Variation and evolution 4.4

Question 1

What is discontinuous variation?

Answer 1

Normally controlled by a single gene. This gene may have two or more alleles. There are no intermediate forms, e.g. light and dark forms in some moth species.

Question 2

What is continuous variation?

Answer 2

Influenced by the combined effect of a number of genes (polygenes). The effect of an individual gene may be small but their combined effect is large.

Question 3

What effect do environmental factors have on variation?

Answer 3

• These will affect characteristics affected by both single and polygenes.
• For some characteristics, the genes inherited by an organism will cause the potential of having a certain phenotype but environmental factors will determine if this is reached.

Question 4

What is non-heritable variation?

Answer 4

If organisms of identical genotype are subject to different environmental influences, they show considerable variety. Because these influences are varied, they are largely responsible for continuous variation in a population.

Question 5

What effect does sexual reproduction have on variation?

Answer 5

• As a result of sexual reproduction, variation may be increased when the genotype of one parent is mixed with that of another.

Question 6

What are the 3 inbuilt methods of creating variety?

Answer 6

1: The mixing of two different parental genotypes where random cross-fertilization occurs.
2: Crossing over between homologous chromosomes during prophase 1.
3: Independent assortment at metaphase 1.

(2 and 3 happen in both parents so there is further increase in the variation of the offspring.)

Question 7

Why are mutations important?

Answer 7

• Mutations are important because they increase variation in a population.
• There are potential advantages from mutations that are beneficial.
• Most mutations are recessive to the normal allele.
• A recessive mutant allele can only be expressed in the phenotype if there are two copies of the allele in the genotype it must await replication in the gene pool, possibly over many generations before chance brings two recessive alleles together, resulting in their expression.

Question 8

What is an example of competition between organisms?

Answer 8

• Organisms must compete for limited resources of all kinds. E.g, plants compete for light , space and mineral ions.

Question 9

What is intra-specific competition?

Answer 9

Competition between individuals of the same species is the basis of the origin of species by natural selection.

Question 10

What is inter-specific competition?

Answer 10

Competition between individuals of different species. This is illustrated by predator-prey relationships.

Question 11

What is non-random breeding?

Answer 11

Most organisms do not breed randomly, many have complex mating rituals and in a large number of species only certain males/females breed at any one time due to hierarchies within a population.

Question 12

What is a selection pressure?

Answer 12

• Selection in the context of evolution, is the process by which organisms that are better adapted to their environment survive and breed, while those less well adapted fail to do so.
• Better adapted organisms are more likely to pass on their characteristics to succeeding generations.
• An organism’s environment exerts a selection pressure and this determines the spread of any allele within the gene pool.

Question 13

How does selection occur when an environment changes?

Answer 13

If the environment changes some phenotypes will be advantageous and will be selected for, whilst others will be at a disadvantage and will be selected against

Question 14

What is directional selection?

Answer 14

When an environment favours all the individuals carrying one gene at the expense of the individuals carrying an alternative gene.

Question 15

What is stabilising selection?

Answer 15

It results in several different types of individuals being maintained in a population so that genetic variation is maintained.

Question 16

What is disruptive selection?

Answer 16

It favours both extremes of variants and leads to divergence of the phenotype i.e to the emergence of two distinct forms. This form of selection is rare but is more likely to result in the formation of a new species.

Question 17

What is genetic drift?

Answer 17

A gene pool is constantly changing, some alleles becoming more frequent and others less frequent. In some circumstances alleles may be totally lost from the gene pool. Sometimes, variations in gene frequencies in populations occur by chance. This is know as random genetic drift.

Question 18

What is the founder effect.

Answer 18

While the founder population remains small it may undergo genetic drift and become even more different from the larger parental population.
- Different mutations occurring in the small and large populations.
- Different selection pressures.
- Different genes may give competitive advantage in the isolated population compared to the original.
- Founder population is isolated from the main population so they can’t interbreed and share genes with the larger gene pool.

Question 19

What is an isolating mechanism?

Answer 19

A barrier which effectively prevents gene exchange between demes.

Question 20

What is reproductive isolation within a population?

Answer 20

• Occurs when organisms inhabiting the same area become reproductively isolated into two groups for reasons other than geographical barriers.

Question 21

What is sympatric speciation?

Answer 21

Species formation occurring in demes in the same geographic area.

Question 22

What are the barriers to breeding?

Answer 22

• Behavioural isolation: in elaborate courtship behaviour the steps in the display of one subspecies fails to attract the necessary response in a potential partner of another subspecies.
• Mechanical isolation: Genitalia of the two groups may be incompatible.
• Gametic isolation: In pollinating plants pollination may be prevented because the pollen grain fails to germinate on the stigma whereas in animals sperm may fail to survive in the oviduct of the partner.
• Hybrid inviability: despite fertilization taking place development of the embryo may not occur.
• Hybrid sterility: When individuals of different species breed, sets of chromosomes from each parent are different. These sets are unable to pair up during meiosis and so the offspring are unable to produce gametes.

Question 23

What is geographical isolation between populations?

Answer 23

• Occurs when the population becomes geographically split into separate demes.
• The evolution of a new species is very probable, given time.
• This sort of speciation is known as as allopatric speciation.

Question 24

Speciation is …

Answer 24

1: Variation due to mutations arising within a population.
2: A barrier separates one part of the population from the other.
3: The gene frequencies in a small population may not be representative of the overall gene frequencies in the whole gene pool.
4: Different selection pressures cause different changes to the gene pools in the separated populations and they evolve along separate lines.
5: If the barrier is removed there may be sufficient differences to prevent the formation of viable offspring - new species evolved.

Question 25

What is evolution?

Answer 25

The process by which new species are formed from pre-existing species over a period of time.