In
biology, evolution is the change in the characteristics of a species over
several generations and relies on the process of natural selection.
According to Hall & Hallgrímsson “Evolution is
change in the heritable characteristics of biological populations over
successive generations.”
- 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 color with black spots.
- This coloring 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.