Variation and Evolution

Question 1

Why is variation between the individuals of a species essential?

Answer 1

It is essential if a species is to survive over a long period of time, so that they can adapt to changes in the environment.

Question 2

What are the two types of variation?

Answer 2

-Discontinuous variation, normally controlled by a single gene with no intermediate forms
-Continuous variation, influenced by the combined effect of a number of genes

Question 3

Describe the properties and representation of continuous variation

Answer 3

It has no distinct categories and tends to be quantitative. It is represented by a line graph.

Question 4

Describe the properties and representation of discontinuous variation

Answer 4

It has distinct categories and tends to be qualitative. It is represented by a bar graph.

Question 5

What type of distribution does continuous variation show?

Answer 5

A normal distribution curve around the mean and the median and mode are the same.

Question 6

What type of distribution does discontinuous variation show?

Answer 6

Discrete, non-overlapping groups, which each show a normal distribution.

Question 7

What are the causes of variation in individuals?

Answer 7

-Environmental factors
-Sexual reproduction
-Mutation
-Competition between organisms
-Non-random breeding
-Selection pressures

Question 8

What are the three ways sexual reproduction creates variety?

Answer 8

1. The mixing of two parental genotypes where random cross-fertilisation occurs
2. The random assortment of chromosomes during metaphase I, and of chromatids during metaphase II of meiosis
3. The crossing over of homologous chromosomes during prophase I of meiosis

Question 9

What is selection?

Answer 9

The process by which organisms that are better adapted to their environment survive and breed. Better adapted organisms are more likely to pass on their advantageous alleles to succeeding generations.

Question 10

What is a gene pool?

Answer 10

All the alleles, of all the genes of all the individuals in a population at any one time.

Question 11

What are the three types of selection shown when an environment changes?

Answer 11

-Directional selection
-Stabilising selection
-Disruptive selection

Question 12

Describe directional selection

Answer 12

Selection against one extreme. It is for one extreme trait and against the other extreme.

Question 13

Describe stabilising selection

Answer 13

Selection against both extremes. It is for moderate traits and against both extremes.

Question 14

Describe disruptive selection

Answer 14

Selection against one extreme. It is for both extremes and against moderate traits.

Question 15

What is the founder effect?

Answer 15

When a few individuals become isolated from the rest of the species and start a new population. The isolated population may have a very different allele frequency compared to the original population.

Question 16

Why does the isolated population change in the founder effect?

Answer 16

They may become very different from the original population due to:
-Different mutations occurring in both populations
-Different selection pressures
-Different alleles giving competitive advantages in the isolated population
-The founder population is isolated so cannot interbreed and share genes with the larger gene pool

Question 17

Why might a surviving population be significantly different to the original one?

Answer 17

-Certain alleles will be overrepresented among survivors
-Some alleles will be underrepresented
-Some alleles will be eliminated completely

Question 18

What are demes?

Answer 18

Breeding sub-units within a population of one species.

Question 19

What is an isolating mechanism?

Answer 19

A barrier which effectively prevents gene exchange between demes.

Question 20

What are the two main forms of isolating mechanism?

Answer 20

Reproductive isolation and geographical isolation.

Question 21

What are the different mechanisms of reproductive isolation?

Answer 21

-Behavioural isolation, in animals with elaborate courtship behaviour, the display of one subspecies fails to attract the necessary response in a potential partner of another sub-species
-Mechanical isolation, the genitalia of two groups may be incompatible
-Gametic isolation, in flowering plants, pollination may be prevented if the pollen grain fails to germinate on the stigma and in animals sperm may fail to survive in the oviduct in the partner
-Hybrid in viability, despite fertilisation taking place, development of the embryo may not occur
-Hybrid sterility, when individuals of different species breed, the sets of chromosomes from each parent are different and these sets are unable to pair up during meiosis so the offspring are unable to produce gametes

Question 21

What are the three forms of speciation?

Answer 21

-Sympatric speciation, where a reproductive/behavioural barrier causes reproductive isolation
-Allopatric speciation, where a physical barrier causes reproductive isolation
-Founder effect, where a small population is estranged from the original

Question 22

Describe the process of speciation

Answer 22

1. Variation due to different, random mutations arises within a population
2. A barrier separates one part of the population from another
3. The allele frequencies in the isolated populations may be very different
4. Different selection pressures cause different changes to gene pools in the separated populations, as they evolve along separate lines
5. If the barrier is removed there may be sufficient differences to prevent the formation of viable offspring and new species have evolved

Question 23

What is evolution?

Answer 23

The process by which new species are formed from pre-existing ones over a period of time.

Question 24

What did Alfred Russel Wallace and Charles Darwin propose?

Answer 24

That natural selection is the force that causes changes in populations.

Question 25

What did Darwin observe in populations?

Answer 25

-in any population, there is variation -individuals within a population have the potential to produce large numbers of offspring, yet the numbers of adults tends to stay the same from one generation to the next

Question 26

What did Darwin deduce after making observations?

Answer 26

-there is a struggle for survival with only the 'fittest' surviving -the individuals that survive and reproduce pass on the advantageous alleles for the characteristics that enable the offspring to succeed

Question 27

Describe the process of natural selection

Answer 27

1. Overproduction- more individuals are produced than can survive to maturity 2. Mutation increasing variation- mutations cause variation in different phenotypes 3. Selection- some individuals with some phenotypes will have a competitive advantage over others 4. Reproduction- the individuals that survive can reproduce and pass on the advantageous alleles to the next generation, with these alleles becoming more common in the gene pool

Question 28

What is adaptive radiation?

Answer 28

The change in the shape of one structure to perform different functions and fill different ecological niches.

Question 29

What does the Hardy-Weinberg principle state?

Answer 29

The frequencies of dominant and recessive alleles will remain constant from one generation to the next.

Question 30

What are the certain conditions the Hardy-Weinberg principle operates under?

Answer 30

-Organisms are diploid -The characteristic is controlled by a single gene with two alleles -Only sexual reproduction occurs -Generations are non-overlapping -Mating is random -Population size is infinitely large -Allele frequencies are equal in the sexes -There is no migration, mutation or selection

Question 31

What can the Hardy-Weinberg equation be used for?

Answer 31

-To calculate allele and genotype frequencies in a population -To predict the number of individuals in a population carrying a certain allele

Question 32

What do p and q represent in the Hardy-Weinberg equation?

Answer 32

p+q=1 Where p= (dominant) frequency/proportion of one allele and q= (recessive) frequency/proportion of the other allele

Question 33

State the Hardy-Weinberg equations

Answer 33

p+q=1 and p^2 + 2pq + q^2 =1 Where p^2 is the proportion of a population who are homozygous dominant 2pq is the proportion of the population who are heterozygous q^2 is the proportion of a population who are homozygous recessive

Question 34

When might a Students T-test be used?

Answer 34

Comparing two sets of data showing normal distribution

Question 35

What can the Students T-test be used to determine?

Answer 35

It can help us decide whether differences between the mean of two groups of data is due to chance or some other factor by measuring the overlap between two sets of data.

Question 36

What does the null hypothesis state for the Students T-test?

Answer 36

There is no significant difference between the means of the two groups, and that any difference is due to chance alone.

Question 37

State the steps to carry out a Students T-test

Answer 37

1. Formulate a null hypothesis 2. Collect the data 3. Calculate the mean 4. Calculate the standard deviation for both samples 5. Calculate the value for t using the equation 6. Calculate the number of degrees of freedom 7. Choose a suitable probability level 8. Find the critical value for t 9. Formulate a conclusion

Question 38

How do you work out the degrees of freedom in a Students T-test?

Answer 38

(Number of readings in sample 1 + Number of readings in sample 2) -2

Question 39

What conclusion do you draw if the calculated value for t is lower than the critical value?

Answer 39

The null hypothesis is accepted, and any deviation between the means of both samples is due to chance alone.

Question 40

What conclusion do you draw if the calculated value for t is higher than the critical value?

Answer 40

The null hypothesis is rejected, and any deviation between the means of both samples is due to some other factor than chance alone.