Chapter 18 - Populations And Evolution

Question 1

What do the terms population, gene pool and allelic frequency mean?

Answer 1

Population - a group of organisms of the same species in a particular place at a particular time + have the potential to interbreed

Gene pool - all the different alleles of a particular gene for all the individuals in a population at a given time

Allelic frequency - number of times a single particular allele occurs in a gene pool

Question 2

What is the study of population genetics?

Answer 2

Looking at how the allelic frequencies for different alleles change over time

Question 3

What is the Hardy-Weinberg Principle and when does it apply?

Answer 3

A mathematical equation used to calculate the frequency of alleles for a particular gene in a population

Only applies:
- when looking at one gene controlling one characteristic
- when one gene only has two different alleles for it

Question 4

What is the first equation of the Hardy-Weinberg Principle? Explain it

Answer 4

p + q = 1.0 (100%)

p = probability of allele A
q = probability of allele a

• looks at the frequency of each allele (dominant or recessive) for a gene in the population

Question 5

What is the second equation of the Hardy-Weinberg Principle? Explain it

Answer 5

p² + 2pq + q² = 1.0

• looks at the frequency of different genotypes in a population

Question 6

To use the Hardy-Weinberg equations, what assumptions must be made?

Answer 6

1. No mutations arise
2. Population is isolated - no movement of alleles into or out of the population (population is not interbreeding with another population)
3. No allelic selection occurs (both alleles are equally as likely to be passed onto the next generation)
4. Large population
5. Mating within the population is random not controlled

Question 7

What causes variation within a population of a species?

Answer 7

Random mutations = results in new alleles of a gene which may or may not be passed onto the next generation

Question 8

What are the 3 genetic factors influencing variation?

Answer 8

Random mutations, meiosis, random fertilisation of gametes

Question 9

What are the two types of variation in population?

Answer 9

Continuous variation and discontinuous variation

Question 10

Explain the properties and examples of continuous variation

Answer 10

Properties:
- no distinct categories
- no limit of the value
- tends to be quantitative

Examples:
- height, weight, heart rate, finger length, leaf length

Question 11

What is continuous variation controlled by and how is it represented?

Answer 11

Controlled by:
- a lot of genes and environments causing a range of phenotypes between 2 extremes (height in humans)

Represented by a line graph

Question 12

Explain the properties and examples of discontinuous variation

Answer 12

Properties:
- distinct categories
- no in-between categories
- tends to be qualitative

Examples:
- tongue rolling
- finger prints
- eye colour
- blood groups

Question 13

What is discontinuous variation controlled by and how is it represented?

Answer 13

Controlled by:
- a few genes which limit number of phenotypes so no intermediates (A,B,AB,O blood groups)

Represented by a bar graph

Question 14

What is an advantageous allele vs a disadvantageous allele?

Answer 14

Advantageous allele = makes it more likely for organisms to survive to reproduce so pass on allele to next generation

Disadvantageous allele = less likely to survive to reproduce so don’t pass on allele to next generation because they die

Question 15

Describe the process of natural selection [4]

Answer 15

1. There is variation present within a population due to random mutations producing new/ different alleles of a gene.
2. Selective pressures arise due to competition for limited resources (food, living space, water, mates)
3. Individuals with an advantageous allele are better adapted to their environment, giving them a selective advantage and are therefore more likely to survive (survival of the fittest).
4. Those that survive can reproduce so pass on advantageous allele to offspring. This results in an increase in frequency of advantageous allele in population and so species becomes better adapted for environment

Question 16

What are the 3 types of selection that affect a population?

Answer 16

1. Stabilising selection - selection for the individuals around the mean and against individuals at either extremes (birth weights)
2. Directional selection - changes the phenotypes of a population by favouring phenotypes that vary in one direction from the mean of the population (antibiotic resistance)
3. Disruptive selection - favours individuals with extreme phenotypes rather than those with phenotypes around the mean

Question 17

What does species mean?

Answer 17

A group of individuals that can interbreed and produce fertile offspring

Question 18

What does speciation mean?

Answer 18

The evolution of new species from existing ones (occurs over many years and has resulted in huge diversity)

• it occurs when a population of a species becomes somehow isolated from other populations of the same species

Question 19

What are the 2 types of isolation?

Answer 19

• Pre-zygotic isolation
• Post-zygotic isolation

Question 20

What are the kinds of pre-zygotic isolation?

Answer 20

• geographical = isolated by physical barriers (oceans, rivers)
• ecological = inhabit different habitats in same area
• temporal = breeding seasons don’t coincide
• behavioural = mutations cause variations in patterns of courtship so females don’t respond to males
• mechanical = physically impossible for penis to enter vagina
• gametic = gametes prevented from meeting due to genetic/ biochemical incompatibility

Question 21

What are the types of post-zygotic isolation?

Answer 21

Hybrid sterility - hybrids formed from the fusion of gametes from different species are often infertile as cannot produce viable gametes

• e.g. in cross between horse (2n=64) and a donkey (2n = 62), the resultant mule has 63 chromosomes so mule is infertile

Question 22

What are the 2 types of speciation?

Answer 22

Allopatric speciation and sympatric speciation

Question 23

What is allopatric speciation?

Answer 23

Results from 2 populations becoming geographically isolated (physical barriers)

Question 24

What is the key example of allopatric speciation?

Answer 24

Darwin’s finches on Galápagos Islands

• large number of finches on these islands which were similar to those on mainland South America

= a small random population of finches became geographically isolated from mainland (founder event)

Question 25

What does the term genetic drift mean?

Answer 25

Change in the frequency of existing alleles in an existing population due to random sampling of organisms

• the loss of genetic variation

Question 26

Explain the genetic drift that occurred using the finches on Galápagos Islands example

Answer 26

• some birds from founder event cross and inhabited the Galápagos Islands
• the environmental conditions differed e.g. food sources available
• created a selection pressure resulting in natural selection occurring
• this resulted in a change in allele frequencies in the populations on different islands
• if enough change occurs in populations, this may lead to reproductive isolation (so new species that can no longer interbreed)

Question 27

Give the model answer for allopatric speciation [6]

Answer 27

1. Geographical isolation of populations = allopatric speciation
2. Therefore there was reproductive isolation between these two populations as no interbreeding could occur
3. There is variation in each population due to random mutations + genetic drift
4. Different environmental conditions in different locations = different selective pressure
5. Advantageous alleles in each population are selected for by natural selection changing frequency of alleles in different populations
6. Eventually each populations may change so much that they can no longer interbreed between the populations and become different species

Question 28

What is sympatric speciation?

Answer 28

Speciation that results within a population in the same area (no physical separation) but something else isolating them

Question 29

What was the key example of sympatric speciation?

Answer 29

Anoles - types of lizard

Question 30

Give the model answer for the Anoles example of sympatric speciation [7]

Answer 30

1. There is variation in a population of anoles due to random mutations (different limb length, toe pad size and dulap colour)
2. The anoles are not geographically isolated as are found in same environment
3. A species living in the same area compete for limited resources so members move in to different habitats in same area to access and use resources not available to others
4. Leads to reproductive isolation
5. Different environmental conditions in different locations = different selective pressures
6. Advantageous alleles in each population are selected for by natural section (survive , reproduce and pass on alleles) which changes the frequency of alleles in different populations
7. Eventually each population may change so much they can no longer interbreed between the populations so have become different species

Question 31

What is a type sympatric speciation in plants called?

Answer 31

Polyploidy

Question 32

Outline what polyploidy is in plants

Answer 32

• pollen from plants anther to stigma on same plant or on another plant

Species A (2n=4) x Species B (2n=6)
Hybrid zygote = 5 (uneven number so produces sterile offspring)

• however a mitotic error can occur in which spindle fibres do not form properly so a diploid hybrid zygote forms (2n=10)
• all homologous pairs go into one cell rather than split into two
• then self fertilisation produces a fertile hybrid plant = a new species