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The cumulative changes that occur in a population over time. The changes produce at the genetic level as organisms' genes mutate and/or recombine in different ways

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The Basics

Biological evolution refers to the cumulative changes that occur in a population over time. These changes are produced at the genetic level as organisms' genes mutate and/or recombine in different ways during reproduction and are passed on to future generations. Sometimes, individuals inherit new characteristics that give them a survival and reproductive advantage in their local environments; these characteristics tend to increase in frequency in the population, while those that are disadvantageous decrease in frequency. This process of differential survival and reproduction is known as natural selection. Non-genetic changes that occur during an organism's life span, such as increases in muscle mass due to exercise and diet, cannot be passed on to the next generation and are not examples of evolution.

Yes. Just as the tree of life illustrates, all organisms, both living and extinct, are related. Every branch of the tree represents a species, and every fork separating one species from another represents the common ancestor shared by these species. While the tree's countless forks and far-reaching branches clearly show that relatedness among species varies greatly, it is also easy to see that every pair of species share a common ancestor from some point in evolutionary history. For example, scientists estimate that the common ancestor shared by humans and chimpanzees lived some 5 to 8 million years ago. Humans and bacteria obviously share a much more distant common ancestor, but our relationship to these single-celled organisms is no less real. Indeed, DNA analyses show that although humans share far more genetic material with our fellow primates than we do with single-celled organisms, we still have more than 200 genes in common with bacteria.

It is important to realize that describing organisms as relatives does not mean that one of those organisms is an ancestor of the other, or, for that matter, that any living species is the ancestor of any other living species. A person may be related to blood relatives, such as cousins, aunts, and uncles, because she shares with them one or more common ancestors, such as a grandparent, or great-grandparent. But those cousins, aunts, and uncles are not her ancestors. In the same way, humans and other living primates are related, but none of these living relatives is a human ancestor.

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Genes are the portions of an organism's DNA that carry the code responsible for building that organism in a very specific way. Genes -- and, thus, the traits they code for -- are passed from parent to offspring. From generation to generation, well-understood molecular mechanisms reshuffle, duplicate, and alter genes in a way that produces genetic variation. This variation is the raw material for evolution.

Evolution and "survival of the fittest" are not the same thing. Evolution refers to the cumulative changes in a population or species through time. "Survival of the fittest" is a popular term that refers to the process of natural selection, a mechanism that drives evolutionary change. Natural selection works by giving individuals who are better adapted to a given set of environmental conditions an advantage over those that are not as well adapted. Survival of the fittest usually makes one think of the biggest, strongest, or smartest individuals being the winners, but in a biological sense, evolutionary fitness refers to the ability to survive and reproduce in a particular environment. Popular interpretations of "survival of the fittest" typically ignore the importance of both reproduction and cooperation. To survive but not pass on one's genes to the next generation is to be biologically unfit. And many organisms are the "fittest" because they cooperate with other organisms, rather than competing with them.

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| | | | | | | The Basics Biological evolution refers to the cumulative changes that occur in a population over time. These changes are produced at the genetic level as organisms' genes mutate and/or recombine in different ways during reproduction and are passed on to future generations.

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In Darwin's theory he states that evolution happens by natural selection. Individual species show variation in physicalities. The species that survive, reprduce and the species that was best suited for the environment are left.

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The theory of evolution is based on the idea that all species are related and gradually change over time.

Evolution relies on there being genetic variation in a population which affects the physical characteristics (phenotype) of an organism.

Some of these characteristics may give the individual an advantage over other individuals which they can then pass on to their offspring.

What is natural selection?

Charles Darwin's theory of evolution states that evolution happens by natural selection.

Individuals in a species show variation in physical characteristics. This variation is because of differences in their genes.

Individuals with characteristics best suited to their environment are more likely to survive, finding food, avoiding predators and resisting disease. These individuals are more likely to reproduce and pass their genes on to their children.

Individuals that are poorly adapted to their environment are less likely to survive and reproduce. Therefore their genes are less likely to be passed on to the next generation.

As a consequence those individuals most suited to their environment survive and, given enough time, the species will gradually evolve.

Natural selection in action: the Peppered moth

Before the industrial revolution in the mid-1700s, the peppered moth was most commonly a pale whitish colour with black spots.

This colouring enabled them to hide from potential predators on trees with pale-coloured bark, such as birch trees.

The rarer dark-coloured peppered moths were easily seen against the pale bark of trees and therefore more easily seen by predators.

As the Industrial Revolution reached its peak, the air in industrial areas became full of soot. This stained trees and buildings black.

As a result, the lighter moths became much easier to spot than the darker ones, making them vulnerable to being eaten by birds.

The darker moths were now camouflaged against the soot-stained trees and therefore less likely to be eaten.

Over time this change in the environment led to the darker moths becoming more common and the pale moths rarer.

What have genes got to do with it?

The mechanisms of evolution operate at the genomic level. Changes in DNA sequences affect the composition and expression of our genes, the basic units of inheritance.

To understand how different species have evolved we have to look at the DNA sequences in their genomes.

Our evolutionary history is written into our genome. The human genome looks the way it does because of all the genetic changes that affected our ancestors.

When DNA and genes in different species look very similar, this is usually taken as evidence of them sharing ancestors.

For example, humans and the fruit fly, Drosophila melanogaster, share much of their DNA. 75 per cent of genes that cause diseases in humans are also found in the fruit fly.

DNA accumulates changes over time. Some of these changes can be beneficial, and provide a selective advantage for an organism.

Other changes may be harmful if they affect an important, everyday function. As a result some genes do not change much. They are said to be conserved.

Different types of evolution
Convergent evolution

When the same adaptations evolve independently, under similar selection pressures.

For example, flying insects, birds and bats have all evolved the ability to fly, but independently of each other.

Co-evolution

When two species or groups of species have evolved alongside each other where one adapts to changes in the other.

For example, flowering plants and pollinating insects such as bees.

Adaptive radiation

When a species splits into a number of new forms when a change in the environment makes new resources available or creates new environmental challenges.

For example, finches on the Galapagos Islands have developed different shaped beaks to take advantage of the different kinds of food available on different islands.

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In biology, evolution is the change in the characteristics of a species over several generations and relies on the process of natural selection.

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Scientists use evolution to prove that species are related.

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Scientists use evolution to prove that species are related.