Transcript Ertertewt ertwetr - Campbell County Schools

Chapter 16 – Evolution of
Populations
16.1 Genes and Variation
Biology
Mr. Hines
Figure 1-21 Levels of Organization
Section 1-3
Levels of organization
Biosphere
The part of Earth
that contains all
ecosystems
Biosphere
Ecosystem Community and
its nonliving
surroundings
Hawk, snake, bison, prairie dog, grass, stream, rocks, air
Community Populations that
live together in a
defined area
Hawk, snake, bison, prairie dog, grass
Population
Group of
organisms of one
type that live in
the same area
Bison herd
Darwin’s handicap
Throughout Darwin’s studies of evolution he had a
handicap – he new nothing about genetics since
Mendell’s work was unknown to him.
2 problems for Darwin
1. He had no idea how traits could be inherited
(genes)
2. He had no idea about how variation appeared.
(mutations)
Mendell and Darwin’s work were merged in
the 1930s.
Here are the answers to Darwin’s handicaps.
1. Genes control heritable traits
2. Mutations in genes cause variation
This chapter will explain evolution at a level
Darwin never knew – genetics meets
evolution
How common is genetic variation?
We know that all genes have 2 forms (alleles)
Remember from earlier, alleles are the
possible outcomes from a cross – big
feet/little feet (Ff)
All living things have different alleles which
can cause variation.
This kind of information can seem invisible
because you need to be a molecular
geneticist to see it.
Living things are between 4 and 15 percent
heterozygous (2 different alleles)
Variation and Gene Pools
Genetic variation is studied in populations.
A population is a group of individuals of the same
species that interbreed.
Animals in a population usually live together within
a habitat.
The tortoises on Hood island live together within the
same habitat, they breed, and are therefore a
population.
Tortoises on other islands are not part of the
population of hood island.
Members of a population share a common gene
pool.
A gene pool consists of all genes, including all the
different alleles, that are present in a population.
Relative frequency of an allele is the number of
times that the allele will appear in the gene pool.
Relative frequency is usually represented by a
percent.
For example, the relative frequency of the
gene for long neck might be 95% on Hood
island.
This means that there is a 95% chance that
offspring on hood island will have a long
neck.
Mouse gene frequency example
In a population of mice, there are black and
brown mice.
Black is dominant – B
brown is recessive - b
Calculate the relative frequency of
brown mice and black mice
(page 394)
•
Count this up on the other board and
demonstrate
there will be 20 black and 30 brown.
Divide each allele count by the total allele
count
This shows that there are more brown mice
than black mice – how? Since black is
dominant?
Go to gene pools 146
Go to gene pools 146
Important fact – the relative gene frequency
has nothing to do with whether the gene is
dominant or recessive.
The dominant gene might not be fit for that
environment.
Black mice might get spotted by a predator in
a brown habitat.
•
What would happen to the gene pool if all
black mice were eaten.
The relative gene frequency of black mice
would fall to zero – therefore never again
will mice be black (in that population).
This is an example of how a gene frequency
can change.
All mice would be brown (in that population)
In genetic terms, evolution is any change in
the relative frequency of alleles in a
population.
In genetic terms, evolution is any change in
the relative frequency of alleles in a
population
Sources of genetic variation
There are 2 main sources of genetic variation.
1. Mutations
2. Gene shuffling
Mutations
A mutation is any change in a sequence of
DNA.
Mutations occur as a result of
1. mistakes during replication
2. Toxic chemicals in the environment
3. Radiation
Mutations do not always affect an animal’s
phenotype.
Some mutations will cause a change in an
animal’s phenotype.
This change might alter its ability to survive
in 2 ways.
1. Beneficial mutation
2. Harmful mutation
Gene shuffling
If you and your siblings have the same parents, and
therefore the same genes, why do you look
different?
Gene shuffling is caused by sexual reproduction.
Why do we need two organisms to create life – why
not just have all females?
Sexual reproduction keeps the genes shuffling and
changing throughout time.
Just think if the black mice never had an alternative
color – there would be no mice.
Sexual reproduction causes gene
shuffling by 2 ways.
1. Chromosomes of a homologous pair move
independently during meiosis II (creation
of gametes)
2. Crossing over in meiosis – this increases
the amount of genotypes that can appear.
Crossing over in Prophase 1
•
Metaphase I
•
Sexual reproduction shuffles genes similar to
one shuffling a deck of cards.
All of the cards are always the same, but each
time a hand is dealt to a player, it will be
different.
The gene pool rarely changes unless a trait is
selected, by nature, to die.
Single gene and polygenic traits
The number of phenotypes produced for a given
trait depends on how many genes control the
trait.
Single gene traits
Single gene traits are phenotypes that have
only 2 alleles.
It is one or the other – sort of like a coin – you
get heads or tails.
Widows peak is a single gene trait. You get
widows peak or a strait hairline – no other
options.
•
Widow’s peak is a dominant gene
Does this mean that widow’s peak is more
common?
Here is the gene frequency for widow’s peak
vs no widow’s peak.
Many traits are controlled by many genes.
This is called polygenic traits.
Height in humans is polygenic.
This would explain why height varies greatly
among humans.
•
•