Selection and speciation A2

Question 1

Changes in ecosystems can lead to speciation. In Southern California 10,000 years ago a number of interconnecting lakes contained a single species of pupfish.
Increasing temperatures caused evaporation and the formation of separate, smaller lakes and streams. This led to the formation of a number of different species of pupfish.
Explain how these different species evolved. (Allopatric speciation)

Answer 1

1. Geographical isolation
2. separate gene pools/no interbreeding (between populations)
3. variation due to mutations
4. different environmental/abiotic/biotic conditions/selection pressures
5. selection for different/advantageous features/characteristics/mutation/allele
6. differential reproductive success/(selected) organisms survive and reproduce
7. leads to change in allele frequency
8. occurs over a long period of time

Question 2

Changes in ecosystems can lead to speciation. A high concentration of copper in soil is toxic to most plants. In some areas where the soil is polluted with copper, populations of grasses are found to be growing. These populations of grass belong to to a species also found growing on unpolluted soils. It has been suggested that a new species of grass may evolve on soil that has been polluted with copper.
Explain how this species may evolve.

Answer 2

1. variation/variety
2. mutation
3. some plants have allele to survive/grow/live in high concentrations of copper/polluted soils
4. (differential) reproductive success/adapted organisms reproduce
5. increasing in frequency of allele
6. no interbreeding (with other populations)/separate gene pool/gene pool differs (from other populations)
7. Sympatric Speciation

Question 3

Define the term genetic variation.

Answer 3

The difference in DNA sequences between individuals within a population.

Question 4

Define the term mutation.

Answer 4

A change in the usual DNA sequence at a particular gene locus.

Question 5

Describe how natural selection results in change to the allele frequency within a gene pool.

Answer 5

Natural selection acts on the gene pool by changing allele frequency within the population. Traits that enhance survival and reproduction will be represented wit increasing frequency in the gene pools.

Question 6

Explain the difference between directional and stabilising selection.

Answer 6

Stabilizing selection results in a decrease of a population ‘s genetic variance when natural selection favours an average phenotype and selects against extreme variations. In directional selection, a population’s genetic variance shifts toward a new phenotype when exposed to environmental changes.

Question 7

Explain how natural selection and geographic isolation can lead to the formation of new species.

Answer 7

If the environments differ, different adaptations are favoured by natural selection. This leads to different characteristics evolving in each group as time passes. Eventually the groups become so different that if they come together again they are unable to interbreed and are now separate species.

Question 8

Define term reproductive isolation.

Answer 8

The inability of a species to breed successfully with related species due to geographical, behavioural, physiological, or genetic barriers or differences.

Question 9

Conditions for Hardy-Weinberg Equilibrium

Answer 9

1. No mutations
2. Random mating
3. No natural selection
4. Extremely large population size
5. No gene flow
   Describes a population that is not evolving. In real populations, allele frequencies often do change over time. Such changes occur when one or more of the conditions for the equilibrium are not met

Question 10

Bottleneck Effect

Answer 10

A change in allele frequency following a dramatic reduction in the size of a population. Could be due to natural disasters or purposeful killing by humans.

Question 11

Directional Selection

Answer 11

Form of natural selection in which the entire curve moves; occurs when individuals at one end of a distribution curve have higher fitness than individuals in the middle or at the other end of the curve. Favours individuals at one extreme end of the phenotypic range. Eg: Long horns

Question 12

Disruptive Selection

Answer 12

Form of natural selection in which a single curve splits into two; occurs when individuals at the upper and lower ends of a distribution curve have higher fitness than individuals near the middle. Favours individuals at both extremes of the phenotypic range. Eg: Rock pocket mice

Question 13

Stabilizing Selection

Answer 13

Natural selection that favours intermediate variants by acting against extreme phenotypes. Eg: Malaria and sickle cell mutation.

Question 14

Genetic Drift

Answer 14

A change in the allele frequency of a population as a result of chance events rather than natural selection. Ex: by chance alone, alleles encoding darker color have become more common over several generations.

Question 15

What is sympatric Speciation?

Answer 15

• takes place with no geographic barrier
• same species living in same place but 2 separate populations
• no gene flow between populations
• ecological factors eg. soil PH etc may effect
• behavioural factors eg. Different feeding patterns
• different selection pressures
• cause natural selection
• can no longer interbreed