StemUp: AQA A level Biology 3.7.3 Evolution may lead to speciation

What might individuals within a population of a species show in terms of phenotype? (1)

May show a wide range of variation in phenotype

What causes similarities and differences between individuals within a species? (3)

May the results of:

- Genetic factors

- Differences in environmental factors

- A combination of both

What are the 4 sources of genetic variation? (4)

- Gene mutations

- Crossing over

- Independent segregation of chromosomes

- Random fertilisation of gametes

How do gene mutations contribute to genetic variation? (2)

- Result in the production of different polypeptides or enzymes

- Which contributes to genetic variation

What role does crossing over during meiosis play in genetic variation? (2)

- Involves the he exchange of alleles of the same genes between the chromosomes of a homologous pair

- This results in a new combinations of alleles

How does independent segregation of chromosomes contribute to genetic variation? (2)

- The separation of the two members of a homologous pair of chromosomes

- Occurs independently

of the separation of other homologous pairs

What is the significance of random fertilisation of gametes in sexual reproduction? (2)

- Contributes to genetic variation

- By combining different sets of alleles from two parents

What are 3 examples of selective pressures present in an environment? (3)

- Predation

- Disease

- Competition e.g. for food and/or shelter

Why do only some offspring survive despite organisms producing a large number of them? (2)

- Environmental conditions

- Favor certain phenotypes over others

How does a change in the environment affect natural selection? (2)

- Results in organisms with a particular phenotype being better adapted to survive and reproduce

- While those who are less adapted do not survive

What is differential survival and reproductive success? (2)

- Refers to the concept that only some organisms in a population survive to reproduce

- Depending on their adaptation to the environment

How do alleles related to selected phenotypes affect future generations? (2)

- Organisms with alleles for selected phenotypes have a greater chance of surviving and reproducing

- Passing these alleles to the next generation

What happens to the frequency of selected phenotypes and favorable alleles over generations? (1)

Increase in the gene pool from generation to generation as selection continues

What are the three 3 types of natural selection? (3)

- Stabilising selection

- Directional selection

- Disruptive selection

What is stabilising selection? (3)

- Occurs when the environment is stable

- Natural selection will favour organisms with alleles for characteristics towards the middle of the range

- E.g. fur length

What is directional selection? (3)

- Occurs when the environment is changing

- Change in the environment selects for those organisms with alleles for a phenotype towards the extreme of a range

- E.g. a decrease in temperature may select for individuals of a species of mammal that possesses longer fur than the mean fur length

What is disruptive selection? (4)

- Involves selection of the phenotypes at the two extremes at the expense of the intermediate phenotypes

- E.g. the environment may change to produce much lower winter temperatures and much higher summer temperatures.

- A lower temperature during the winter would select for animals in the population with a longer fur length

- A higher temperature during the summer would select for animals with a shorter fur length

What is speciation? (1)

The evolution of new species from existing ones

How does evolution occur in the context of speciation? (1)

Occurs as a result of a change in the allele frequencies in a population

What role does reproductive isolation play in speciation? (2)

- Can lead to the accumulation of differences in their gene pools

- Which contributes to speciation

When do new species arise in the process of speciation? (3)

- When genetic differences

- Lead to an inability of the members of the populations to interbreed

- To produce fertile offspring

What are the two types of speciation? (2)

- Allopatric

- Sympatric

What is allopatric speciation? (3)

- Where a population is split into two or more separate groups

- By geographical isolation

- Leading to the evolution of new species

How is a population split in allopatric speciation? (3)

- By geographical isolation

- Such as physical barriers

- That prevent breeding between the separated populations

What happens to gene flow between populations in allopatric speciation? (1)

Separated populations are isolated with no gene flow between their gene pools

How do different environments influence populations in allopatric speciation? (2)

- Different selection pressures in the different environments

- Result in selection for different phenotypes in the isolated populations

What is the outcome of selection for different phenotypes in allopatric speciation? (3)

- Organisms with selected phenotypes survive and breed

- This allows them to pass on their advantageous alleles to successive generations

- Leading to an increase in the frequency of these phenotypes and alleles in each isolated population

What is sympatric speciation? (3)

- The evolution of new species from a single population that remain in the same habitat

- But become reproductively isolated

- Due to genetic changes

How can random mutations lead to sympatric speciation? (2)

- Cause a group of organisms to stop reproducing with others of the same species in the same habitat

- Such as due to different flowering or mating seasons

What is the status of gene flow between populations in sympatric speciation? (3)

- Although populations live in the same habitat

- They are reproductively isolated

- With no gene flow between their gene pools

Why do allelic frequencies change in sympatric speciation? (2)

- Mutations occur independently in each isolated group

- Leading to divergent genetic changes

What happens to the gene pools over time in sympatric speciation? (3)

- Become so different

- That new species develop

- Organisms from different gene pools cannot interbreed to produce fertile offspring

What is genetic drift? (3)

- A process that occurs by chance

- Not by natural selection

- Leading to changes in allele frequencies in a population

How does genetic drift affect allele frequencies? (2)

- By chance, certain alleles are passed on to offspring more frequently than others

- Causing the frequency of these alleles to increase in successive generations

In which populations does genetic drift have a greater effect? (2)

- Smaller populations

- Where there is a smaller variety of alleles in the gene pool and lower genetic diversity

How does genetic drift influence speciation in small versus large populations? (3)

- In small populations, genetic drift can lead to speciation more rapidly

- Due to the increased effect on allele frequencies

- While in large populations, the effect is less pronounced, and speciation occurs more slowly