evolution leading to speciation

Define evolution

change in allele frequencies in a population

For evolution to take place, variation must occur in original population, what are main sources of variation?

Genetic variation from:

o New mutations

o Meiosis (crossing over and independent segregation)

o Sexual reproduction (the random fertilisation of gametes)

• Variation caused by the environment e.g. food availability affecting growth, access to

light (for plants), temperature (which can control the sex of an organism) injuries

which have been picked up during a lifetime

What force causes allele frequency to change?

Natural selection

define natural selection

When individuals with certain phenotypes (are better adapted to their environment), so they are more likely to survive and reproduce and pass on their alleles to their offspring

What is ‘selection prtessure’?

A feature of the abiotic or biotic environment which makes some individuals more likely to survive than others

What are the four key forces which create selection pressures and therefore cause natural selection?

A feature of the abiotic or biotic environment which makes some individuals more likely to

survive than others

What are the four key forces which create selection pressures and therefore cause natural selection?

Predation

Disease

Competition for resources needed for survival

Competition for mates

How does natural selection occur and what is the consequence?

The best adapted individuals (the ones with a selective advantage) are more likely to

survive and reproduce and pass on their advantageous alleles to the next generation

This results in a change in allele frequency in the next generation

What is directional selection?

Selection that favours a phenotype that is away from the mean of the population range.

An extreme phenotype has a selective advantage.

This changes the characteristics of the population over time.

When experiencing directional selection, what happens to the bell shaped normal distribution curve?

It could shift either to the left or right

What effect does directional selection have on the mean characteristic of the population?

Mean changes (increases or decreases)

What effect does directional selection have on the standard deviation of the characteristics of the population?

Standard deviation stays the same

What is stabillising selection?

When the average phenotype has a selective advantage over the extreme phenotypes and is selected for

what happens to the bell curve after the population experiences selection?

what impact does this have on the:

1. mean characteristics of the population?

2. standard deviation of the characteristic of the population?

The graph will have a sharper middle peak

1. Mean of the population will stay the same

2. standard deviation decreases

Define disruptive selection

Selection that favours both the extreme phenotypes and selects against the individuals with average phenotypes.

This changes the characteristics of the population over time.

What happens to the graph of bell shape- normal distribution in disruptive selection?

What effect does stability selection have on the standard deviation of characteristics of the population?

Standard deviation increases

what is artificial selection?

Artificial selection is when humans select which organisms breed. Humans select the organisms which have characteristics that are useful for the humans.

This results in a change in allele frequency of the population over time.

Describe the process of artificial selection

• Humans select the individuals with the desired allele(s) which makes the individual

• ….. (always link to the context of the question and state what characteristic the desired allele codes for)

• These individuals are bred with each other

• So they pass on their desirable alleles (more than other individuals)

• Therefore, the frequency of the desirable alleles increases in the population

is artificial selection different to natural selection? (4)

natural selection, it is the best adapted individuals with the advantageous alleles that give them a selective advantage which survive and reproduce

Natural selection = SURVIVE AND REPRODUCE

But in artificial selection is it the individuals with alleles that give desirable characteristics for humans are selected to reproduce

Artificial selection = SELECT AND BREED

Sometimes artificial selection selects individuals who would not actually survive very well in the wild!

define species

A group of organisms with similar characteristics who can interbreed to produce fertile offspring

what does ‘speciation mean’?

The development a new species

What is meant by geographic isolation?

When one population is divided into two populations which are geographically separated from each other, so that the individuals in each population can no longer meet each other and interact

Why is geographic isolation key to the development of new species?

Because it prevents the individuals meeting and cross breeding and thus it prevents all gene flow between the populations

what is allopatric speciation?

When new species evolve from a single ancestral species as a result of a physical separation between individuals of the original population

what is sympatric speciation?

When a new species evolves from a single ancestral species within the same geographic location.

compare symaptric and allopatric speciation (at least 4 points)

• describe how speciation allows 2 species separated by a river to be formed. (5)

1. geographical isolation/ allopathic speciation

2. reproductive separation/ isolation

   • OR NO GENE FLOW

   • OR GENE POOLS SEPERATE

3. different selection pressures

   • OR different environmental conditions

4. alleles passed on are selected

   • OR change in frequency of alleles

5. eventually different species, cannot produce fertile offspring