Speciation and Selection

Question 1

Hardy Weinberg assumptions

Answer 1

No mutations
No natural selection
No migration (gene flow in/out)

Large population (genetic drift has little affect)
Random mating

Question 2

What causes variation within a population

Answer 2

Genetics

Environmental factors

Question 3

Sources of genetic variation

Answer 3

Random mutations
Meiosis
Random fertilisation of gametes

Sexually reproducing organisms increase variation by all 3

Question 4

When do species become extinct

Answer 4

Rapid change in environment where previous advantageous allele is now selected against
Mutations are not quick enough for new characteristics to be made
Population dies
More likely if variation is low/small population

Question 5

What 3 things can a mutation cause

Answer 5

A selective advantage
Neutral
A selective disadvantage

Question 6

Monogenic

Answer 6

Variation caused predominantly by genetic factors are discontinuous
So produce categoric data like bar chart
Usually caused by a single gene (monogenic)
Blood groups in humans (4 distinct groups linked to a particular allele)

Question 7

What is a mutation

Answer 7

A sudden, random change in the base sequence of DNA

Introduces new alleles to a population

Question 8

How does environment affect variation

Answer 8

Influences the ways an organisms genes are expressed in the phenotype
Genotype sets limits for characteristics but environment determines where within these limits an organism lives

Question 9

Examples of environmental influences

Answer 9

Climate conditions (temperature, light intensity, humidity)
Abiotic/biotic factors

Question 10

Polygenic

Answer 10

Characteristics controlled by more than one gene
Often form continuous variation
Environmental factors play a big role in where the organism will lie
Normal distribution curve
Height and Mass

Question 11

What is a normal distribution curve

Answer 11

A bell shaped curve that measures a phenotype against its frequency in a large population

Question 12

What is selection

Answer 12

A process in which organisms that are better adapted to their environment survive and reproduce in greater numbers
Resulting in an increase of the frequency of the advantageous allele within the population
Over time

Question 13

Natural selection

Answer 13

Variation due to a mutation
Selection pressure causes advantageous alleles to be selected for
Those with them survive and reproduce in greater numbers
To produce offspring with same alleles
Those without more likely to die
Over time frequency of advantageous allele increases in population

Question 14

Stabilising selection

Answer 14

Occurs in population where environment is stable
Selection pressure occurs at both ends of distribution (
Favouring the average
Usually eliminating extremes
Reduces variation and standard deviation
Reduces opportunity for evolutionary changes

Question 15

Example of stabilising selection

Answer 15

Birth mass
Very heavy and very light babies have a high neonatal mortality rate than those with medium mass
Over time selection operates to reduce number of very heavy and very light babies

Question 16

Directional selection

Answer 16

Mean in population represents optimum phenotype for existing conditions
Environmental change may produce a selection pressure favouring an extreme phenotype due to advantageous allele
Some organisms possess new optimum phenotype and so survive and reproduce in greater numbers
Over time, this allele becomes predominant
Mean phenotype will shift

Question 17

Important to note about selection and mutations

Answer 17

Organisms do not develop a phenotype because of a change in the environment
Phenotype will usually be there before the selection pressure and change in environment due to a random mutation

Question 18

Example of directional selection

Answer 18

Thicker fur in mice is an advantage in a cold climate
Over time selection operates against disadvantageous allele for thin fur
Mean shifts towards the favoured extreme
Frequency of alleles causing long hair increase

Question 19

Least common but most important type of selection in evolution

Answer 19

Disruptive selection

Question 20

Disruptive selection

Answer 20

Environment has 2 selection pressures that favour the 2 extreme phenotypes
Optimum phenotype for existing environment selected against
Some organisms will possess the extreme new optimum phenotype and so survive and reproduce
Over time selection means that the two extremes will dominate and the mean will shift in both directions

Question 21

Example of disruptive selection

Answer 21

Mice fur at a beach with light-coloured sand tall patches of grass
Light-coloured agouti mice blend in with the sand so favoured
Dark-coloured mice can hide in the grass so favoured
Medium-coloured mice would not blend in with either and would be more likely to be eaten by predators

Question 22

How do 2 populations become genetically distinct

Answer 22

Must undergo a period of reproductive isolation (prevented from interbreeding)
Via an accumulation of mutations and different selection pressures

Question 23

What does natural selection lead to

Answer 23

Change in frequency of alleles in a population

A mechanism by which new populations of a species can arise and even evolve into 2 distinct species

Question 24

Genetic drift

Answer 24

Random increase/decrease in allelic frequency
Important in small populations where random loss of individuals with certain alleles causes significant change in frequency of alleles in population and future generations
Not a selection pressure but a random event

Question 25

Gene flow

Answer 25

Change in allele frequency due to the migration of fertile individuals
Founder effect or genetic bottle neck

Question 26

Founder effect

Answer 26

Example of gene flow
Migration can lead to new isolated populations with mixed or same alleles due to chance
Always draw a diagram

Question 27

Explain how geographical isolation can lead to new species

Answer 27

A physical barrier (river, road, mountain) prevents population meeting therefore reproductive isolation and can’t interbreed
Isolated populations subject to different selection pressures
Variation in both caused by random mutations
Different alleles selected for/against in each so organisms with advantageous alleles survive and reproduce
Leads to increase in advantageous allele frequency
Over a long time, gene pools become so different that they can no longer breed together to produce fertile offspring

Question 28

Allopatric speciation

Answer 28

Speciation associated with geographical isolation
Selection occurs on different sides of a geographical barrier that stops them interbreeding
Organisms change via such an extent that they become a different species
Can’t produce fertile offspring when bred.

Question 29

Sympatric speciation

Answer 29

Same country and so organisms in the same area become reproductively separated
Through choice of food or season to mate or hibernation etc

Question 30

Example of allopatric speciation

Answer 30

Darwin’s finches

Question 31

Example of sympatric speciation

Answer 31

Apple maggot fly
Flies mate exclusively on or near fruit of their host
Difference in host preference can result in virtually complete premating isolation among species
Flies feeding on Hawthorn do not mix with flies that feed on the apples
Isolated genetically and can develop through selection to become different species

Question 32

Types of isolation mechanisms

Answer 32

Temporal
Ecological
Behavioural
Gametic
Hybrid
Mechanical

Question 33

Temporal isolation

Answer 33

Organisms breed at different times of year

Question 34

Ecological isolation

Answer 34

Different habitats within the same area

Question 35

Behavioural isolation

Answer 35

Different behaviour patterns like courtship and birds songs or plumage

Question 36

Mechanical

Answer 36

Anatomical differences making it impossible for gametes to come together

Question 37

Gametic isolation

Answer 37

Incompatibility (genetic or physiological) between gametes prevent hybrid from forming
Sperm may be destroyed in birthing canal if its too acidic

Question 38

Hybrid isolation

Answer 38

Organisms interbreed but offspring are infertile
Horse and donkey produce an infertile mule
Number of chromosomes of mule is odd and so bivalent can’t form in prophase 1 due to not having homologous chromosomes and meiosis can’t occur to produce gametes