Chapter 16 – Evolution of Populations

16.1 Genes and Variation

Biology

Mr. Hines

Biosphere

Ecosystem

Community

Population

The part of Earththat contains allecosystems

Community and its nonliving surroundings

Populations thatlive together in a defined area

Group of organisms of onetype that live in the same area

Biosphere

Hawk, snake, bison, prairie dog, grass, stream, rocks, air

Hawk, snake, bison, prairie dog, grass

Bison herd

Section 1-3

Figure 1-21 Levels of Organization

Levels of organization

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 shufflingIf 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 traitsThe 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.

•

•