acquired. These involve a change to the structure or function of parts of the body. This could be caused by repeated use of a body part (such as exercise), a disease or accident or by other environmental influences. These acquired characteristics are not inherited – they cannot be passed down to their offspring. A weightlifter may develop stronger muscles through regular training but that doesn’t mean their children will be automatically have bigger muscles. If you cut tails off mice, their offspring will still be born with tails (someone actually tried this!)
Some inherited characteristics can be altered – hair can be dyed a different colour and plastic surgery can alter our bodies but our genes will stay the same.
Adaptation
The phrase “Survival of the fittest” means that those organisms that are better suited to the environment will have a better chance to survive and breed. It is a misconception that this means that only the strongest or the “best” survive.
Polar bears evolved from brown bears. This can be explained through evolution through natural selection. The Polar Bear is very well suited for its environment. It has adapted for life in the arctic. It has large paws with claws to help it walk on snow, white fur for camouflage, small ears to reduce heat loss and lots of body fat to insulate it from the cold.
Polar Bears evolved relatively recently in evolutionary terms. It’s thought that about 150,000 years ago a mutation occured within a population of brown bears which gave some bears white fur. In their very cold northerly climate, bears with white fur were better camouflaged. This meant they had a better chance of surviving and reproducing, whilst bears with brown fur in these areas struggled and died.
Natural selection had promoted the white mutation, and gradually the polar bear evolved. Later mutations meant that some of these bears had smaller ears or more body fat. This helped them to survive better and the polar bears continued to evolve into the species we know today.
Darwin’s Finches
During his time on the Galapagos Islands, Darwin collected specimens of the different species of finch living on the island. It wasn’t until he returned to the UK that he studied these specimens and realised how important they were.
By noticing that finches on the different islands had beaks that were adapted to their environment, and realizing that finches whose beaks weren’t adapted wouldn’t survive, Darwin was able to start working out his theory of evolution.
The Peppered Moth
The peppered moth is another good example. It adapted to a change in its habitat within a relatively short time span, and at a time when humans could study it.
The majority of moths were light coloured, only a tiny proportion had the gene which made them dark coloured. The pollution from the industrial revolution coated the trees in soot and also killed the lichen often found on tree trunks. The light peppered moth was no longer able to camouflage itself on the tree trunks, and so became easy to spot by birds. There was now an advantage to being a dark moth, as they were harder to spot.
As a result, the dark moths were able to survive long enough to breed and pass on the dark gene to their offspring. Within 100 years, 98% of all peppered moths were dark. With the introduction of controls for pollution, the number of dark moths felling and light moths increased again.
So, if we evolved from monkeys, why do we still have monkeys?
It’s a myth that we evolved from the monkeys that you see around us today. But it is true that apes and chimps are our relatives, in the same way that you are related to your cousins or second cousins. You all share a common ancestor. You share a grandparent with your cousin, or a great grandparent with a second cousin. In the same way, if you go back far enough in time you will find an animal that is our shared ancestor with chimpanzees.
The common ancestor betwen humans and chimpanzees lived in Africa roughly 7 million years ago. This population then split and evolved into two different species. One species eventually evolved into modern chimps whilst the other eventually evolved into modern humans. The DNA of humans and chimps is 98% identical.
It’s thought that our early ancestors were adapted for a life in the trees and walked on all fours like most other mammals. Walking on two legs helped early humans survive in different ways. It has been suggested that it made it easier to pick fruit and other food from low-lying branches; it freed up hands for carrying food, tools, or babies; it enabled early humans to appear larger and more intimidating; and that is was a more efficient way to move, helping early humans cover wide, open landscapes quickly.
Some of the early human species include Homo habilis which lived around 2.5 million years ago and Homo erectus which lived around 1.8 million years ago. Our own species is called Homo sapiens and it’s thought that we first appeared around 200,000 years ago.
Evolution is more than “Just” a Theory
A theory to a scientist means a different thing than it does to everyone else. In everyday use, a theory means a guess or a hunch. These theories are often unproven.
In science a theory is a well-accepted explanation for a phenomenon based on good evidence. A theory ties together all the facts and can even be used to make predictions. In science a theory is a close to proven as it is possible to be with the evidence available at the time. Gravity may be a “theory” but are you prepared to test it by jumping out of a window?
There is a wealth of evidence that supports the theory of Evolution by Natural Selection. This theory has been tested and scrutinised for over 150 years and has been supported by an overwhelming amount of evidence. Scientists can look at the fossil record of the changes in animals over time, changes in DNA and even observe evolution taking place in bacteria in the lab.
For more on this – visit Not Just a Theory.
Fossils
Fossils are the remains of organisms preserved in the rocks. We are able to date fossils, and so they provide a record of what lived on Earth many millions of years ago. They show a gradual change in living things, from simple to more complex forms, and they also provide evidence for intermediate forms between different groups of organisms. For example scientists can find fossils of lizards that gradually show more bird like characteristics to show that birds evolved from dinosaurs.
We can also observe similarities between living organisms. For example, the skeletons of all mammals are remarkably similar and also not very different from those of birds and reptiles! In a similar way the genes of closely related species are much more similar than those of very different ones, supporting the idea of common ancestry.
Artificial Selection
Humans are unique in that they are able to deliberately drive the evolutionary process in other species. This is called artificial selection and it involves humans, rather than nature, selecting those organisms which will breed and pass on their genes.
It’s taken over 30,000 years for humans to selectively breed wolves into all the breeds of dog that we know today. Farmers have been selecting their best animals for breeding to improve the quality of the offspring. This includes characteristics like more meat, more wool, higher milk production or disease resistance.
All the different types of brassica, such as broccoli, savoy cabbage, kohl rabi and kale, were all bred by humans out of a single species of wild cabbage.
Glossary
Adaptation: The process by which an organism evolves over time to become better suited to its environment.
Acquired characteristic: A characteristic that is changed through repeated use, disease or other environmental influences. Acquired characteristics are not inherited.
Evolution: Gradual change in living things over many generations, due to changes in inherited characteristics.
Gene: Part of the DNA inside cells, which provides the instructions to build the cell and control the way it works.
Inherited characteristic: A characteristic that is passed from generation to generation through their genes. For example eye colour, hair colour and tongue rolling.
Natural Selection: The process by which favourable characteristics become more common over time.
Species: A class of living organism that can breed with others of the same class, to produce fertile offspring
0 Comments