Slide 1
Gene flow
is another factor that can cause evolution in a population. Gene flow
refers to the movement of individuals into and out of a population. Gene
flow is essentially immigration and emigration. As individuals move from
one population to another - they take their alleles with them and have the
ability to change allele frequencies. In this graphic, a population of
beetles that is predominantly green experiences immigration from a population
that is predominantly brown. The result is that the population
experiences an increase in the frequency of the brown alleles.
Slide
2
A mutation refers to a change in an organisms DNA. Mutation is a
normal process - when a DNA molecule is making a copy of itself - it is
possible for mistakes to happen.
These mistakes or mutations are the raw material for evolution - because mutations create new alleles. Think back to the example of eye color in fruit flies. The wild-type allele results in red eyes. A mutation occurred in the gene for eye color and the result is that inviduals who inherited that new allele have white eyes. Mutation has created genetic variation for natural selection to act upon.
While mutations are an important evolutionary factor - many mutations have little effect on allele frequencies. Why is this? Because most mutations are bad for the organism - many mutations result in death of the organism and dead organisms cannot pass on their genes. Therefore the new allele does not get established in the population. However, every now and then a mutation is beneficial - and the organism that has the mutation benefits. In this case the mutation can get established in the population and affect allele frequencies.
However, to be passed to the new generation the mutation most occur in germ cells (or sperm and eggs).
Slide 3
As an example of a mutation, we can look
at the allele that causes sickle cell disease. The allele that causes
the disease is the result of a single base pair substition in the DNA.
The normal allele has a single base pair different from the disease
allele (in all we have 3 billion base pairs that make up our DNA so a single
base pair changes is incredibly small but can have a huge impact on the
person's phenotype). We can see this single base pair change here - the
normal DNA has a T in this position while the mutated DNA has an A. The
result is a single amino acid change in the hemoglobin protein - a glutamic
acid in the normal protein and a valine in the mutated protein.
Slide
4
Mutations are happening all the time as the result of exposure to DNA
mutating agents - such as chemicals in the environment (both natural and
synthetic) and radiation. For example, sunlight (specifically UVB rays)
cause a specific type of mutation known as a thymine dimer which can lead to
skin cancer. Luckily, we also have DNA repair enzymes that can fix some
mistakes - occasionally, however, the damage overwhelms these systems and
cancer can result.
Slide 5
Non-random mating is the norm in
populations. This means that there is some sort of mate selection
occuring. In the example on this screen, The brown beetle on the left is
exhibiting random mating - he is equally likely to mate with any of the other
color beetles. The beetle on the right, however, is exhibiting nonrandom
mating - he only wants to mate with other brown beetles.
Humans exhibit non-random mating - people of the same cultural, educational and socioeconomic backgrounds tend to partner more than people of different backgrounds do.
Non-random mating changes genotype frequencies but not allele frequencies because it does not introduce new alleles to a population.
Slide 6
Our final
factor in evolutionary change is one that we have already talked about -
natural selection. Natural selection should have a big star next to it
in your notes - because it is the only factor in evolution that results in a
population becoming better adapted to its environment than previous
generations.
In this population of beetles there are two color morphs -
green and brown. A new species of bird moves into the environment - and
it tends to eat the green beetles because
they are easier for it to
see. As a result, the brown allele increases in frequency in the
population and the population becomes predominantly brown. This
population is now better adapted to its environment because most of the
individuals in the population are hard for the predator to see.
Slide
7
Remember that our most concise definition of natural selection is
differential reproduction. Those individuals that are better adapted to
their environment leave more offspring. The number of offspring an
individual leaves is a measure of its fitness.
If different genotypes have differing fitness - the more fit genotypes will become more common. In our beetle example, the brown beetles were more likely to survive and reproduce and thus the genotype associated with brown color became more common.
Slide 8
Fitness is an indicator of reproductive success -
the individual that leaves the most offspring has the highest biological
fitness. There is a distinction between survivorship and reproductive
success.
An individual can die young but be fit - by having a large number of offspring. Or an individual can die old but lack fitness - by not leaving any offspring.
Comparisons of fitness can only be made
within populations. You cannot compare the fitness of an elephant with
the fitness of a mayfly. A female elephant, for example, does not become
reproductive until age 13. She can live to be 70 but typically has 5-6 babies
during her life. A mayfly by comparison lives minutes to days depending
on the species but in that time can have hundreds of offspring. It would
not be biologically meaningful to compare the fitness of an elephant with the
fitness of a mayfly!