Selection

Question 1

What is Natural Selection ?

Answer 1

Differential survival and reproduction due to predation, disease and competition

Question 2

What is a mutation ?

Answer 2

random change to the nucleotide base sequence of DNA.

Question 3

What is the Gene pool ?

Answer 3

all the genes with all the alleles in a population at a given time

Question 4

What is Allele frequency ?

Answer 4

how often different alleles occur in the gene pool

Question 5

What is variation ?

Answer 5

the differences that exist between organisms- either between different species or between individuals within a species

Question 6

What are the two factors that influence variation ?

Answer 6

Genetics
Environment

Question 7

What 3 factors increase Genetic variation ?

Answer 7

Meiosis (crossing over and independent segregation of chromosomes)
random fusion of gametes
mutation

Question 8

What is independent segregation of chromosomes ?

Answer 8

Homologous pairs of chromosomes are randomly aligned at the equator during metaphase. Therefore which homologous chromosome ends up in each daughter cell occurs at random. Therefore each daughter cell(gamete) contains different combinations of chromosomes

Question 9

What is crossing over in meiosis ?

Answer 9

occurs in prophase.
process by which a chromatid breaks and re joins the centromere of its homologous chromosome so alleles can be exchanged.
Cross over points (chiasmata) occur at random.
This means each chromosome contains new and different combinations of alleles.

Question 10

How does random fusion of gametes increase genetic variation ?

Answer 10

any male gamete is able to fuse with any female gamete

Question 11

What is a selection pressure ?

Answer 11

External agents which affect an organisms ability to survive in a given environment

Question 12

Give examples of different selection pressures

Answer 12

predation
disease
competition

Question 13

What is competition within a species known as ?

Answer 13

Intraspecific competition

Question 14

Describe the process of natural selection

Answer 14

random mutation results in new alleles of a gene being formed.
This allele is advantageous, it means the individuals with this allele are better adapted to the environment. This means individuals with this allele are more likely to survive and outcompete others. Therefore they are more likely to survive, reproduce and pass on genes to offspring.
This occurs from generation to generation
allele frequency increases in the population over time.

Question 15

Why is variation in a population important ?

Answer 15

allows organisms to adapt to changing environments. Makes population more stable.

Question 16

What is directional selection ?

Answer 16

acts against one of the extremes in a range of phenotypes.
changes the characteristics of a population
tends to occur when the environment has changed.
One allele confers an advantage and one allele confers a disadvantage.
the mean alleles are no beneficial
the individuals with the selective advantage are more likely to survive and reproduce
Allele frequency will shift over time in the direction of that phenotype.

Question 17

for a graph of directional selection where would the mode of the graph be shifted to ?

Answer 17

to the direction of the advantage (right/left)
standard deviation of the curve is less

Question 18

What is an example of directional selection ?

Answer 18

Antibiotic resistance in bacteria

Question 19

Explain natural selection in bacteria

Answer 19

a few bacteria have antibiotic resistance genes.
a selection pressure is applied when antibiotics are given.
antibiotic resistance alleles are selected for.
The resistant bacteria are more likely to survive, reproduce and pass on resistant alleles to the next generation
allele frequency increases in the population.
Over many generations, bacteria evolve to become resistant

Question 20

What is stabilising selection ?

Answer 20

selects against the extremes in a range of phenotypes and selects for the mean.
the mean allele confers an advantage, the extremes are not beneficial.
The individuals with the selective advantage are more likely to survive, reproduce and pass on genes to offspring.
As a result, extreme phenotype becomes rare and a mean phenotype becomes common.

Question 21

For a graph of stabilising selection what would happen to the mode ?

Answer 21

mode would stay the same
as stabilising selection has reduced variation about the mean as both extremes are being selected against.
(standard deviation or variation about the mean is reduced)

Question 22

What is an example of stabilising selection ?

Answer 22

Average birth weight in humans.
Babies of low weight lose heat more quickly and get ill from infectious diseases more easily, where as babies of large body weight are more difficult to deliver through the pelvis.
Infants of a more medium weight survive more often.

Question 23

What is disruptive selection ?

Answer 23

Disruptive selection favours both extremes.
May lead to evolution of a new species
selects against the mean in a range of phenotypes and selects for the extremes.
Both extreme alleles confer an advantage, the mean alleles are not beneficial.
The individuals with the selective advantage are more likely to survive and reproduce.
As a result, the extreme phenotypes become common and a mean phenotype becomes rare.
This is repeated over many generations and the individuals become better adapted and more different.

Question 24

What is Evolution ?

Answer 24

the change in allele frequencies in a population