Topic 7 Genetics, populations, evolution and ecosystems

Question 1

Define community

Answer 1

Populations of different species living in a habitat

Question 2

Define ecosystem

Answer 2

A community and the non-living components of its environment

Question 3

Define Niche

Answer 3

The role of a species within its habitat e.g. what it eats, when it feeds

Question 4

Why can two species not occupy the same niche?

Answer 4

• If two species try to occupy the same niche, interspecific competition will take place for the limiting
  resources (abiotic and biotic) → the better adapted species will outcompete the other, until only one
  species is left (competitive exclusion principle).
• (sometimes it may look like two species are occupying the same niche - but there will be slight differences - e.g. both
  species may be eaten by the same predator, but there are variations in what the two species themselves eat).

Question 5

Define carrying capacity

Answer 5

The maximum stable population size of a species that an ecosystem can support.

Question 6

Intraspecific competition

Answer 6

Occurs between organisms of the same species, occurs when resources or
mates become limited → leads to natural selection and adaptation.

Question 7

Interspecific competition

Answer 7

Occurs between organisms of different species when a resource (e.g. food / habitat) is in limited supply → if two species are competing for the same resources, the one that
is better adapted to the environment will out-compete the other species.

Question 8

Predator-prey relationship

Answer 8

• Prey increases in number.
• More food available for predator.
• Predator increases in number (because there is more energy available for reproduction & growth).
• Predators eat more of the prey.
• Prey decreases in number.
• Less food available for predator.
• Predator decreases in number.
• Fewer prey are eaten.
• Prey starts to increase in number [cycle repeats].

Question 9

How to sample non-motile species

Answer 9

• Divide a map of the area into a grid.
• Select random sets of coordinates using a random number generator = random sampling (removes sampling
  bias).
• Place a quadrat at each coordinate.
• Identify plant(s) using a key or photographs.
• Measure abundance of the plant species in each quadrat:
• Count individual plants or
• If plants are too small, calculate percentage cover by calculating the percentage of the squares in the quadrat
  that are at least 50% covered by the species.
• Calculate mean abundance and multiply size of quadrat by size of total area to estimate total population size.

Question 10

How to sample plants along a BIOLOGICAL GRADIENT

Answer 10

• Use a belt transect.
• Place a tape along the path and at regular intervals along the tape place a quadrat (interrupted belt transect →
  there is space between the quadrats).
• This is systematic sampling (not random sampling) because samples are taken at regular intervals
  (useful for investigating a correlation with an abiotic factor* or looking at the stages of succession).
• Measure abundance within the quadrat → measure the percentage cover of each quadrat by counting how much
  of each quadrat is covered by the species or count individual species.
• [*You could also measure a particular abiotic factor (e.g. light intensity, soil pH) at each interval to see if there is a
  correlation between the abiotic factor and abundance.]

Question 11

How to sample motile organisms

Answer 11

• Mark-release-recapture technique.
• Capture the animal species [sample 1].
• Mark them in a way that is not harmful (tag or fluorescent marker).
• Release the first sample.
• After some time (leave enough time for sample 1 to mix with the whole population), capture a second sample.
• Count the number of organisms that are marked in the second sample.
• Estimate population size using: number in sample 1 x number in sample 2 divided by
  number marked in sample 2

Question 12

Assumptions of the mark-release-recapture method

Answer 12

• No/few births or deaths.
• No migration.
• Marked animals have had time to mix evenly with population.
• Mark is not harmful and mark does not come off.
• Mark does not affect chance of survival (i.e. it doesn’t make the animal more visible to predators).
• Large population (mark-release-recapture not suitable for endangered species).

Question 13

Define succession

Answer 13

The change in an ecosystem overtime

Question 14

What is the difference between primary and secondary succession?

Answer 14

• Primary (occurs on new land e.g. bare rock, sand).
• Secondary (occurs on land where a layer of soil is already present e.g. after a forest fire).

Question 15

Primary succession

Answer 15

• Pioneer species colonise bare rock / sand.
• Pioneer species e.g. lichen, are:
• Adapted to extreme abiotic conditions (extreme wind & extreme temperatures on exposed land - no
  shade/shelter).
• Xerophytes.
• Can anchor to rock/sand.
• Pioneer species die and decompose adding humus and nutrients to the soil.
• Pioneer species change the environment so it is less hostile so small plants can now grow and they out
  compete the pioneer species.
• Over time, the small plants die and decompose adding more humus and nutrients to the soil.
• Large plants/trees can now grow, they out compete the small plants.
• Process continues until the climax community is reached.
• The climax community contains the best adapted species to the environment (no more succession).

Question 16

What are features of a climax community?

Answer 16

• Stable community.
• Abiotic factors constant over time.

Question 17

Secondary succession

Answer 17

• Starts from small plants not pioneer species (because soil and nutrients are already present).
• Faster than primary succession because succession can begin at a later stage.

Question 18

Properties of succession

Answer 18

• Species diversity increases (peaks just before climax – species in climax will out compete others).
• Habitat diversity increases.
• Environment becomes less hostile.
• Food chains become more complex.
• Biomass increases.

Question 19

Define gene

Answer 19

Sequence of bases on a DNA molecule that codes for a polypeptide

Question 20

Reasons why observed ratios may be similar to, but not exactly
the same as, expected ratios?

Answer 20

1. Random fertilisation of gametes
2. Small sample size
3. Offspring ratios arise by chance

Question 21

Reasons why observed ratios may be significantly different
from the expected ratios?

Answer 21

1. Sex linkage
2. Autosomal linkage
3. Epistasis

Question 22

Expected phenotypic ratio for a dihybrid cross (if both parents are heterozygous for both genes)

Answer 22

9:3:3:1

Question 23

Why are males more likely to show recessive sex-linked characteristics?

Answer 23

• MALES only have one copy of the X chromosome, so the RECESSIVE PHENOTYPE is always expressed if
  they INHERIT a RECESSIVE ALLELE from their MOTHER.
• Females have two copies of the X chromosomes and so would need two copies of the recessive allele in order
  to express the recessive phenotype

Question 24

Autosomal linkage

Answer 24

• Genes on the same autosome are linked.
• They will stay together during independent segregation of chromosomes in meiosis I
• So their alleles will be passed on to the offspring together (unless crossing-over splits them up) → therefore
  gametes usually have the same combination of alleles as the parent.
• The closer together two genes are, the more closely linked they are because crossing over is less likely
  to split them up.
• Therefore, genetic crosses involving linked genes do not produce the same phenotypic ratio as genetic crosses
  involving non-linked genes.

Question 25

Epistasis dominant expected ratio

Answer 25

12:3:1

Question 26

Epistasis recessive expected ratio

Answer 26

9:3:4

Question 27

What is assumed in Hardy Weinberg equation?

Answer 27

1. There are no mutations
2. No natural selection (i.e. all alleles are equally advantageous)
3. The population is large (no genetic drift)
4. The population is genetically isolated/no migration
5. Mating is at random

Question 28

Hardy Weinberg equations

Answer 28

p + q = 1
p2 + 2pq + q2 = 1

Question 29

Natural selection

Answer 29

1. New alleles arise by random mutation
2. Individuals of the same species vary because they have different alleles
3. Environment applies a selection pressure on the population
4. Those with the advantageous allele are more likely to survive and reproduce,
5. Passing on the favourable allele to their offspring (= reproductive success)
6. Over a long time/many generations, the frequency of the advantageous allele within the population
   increases

Question 30

Disruptive selection

Answer 30

1. Disruptive selection
   * Environment changes between both extreme conditions.
   * Individuals at both extremes are favoured at different times and
   increase in number.
   * Those in the middle (average) decrease in number.
   * Disruptive selection can drive speciation

Question 31

Stabilising selection

Answer 31

2. Stabilising selection
   * Occurs when there is NO change in the environment (stable
   environment).
   * Individuals with phenotypes closest to the mean are favoured
   (individuals with either extreme of phenotype are at a selective
   disadvantage → extremes of phenotype are lost over time).
   * Mean phenotype remains the same.
   * Standard deviation decreases.

Question 32

Directional selection

Answer 32

3. Directional selection
   * Occurs when there is a change in the environment.
   * One of the extremes of phenotype has a selective advantage.
   * The mean shifts in the direction of the individuals with the selective
   advantage.

Question 33

Allopatric speciation

Answer 33

Geographical isolation.
* Separates gene pools.
* No interbreeding between populations.
* Variation exists in populations due to random mutations.
* Geographically isolated populations are subject to different environmental conditions and different
selection pressures → different alleles are advantageous.
* The better adapted organisms survive and reproduce, passing on advantageous alleles to offspring.
* Over a long period, this leads to change in allele frequency.
* Eventually the two populations accumulate enough different mutations that they cannot interbreed to
produce fertile offspring → they have become separate species.

Question 34

Sympatric speciation

Answer 34

• Occurs in same habitat.
• Random mutation results in reproductive isolation due to:
• Temporal mechanism e.g. different breeding seasons
• or behavioural mechanism e.g. different courtship behaviour
• or mechanical mechanism e.g. mismatch of reproductive parts
• So no gene flow between populations.
• Different alleles passed on to offspring.
• Change in allele frequency (gene pools diverge).
• Over a long period of time, the two reproductively isolated populations accumulate enough different
  mutations that they cannot interbreed to produce fertile offspring → they have become separate
  species.

Question 35

Genetic drift

Answer 35

random changes in allele frequency that occur in all populations, chance dictates which alleles
are passed onto the next generation.

Question 36

Why is genetic drift only important in small populations?

Answer 36

• Evolution by genetic drift has a greater impact in smaller populations,
• Because random changes in alleles cause a greater percentage change in allele frequencies
  than they would in larger populations.
• In a large population, chance variations in allele frequencies even out across the whole population.

Question 37

How has evolutionary change over a long period resulted in a great diversity of species?

Answer 37

• One population of organisms were separated.
• Speciation occurred and the two populations evolved into separate species.
• The new species were divided again and each of these new populations eventually evolved into a separate
  species due to speciation.
• Process repeated over millions or years to create the millions of species that exist today.