3.3.8 Evolution may lead to Speciation Flashcards Preview

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What is evolution?

Is the change in allele frequencies in a population over time


Name 2 methods in which evolution occurs

  1. Natural selection
  2. Genetic drift


Describe how natural selection occurs by evolution

  1. Selection pressures (e.g. predation, disease, competition) create struggle for survival
  2. Genetic variation between individuals
  3. Individuals with (beneficial) allele that makes them better adapted to selection pressure than others
  4. This increases their chance of survival & more likely to reproduce and pass on this (beneficial) allele
  5. Increased freqency of beneficial alleles in next generation 
  6. Offspring more likely to survive, reproduce and pass on their alleles
  7. ∴ frequency of beneficial alleles in gene pool increases from generation to generation


Name 4 observations Darwin made

  •  Population remain stable
  • Offspring appear similar to their parents
  • No two individuals are the same
  • Organisms have the ability to reproduce


What type of variation results in evolution?

Only genetic variation


What is a selection pressure?

Anything that affects an organism's chance of survival and reproduction


Name 3 selection pressures 

  • Human activity / pollution 
  • Availability food sources 
  • Diseases
  • Competition 
  • Predators 
  • Land clearance 


Types of natural selection affect ____ ______  in different ways

allele frequency


Name the 3 types of natural selection

  • Stablising Selection 
  • Directional Selection 
  • Disruptive Selection 


Define Stablising Selection 

Where individuals with alleles for characteristics towards the middle of the range are more likely to survive and reproduce

(Favours the average = averages increase and extremes decrease)


When does stablising selection occur?

Occurs when environment isn't changing = reduces range of possible phenotypes


Define Directional Selection 

Where individuals with alleles for single extreme phenotype are more likely to survive and reproduce


When does directional selection occur?

Occurs in response to environmental change


Describe Disruptive Selection 

Where individuals with alleles for extreme phenotypes at either end of the range are more likely to survive and reproduce


When does speciation occur?

When populations of same species become reproductively isolated


Name the 2 types of speciation

  • Allopatric Speciation 
  • Sympatric Speciation 


What does allopatric speciation involve?

Geographical isolation


Describe allopatric speciation (6x)

  1. Populations that are geographically separated will experience slightly environmental different conditions
  2. Populations will experience different selection pressures ∴ = selection for different alleles
    1. Variation due to mutation 
  3. Selection for advantageous allele = selected organisms survive and reproduce 
  4. Leads to changes in allele frequency
  5. Eventually, individuals from different populations changed so much = can't breed to produce fertile offspring
    • Become reproductively isolated
  6. 2 groups = become separate species


Allopatric Speciation 

Name 3 ways in which different changes in allele frequencies could occur

  • Different alleles could be more favourable in new conditions and directional selection could take place
    • e.g. colder climate: directional selection will act on alleles for fur length = increasing frequency of allele for longer fur length
  • Mutations could occur in each population
  • Genetic drift


What is sympatric speciation caused by?

Mutations, changes in mating patterns


Describe how sympatric speciation could occur due to mutations

  1. Most eukaryotic organisms = diploid
    1. Sometimes mutations occur that increase no. of chromosomes
    2. Known as polyploidy 
  2. Individuals with different no. of chromosomes can't reproduce sexually to give fertile offspring
    • e.g. polyploid organism in diploid population = reproductively isolated
  3. If polyploid organism reproduces asexually = new species develops


Sympatric speciation is more common in ____ than ______

plants than animals


Why does reproductive isolation occur?

Occurs ∵ changes in alleles and phenotypes in some individuals prevent breeding successfully with individuals without these changes


Name 3 examples of changes that occur to individuals that leads to reproductive isolation 

  • Seasonal 
    • Different flowering or mating seasons
    • Or become sexually active at different times of year
  • Mechanical 
    • Changes in genitalia prevent successful mating
  • Behavioural 
    • Develop courtship rituals that aren't attractive to main populations


What is genetic drift?

When chance rather than environmental factors dictates which individuals survive, breed and pass on their alleles


Give an example of how genetic drift could lead to speciation 

  1. Individuals within population show variation in their genotypes
  2. By chance, allele for one genotypes (B) is passed on to offspring more often than others
  3. ∴ no. of individuals with the allele increases
  4. Changes in allele frequency in 2 isolated populations could eventually lead to reproductive isolation and speciation


Evolution by genetic drift has greater effect in ____ populations



Why does genetic drift have a greater effect in smaller populations?

∵ chance has greater influence in smaller populations

(In larger populations, any chance variations in allele frequency tend to even out across whole population)


Explain how an insect population can become resistant to an insectidice (3)

  • mutations
  • producing alleles giving resistance
  • natural selection / leads to increased frequency of alleles in population 


Copper-tolerant grass produces its flowers eariler in the year than non-tolerant grass of the same species. Explain how this might produce two different species of grass. (5)

  1. (populations are) reproductively isolated (due to different flowering times)
  2. ∴ no flow of alleles between the populations/different (random) mutations in each group
  3. different selection pressures
  4. frequency of alleles will change
  5. (genetic differences lead to) members of populations become unable to interbreed and produce fertile offspring

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