13.9 Selection and Speciation

Question 1

What are the causes of variation due to?

Answer 1

Genetic and environmental factors

Question 2

Causes of variation in sexual reproduction include:

Answer 2

1. Mutation
2. Meiosis
3. Random fertilisation/fusion of gametes during sexual reproduction producing further genetic variation.

Question 3

Describe how bacteria can become resistant to antibiotics

Answer 3

• Variation due to mutation;
• Some bacteria have allele to survive and are resistant;
• (Differential) reproductive success / adapted organisms reproduce;
• Increase in frequency of allele;
• No interbreeding (with other populations) / separate gene pool / gene pool differs (from other populations);

Question 4

Define interspecific variation

Answer 4

Differences between any two species.

Question 5

Define intraspecific variation

Answer 5

Differences within a species

Question 6

Define continuous variation

Answer 6

Where there are two extremes and a full range of values between.
Controlled by more than one gene (polygene) and influenced by the environment
e.g. height or weight

Question 7

Define discontinuous variation

Answer 7

Where there are distinct categories with no intermediate values
Usually caused by a single gene and predominantly by genetic factors, aren’t under environmental influence.
Individuals tend to be distributed evenly between categories
e.g. blood group

Question 8

Define natural selection

Answer 8

The process by which organisms better adapted to their environment will survive and reproduce in greater numbers, increasing the frequency of advantageous alleles within the population.

Question 9

Describe stabilising selection

Answer 9

• Occurs in all populations where the environment is stable.
• Selection pressures at both ends of the distribution.
• Favours the average.
• Tends to eliminate extremes.
• Reduces variability (the range within pop smaller)
• Reduces opportunity for evolutionary change.
  E.G. human birth mass. Heavy and light babies show higher neonatal mortality= over time reduction in the number of heavy and light babies.

Question 10

Describe directional selection

Answer 10

• Mean in population represents optimum phenotype for existing conditions.
• Environmental change may produce new selection pressure that favours an extreme phenotype.
• Change in conditions will favour the allele combination/extreme necessary for survival.
• Some organisms possess optimum allele combinations.
• Over time, selection will mean this allele combination will predominate and mean phenotype will shift.
  E.G. light coloured moths turning to dark coloured moths after industrial revolution

Question 11

Describe disruptive selection

Answer 11

• Opposite of stabilising selection.
• Environment has selection pressures that favour 2 extreme phenotypes.
• When conditions change, the optimum phenotypes necessary for survival will also change.
• Some organisms will possess new extreme phenotypes, and over time selection means that these 2 extremes will dominate and mean will shift in both directions towards 2 extremes.
• Least common type of selection, most important in evolution.
  E.G. mice living at a beach with light-coloured sand and tall patches of grass so two extremes of light-coloured mice and dark-coloured mice camouflage in two areas.

Question 12

Define species

Answer 12

Group of individuals with similar characteristics and same genes capable of interbreeding to produce fertile offspring.

Question 13

Define speciation

Answer 13

Evolution of a new species from an existing species.

Question 14

How can two populations become genetically distinct?

Answer 14

If 2 populations can interbreed they will not evolve into two distinct species.
The two populations must undergo a period of reproductive isolation (interbreeding prevention period) to become genetically distinct via an accumulation of mutations and different selection pressures.

Question 15

Describe what is meant by speciation (allopatric)

Answer 15

• Geographical isolation;
• Separate gene pools / no interbreeding (between populations);
• Variation due to mutation;
• Different environmental/abiotic/biotic conditions / selection pressures;
• Selection for different/advantageous, features/characteristics/mutation/ /allele;
• Differential reproductive success / (selected) organisms survive and reproduce;
• Leads to change in allelic frequencies;
• Cannot breed/mate to produce fertile offspring.

Question 16

Describe what is meant by speciation (sympatric)

Answer 16

• NOT Geographical isolation;
• Leads to reproductive isolation
• Separate gene pools / no interbreeding (between populations);
• Selection for different/advantageous, features/characteristics/mutation/ /allele;
• Differential reproductive success / (selected) organisms survive and reproduce;
• Leads to change in allelic frequencies;
  Cannot breed/mate to produce fertile offspring.

Question 17

Define genetic drift.

Answer 17

Change in allele frequency within a population between generations.

Question 18

Explain the importance of genetic drift in causing changes in allele frequency in small populations.

Answer 18

• There will always be a genetic drift from each generation but there will be a substantial one in the evolution of a smaller population because there is a bigger impact of allele frequency changes, so evolution can occur faster.
• Each individual in a small population represents a larger proportion of the entire population