D2 Species and Speciation

Question 1

Define allele frequency

Answer 1

The proportion of an allele within a population

Question 2

Define gene pool

Answer 2

All the alleles present in an interbreeding population

Question 3

Outline how natural selection results in a change in allele frequency in a population’s gene pool over a number of generations

Answer 3

• By natural selection, alleles encoding for beneficial adaptations will result in a survival advantage and lead to improved reproduction
  
  • These alleles are consequently more likely to be inherited and thus the population’s gene pool will change over generations

Question 4

Discuss limitations of the definition of ‘species’

Answer 4

• Certain organisms (e.g. bacteria) produce asexually and therefore they do not interbreed
  
  • Some organisms are only known through fossil records, making it impossible to ascertain breeding capacity
  • ‘Sibling species’ look identical but don’t interbreed
  • Some distinct species can interbreed and produce fertile offspring
  • Geographically isolated organisms may never come into contact, meaning there is no information regarding their ability to interbreed

Question 5

State three barriers to gene pools

Answer 5

Temporal isolation, behavioural isolation, mechanical isolation

Question 6

Describe temporal isolation

Answer 6

Occurs when two species mate or flower at different times

E.G. Different frog species live in the same pond but breed at different times

Question 7

Describe behavioural isolation

Answer 7

Occurs when two species respond to different specific courtship patterns
E.G. Some crickets are morphologically identical but only respond to species specific mating songs

Question 8

Describe mechanical isolation

Answer 8

Occurs when genital differences prevent copulation (animals) or when flowers are pollinated by different animals (plants)
E.G. Galagos have distinctly shaped genitalia that will only fit other members from the same species

Question 9

Explain how polyploidy can contribute to speciation

Answer 9

• Polyploidy is a condition in which an organism has more than two complete sets of chromosomes in all somatic cells
  
  • It is far more common in plant species as theylack separate sexes and are capable of asexual reproduction (self-pollination)
  • It may occur as a result of the failure of a meiotic cell to undergo cytokinesis (so chromosome replication occurs minus cell division)
  • Consequently gametes are diploid (2n) and resulting offspring are tetraploid (4n)
  • Because tetraploid offspring can no longer mate with diploid organisms(triploid offspring tend to be infertile), speciation has occurred

Question 10

Outline allopatric speciation

Answer 10

Caused by geographical separation of populations
Populations occupy different geographic areas
E.G. Adaptive radiation of Galapagos finches

Question 11

Outline sympatric speciation

Answer 11

Involves a reproductive, temporal or behavioural separation
Populations occupy the same geographical areas
Example: Polyploidy in wheat strains

Question 12

State a similarity between allopatric and sympatric speciation

Answer 12

Both involve the formation of a new speciesvia isolation of the gene pool from an existing species

Question 13

Define speciation

Answer 13

The formation of new and distinct species in the course of evolution by the splitting of an existing species

Question 14

Outline the process of adaptive radiation

Answer 14

• Adaptive radiation describes a rapid evolutionary diversification of a single ancestral lineage
  
  • It occurs when members of a single species occupy a variety of niches with different environmental selection pressures
  • Consequently, members evolve different morphological adaptations as a result of natural selection
  • Adaptive radiation results in speciation (many species from an ancestral line) and may be further enhanced by reproductive isolation
  • An example of adaptive radiation can be seen in the variety of beaks seen in the Galapagos finches

Question 15

Outline convergent evolution

Answer 15

Different ancestor
Converge to produce analogous structures
Species appearance becomes more similar over time
Species are unrelated (genetically different)
Example: Wings in insects, birds and bats

Question 16

Outline divergent evolution

Answer 16

Common ancestor
Diverge to produce homologous structures
Species appearance becomes more different over time
Species are closely related (share genetic homology)
Example: Pentadactyl limb structure in vertebrates

Question 17

Outline gradualism as a pace of evolution

Answer 17

• Continuous change at aconstant paceover a long period of time
  
  • Arises as a result of the gradual accumulation of mutations / variations
  • Examples in the fossil record of gradual change with intermediate forms support this theory
  • An example is the evolution of the modern horse (gradual change in size and foot structure with changing habitat)

Question 18

Outline punctuated equilibrium as a pace of evolution

Answer 18

• Evolution proceedsrapidly in burstsfor short periods of time, intermittent with long periods of stability
  
  • In periods of stability organisms become well suited to the environment, with natural selection acting to maintain characteristics
  • Equilibrium is punctuated by a rapid environmental change (e.g. volcanic eruption, meteor impact) which leads to directional selection
  • Gaps in the fossil record and the lack of intermediate forms for many species support this theory
  • Strata in the fossil record that show the appearance of many new species following a mass extinction support this theory

Question 19

Define transient polymorphism

Answer 19

• A situation when there are two alleles in a gene pool (polymorphic) and one allele is gradually replacing another
  
  • This is due to a strong environmental selective pressure causing directional selection to eliminate one allele

Question 20

Outline an example of transient polymorphism

Answer 20

Antibiotic resistance in bacteria

Question 21

Define balanced polymorphism

Answer 21

• A situation when there are two alleles in a gene pool (polymorphic) and the frequency of the two alleles in not changing
  
  • Occurs when selective pressures promote the coexistence of the two alleles (i.e. heterozygous advantage) causing a stabilising selection

Question 22

Outline an example of balanced polymorphism

Answer 22

• Sickle cell anaemia is controlled by a single gene mutation
  
  • Individuals homozygous for the sickle cell allele have abnormally shaped red blood cells that cause anaemia
  • Individuals homozygous for the normal blood cell allele are highly susceptible to malarial infection
  • Malaria parasite is less successful at infecting sickle-shaped blood cells
  • In areas where malaria is common, heterozygous individuals experience an advantage over either form of homozygote
  • These individuals are more likely to survive and reproduce, leading to a balance in the frequency of the two alleles (heterozygous advantage)