2 Populations And Evolution

Question 1

Define species.

Answer 1

A group of similar organisms that can reproduce to give fertile offspring. They can exist as one or more populations.

Question 2

Define population.

Answer 2

A group of organisms of the same species living in a particular area at a particular time that can potentially interbreed.

Question 3

Define gene pool.

Answer 3

The complete range of alleles present in a population.

Question 4

Define allele frequency.

Answer 4

How often an allele occurs in a population

Question 5

What is the Hardy-Weinberg principal?

Answer 5

A mathematical model that predicts that the frequencies of alleles in a population won’t change from one generation to the next.

Question 6

What is the hardy-Weinberg principle used to predict?

Answer 6

• allele frequency
• genotype and phenotype frequency
• percentage of population that has certain genotype

Question 7

What is the hardy-Weinberg equation to predict allele frequency?

Answer 7

p + q = 1
p= frequency of one allele, usually dominant one
q= frequency of other allele, usually recessive one.

Question 8

What is the hardy-Weinberg equation to predict genotype and phenotype frequency?

Answer 8

p ² + 2pq + q ² = 1
p ² = frequency of homozygous dominant genotype
2pq = frequency of heterozygous genotype
q ² = frequency of homozygous recessive genotype.

Question 9

Explain stabilising selection.

Answer 9

Stabilising selection is where individuals with alleles for characteristics towards the middle of the range are more likely to survive and reproduce. Occurs when environment isn’t changing, and it reduces the range of possible phenotypes.

Question 10

Explain directional selection.

Answer 10

Directional selection is where individuals with alleles for a single extreme phenotype are more likely to survive and reproduce. This could be in response to an environmental change.

Question 11

Explain disruptive selection.

Answer 11

Where individuals with alleles for extreme phenotypes at either end of the range are more likely to survive and reproduce. characteristics towards the middle of the range are lost. It occurs when the environment favours more than one phenotype.

Question 12

Define variation.

Answer 12

The differences that exist between individuals.

Question 13

Describe variation due to genetic factors. Mutations

Answer 13

sudden changes to genes + chromosomes may, or may not, be passed on to next generation.

Question 14

What is the main source of genetic mutation?

Answer 14

• mutation eg. Changes in DNA base sequence lead to production of new alleles.
• Meiosis through crossing over of chromatids and independent segregation of chromosomes and ransoms fertilisation of gametes during sexual reproduction.

Question 15

Define evolution

Answer 15

A change in allele frequencies in a population.

Question 16

Explain evolution.

Answer 16

• individuals of same species vary as have different alleles, so some better adapted to selection pressures than others.
• selection pressures create struggle for survival
• different levels of survival and reproductive success in population. Individuals with phenotype that increases chance of survival are more likely to survive and reproduce and pass on their genes
• means greater proportion of next generation inherit beneficial alleles
• they in turn more likely to survive, reproduce and pass on their genes
• frequency of beneficial alleles in gene pool increases from generation to generation.

Question 17

Describe selection pressures

Answer 17

Predation, disease and competition. These determine frequency of all alleles within the gene pool

Question 18

Describe variation due to genetic factors. Meiosis.

Answer 18

This special form of nuclear division produces new combinations of alleles before they’re passed into gametes, all of which are therefor different.

Question 19

Describe variation due to genetic factors. Random fertilisation of gametes.

Answer 19

In sexual reproduction this produces new combinations of alleles and offspring as therefore different from parents. Which gamete fuses with which at fertilisation is random process further adding to variety of offspring 2 parents can produce.

Question 20

Describe variation due to environmental factors

Answer 20

Environment experts influence on all organisms. These influences affect way an organisms genes are expressed. The genes set limits but largely environment that determines where, within those limits, an organism lies. Environmental influence include; climate conditions, soil conditions, pH, and food availability.

Question 21

Give an example of variation due to environment factors.

Answer 21

In buttercups, one plant may be determined by its genes to grow much taller than other plants. If however, the seed germinated in environment of poor light and low soil nitrate, plant may not grow properly and be short.

Question 22

Explain the role of overproduction of offspring in natural selection.

Answer 22

Darwin realised that, in nature, populations rarely increased in size at such a rate. He concluded that death death rate of even most slow breeding species must be extremely high. High reproductive rates have evolved an many species to ensure sufficiently large population survives to breed + reproduce next generation. Compensates for high death rates from predation, competition for food + water, extremes of temp, and natural disasters.

Question 23

What is the link between over-production of offspring and natural selection?

Answer 23

It lies in the fact that, where there are too many offspring for available resources, there’s intraspecific competition for limited resources available. Greater the numbers, greater this competition + the more individuals will die. These deaths not total random. Individuals in pop. best suited to conditions more likely to survive + reproduce, passing on more favourable allele combinations to next gen.

Question 24

Explain the role of variation in natural selection.

Answer 24

Conditions change overtime and having a wide range of genetically different in population means some will have combination of genes needed to survive in almost any new set of circumstances populations showing little individual genetic variation often more vulnerable to new diseases + climate changes.
Larger pop is, more genetically varied individuals within it, greater chance 1 or more individuals will have combination of alleles that lead to advantageous phenotype. These individuals more likely to survive + reproduce. Variation provides potential for pop to evolve + adapt to new circumstances.

Question 25

Describe how genetic variation leads to natural selection in copper tolerance in grasses (out of spec content).

Answer 25

Soil around old copper workings is heavily contaminated with copper. If seeds of Agrostis spp. are planted in this soil, only those plants with with high tolerance to copper ions will survive. These plants survive and reproduce, passing on alleles to next gen. amongst these alleles will be those giving resistance to copper ions. Over time, frequency of these alleles in gene pool of pop of Agrostis spp increases as result of the selection pressure from copper ions in soil. Frequency of certain alleles changed so gene pool changed + population of Agrostis spp has evolved.

Question 26

Explain how selection affects allelic frequencies.

Answer 26

Number of times an allele occurs within the gene pool referred to as allelic frequency. This is affected by selection and selection due to environmental factors. Environmental changes therefore affect probability of an allele being passed on in a population and hence the number of times it occurs within the gene pool. Environmental factors don’t affect probability of a particular mutant allele arising, they affect frequency of a mutant allele already present in gene pool.

Question 27

How are new species formed?

Answer 27

• population becomes separated in some way from other populations + undergoes different mutations, become genetically different from the other pops
• each pop experience different selection pressures because environment of each slightly different.
• natural selection then lead to changes in allelic frequencies of each pop
• different phenotypes each combination of alleles produces subject to selection pressures that’ll lead to each population becoming adapted to local environment.
• this known as adaptive radiation + results in changes to allele frequencies of each pop(each pop evolves). Pops would be unable to interbreed successfully. Each pop now a different species with its own gene pool.

Question 28

How are new species formed through genetic drift?

Answer 28

• take place in small pops
• as few individuals breed, genetic diversity of pop restricted to those few alleles in original pop
• as only small number of different alleles there’s not an equal chance of each being passed on
• those passed on quickly affect whole pop as frequency is high.
• any mutation to 1 of these alleles that’s selectively favoured quickly affect whole pop because high frequency.
• effects of genetic drift will be greater + pop will change relatively rapidly, making it more likely to develop into separate species.

Question 29

Describe geographical isolation.

Answer 29

May be result of any physical barrier between 2 pops which prevents them interbreeding. These barriers include oceans, rivers, mountain ranges and deserts.

Question 30

Describe allopatric speciation.

Answer 30

• geographical isolation means pops will experience different selection pressures + so different changes in allele frequencies
• different alleles more advantageous in different pops
• allele frequencies will also change as mutations occur independently in each population
• genetic drift may also affect allele frequencies in 1 or both pops
• changes in allele frequency lead to differences accumulating in gene pools of separated pops, causing changes in phenotype frequencies
• eventually, individuals from different pops will have changed so much that they won’t be able to breed with one another to produce fertile offspring, become reproductively isolated.
• 2 groups become separate species

Question 31

Describe sympatric speciation.

Answer 31

Sympatric means same country and describes form of speciation that results within a population in the same area leading to them becoming reproductively separated.

Question 32

What are the conditions for the Hardy-Weinberg principle

Answer 32

has to be large population where there’s no immigration, emigration, mutations or natural selection, also needs to be random mating