Variation And Evolution🦋

Question 1

What is a gene mutation?

Answer 1

• A random event resulting in a new allele
• Occasionally a mutation provides a new phenotype such that the mutant has a selective advantage over other individuals in a population
• Over many generations, populations change so individuals with the mutant allele become more frequent (micro evolution)
• Evolutionary changes also result in the formation of a higher taxonomic group such as a new genus or class (macro evolution)

Question 2

What is selection pressure?

Answer 2

• Environmental factors that keep populations in check
• Created by selective agents
• Include disease, competition for resources, predation, lack of light, water or oxygen, changes in temperature

Question 3

What is a selective advantage?

Answer 3

Organisms that are best adapted to survive these selection pressures

Question 4

What is a gene pool?

Answer 4

All the alleles of all the genes in a population

Question 5

What is allele frequency?

Answer 5

• The environment exerts selection pressures, which regulate the frequency of alleles in the gene pool
• Allele frequencies can be expressed as a proportion or a percentage of the total number of copies of all alleles for that gene

Question 6

What is discontinuous variation?

Answer 6

• One pair of alleles usually controls discontinuous variation
• It is categoric - fits into one of several categories and are not affected by environmental conditions
• Discrete non-overlapping groups mainly cause by single genes
• Not influenced by environment
• Examples include blood groups and earlobe attachment

Question 7

Continuous variation

Answer 7

• Controlled by a number of different alleles
• If the organism inherits alleles for this characteristic then it may be expressed in their phenotype
• The environment plays a part - an individual may inherit alleles for being tall but if the individual has a poor diet they will not reach their potential height
• Can have any value on a scale between a maximum and minimum

Question 8

What is heritable variation?

Answer 8

• A variation in the genes in the gametes of an organism will be passed on to the offspring
• Due to different DNA nucleotide sequences or epigenetics

Question 9

What is non-heritable variation?

Answer 9

• Caused by environmental conditions
• Does not lead to natural selection
• E.g. differences in diet can lead to a large variation in body shape and size within a population of animals, but this variation is not passed onto the next generation because the genes of the organism are not altered

Question 10

RECAP EVOLUTION NOTES

Answer 10

FROM BIODIVERSITY

Question 11

Natural selection

Answer 11

• Mutations are important in natural selection - create genetic variation
• Variation leads to some individuals being better adapted to survive in the environment than other individuals - selective advantage
• Organisms with a selective advantage are more likely to survive and reproduce
• By surviving and breeding, they pass in the alleles that give them their selective advantage to their offspring
• Offspring are more likely to survive
• Occurs over a long period of time and results in the characteristics that gave the selective advantage becoming widespread in the population
• Through natural selection the species has evolved

Question 12

Important aspects of natural selection

Answer 12

•Overproduction - more offspring are produced than are required to replace the parents
•However, over time populations remain relatively stable because of:
-interspecific competition
-intraspecific competition

Question 13

Industrial melanism - example of natural selection

Answer 13

•Peppered moth is an example polymorphism
•One species has pale wings and one has dark wings
•Increase in allele frequency of black (melanic) forms was linked to air pollution and is called industrial melanism
•This is due to selective predation by birds
•The birds produced the main selection pressure by feeding differentially on moths according to their background
-in unpolluted areas, the pale moth had the selective advantage of not being visible
-in polluted areas, the melanic moth had the selective advantage
•Allele frequency of melanic moths was high in industrialised areas
•Allele frequency of pale moths has increased over time

Question 14

What is the Hardy-Weinberg principle?

Answer 14

Predicts that the frequency of alleles of one gene in a population will stay the same from generation to generation

Question 15

What assumptions does the Hardy-Weinberg principle make?

Answer 15

• The population is large (100+ individuals)
• There is no movement of organisms in (immigration) or out (emigration) of the population
• There is random mating between individuals in the population
• All genotypes must have the same reproductive success - no selection for or against any phenotype
• No gene mutation

Question 16

What is the Hardy-Weinberg equation?

Answer 16

p^2 + 2pq + q^2 = 1

p+q=1

Question 17

What do the letters in the Hardy-Weinberg equation mean?

Answer 17

• p=frequency of dominant allele A
• q=frequency of recessive allele a
• p^2=frequency of homozygous dominant AA genotype
• q^2=frequency of homozygous recessive aa genotype
• 2pq=frequency of heterozygous Aa genotype

Question 18

Genetic biodiversity

Answer 18

•Variety of alleles in the gene pool of a population
•Can be assessed by:
-at the population level using Simpson’s diversity index
-at the genetic level by assessing the proportion of polymorphic loci across the species genome
-at the molecular level by DNA fingerprinting and sequencing techniques

Question 19

What is speciation?

Answer 19

• The evolution of 2 or more new species from a genetically similar population that undergo genetic differentiation and reproductive isolation
• Become morphologically, physiologically and behaviourally different from members of the original species
• No longer able to breed with members of original species to produce fertile offspring

Question 20

What can cause speciation?

Answer 20

• Genetic drift (random)
• Founder effect (random)
• Genetic bottleneck (random)
• Selection pressures leading to natural selection (not random)

Question 21

What is genetic drift?

Answer 21

• A change in the gene pool/allele frequency that occurred by chance
• Alleles are also lost by genetic drift and not due to selection pressure

Question 22

How is genetic drift different from natural selection?

Answer 22

Allele frequency is changed through random chance, not because the allele confers or does not confer a selective advantage

Question 23

What are the potential impacts of genetic drift?

Answer 23

• May contribute to the extinction of a population or species
• Could lead to production of a new species - important evolutionary mechanism

Question 24

What is the founder effect?

Answer 24

• A specific type of genetic drift

* Occurs when a small group from a population colonises a new area and forms a new population

Question 25

What could result from the founder effect?

Answer 25

• Because of the chance makeup of the new population there may be a reduction in genetic diversity compared with that of the original population
• Over time, this reduction in genetic diversity and in different alleles within the gene pool of the new population can lead to large genotypic and phenotypic differences between the new and old population

Question 26

What is the effect of natural disasters?

Answer 26

• Can lead to extreme examples of genetic drift
• If a large proportion of a population is wiped out randomly by a natural disaster then this caused a random and severe change in the allele frequency of the population’s gene pool
• This is called a genetic bottleneck

Question 27

How can selection pressure leading to natural selection cause speciation?

Answer 27

• Selection changes the frequency of alleles in a population
• Eventually the allele frequencies between two populations may change so much that the organisms with altered phenotypes can no longer successfully breed - a new species is formed

Question 28

What are the two main types of speciation?

Answer 28

• Allopatric speciation

* Sympatric speciation

Question 29

What is allopatric speciation?

Answer 29

•When populations that once interbred become geographically separated into two groups - barriers to gene flow
•These groups are called demes
•In order for speciation to occur, there can be no exchange of genes between the two demes
•Over time, the gene pools of the two demes will alter
-if the demes are experiencing different selection pressures then natural selection will speed up this process
-however, the gene pools can also alter through random genetic drift

Question 30

What is gene flow?

Answer 30

• The transfer of genetic variation from one population to another
• If the rate of gene flow is high enough, then two populations are considered to have equivalent allele frequencies and therefore effectively be a single population.

Question 31

What are demes?

Answer 31

Local populations that interbreed and share a distinct gene pool

Question 32

What is the process of allopatric speciation?

Answer 32

•There is variation in both demes
•Selection pressure leads to some individuals having alleles that give them a selective advantage
-as the selection pressure is different for each population, the alleles conferring the selective advantage are also different
•The organisms with the selective advantage survive to breed and pass on the advantageous alleles to their offspring
•This is repeated many times over a very long period of time and through many generations
•Eventually, the genes of organisms from the two populations become so different that even if the animals were able to interbreed and produce offspring, the homologous chromosomes in the offspring would be so different from each other that they would not be able to pair up during prophase 1
•Therefore, offspring would not be able to produce gametes and so would be infertile
•As the two populations could no longer interbreed to produce fertile offspring, they would now be considered two different species

Question 33

What is sympatric speciation?

Answer 33

Speciation that arises from different forms of isolation

Question 34

What are some possible isolating mechanisms in sympatric speciation?

Answer 34

Pre-zygotic mechanisms:
•Morphological isolation 
•Behavioural isolation 
•Seasonal isolation 
•Gametic isolation
Post-zygotic mechanisms:
•Hybrid inviability 
•Hybrid sterility 
•Hybrid breakdown

Question 35

What is morphological isolation? (Sympatric speciation)

Answer 35

• Variation in the sexual organs of organisms of the same species can lead to them being unable to mate successfully
• Seen in both insects and plants

Question 36

What is behavioural isolation? (Sympatric speciation)

Answer 36

• Variations in courtship and mating behaviour can lead to groups of a particular species becoming isolated from one another
• Has been observed in a species of the fruit fly Drosophila

Question 37

What is seasonal isolation? (Sympatric speciation)

Answer 37

• Reproductive organs mature at different times of year so organisms aren’t able to interbreed
• Particularly seen in plants that flower at different times of year

Question 38

What is gametic isolation? (Sympatric speciation)

Answer 38

• Although the gametes of the two different organisms have the potential to meet, fertilisation does not occur
• Common in marine invertebrates

Question 39

What is hybrid inviability? (Sympatric speciation)

Answer 39

Fertilisation occurs but the embryo is unable to develop into a living organism

Question 40

What is hybrid sterility?

Answer 40

• A hybrid organism is formed but it is sterile because its homologous chromosomes are unable to pair up during meiosis due to different chromosome numbers/structures
• Therefore gametes cannot form
• E.g. of a sterile hybrid is a mule
• E.g. of a fertile hybrid is wheat

Question 41

What is hybrid breakdown? (Sympatric speciation)

Answer 41

• Occurs in F2 generations
• Reproductive failure
• Caused by incompatibility between interacting genes

Question 42

Hybrid fertility

Answer 42

•Sympatric speciation can produce polyploidy
•New species formed because the tetraploid and the original diploid plant cannot interbreed
•Allopolyploids are formed from 2 sets of chromosomes from one species and the other 2 sets from a closely related species
•Meiosis happens more easily because chromosomes from one species pair with each other
•Plenty of gametes are made
-fertile but cannot breed with individuals of parent species - new species