B3.5

B3.5

1. Mechanisms of Speciation

   • Mutation
     • Definition: Is a change in the base sequence of DNA
     • Effect on Gene Pool: if it is inheratible, non-somatic, it introduces a new allele in to the gene pool, which could become more favourable
   • Gene Flow
     • Definition: the movement of alleles between members of species by reproduction
     • Effect on gene pool: there is a higher probability of new alleles being introduced into the gene pool,population.
   • Genetic Drift
     • Definition: is the change in the relative frequency in which the allele occurs in a population due to random sampling and chance.
     • Bottleneck effect
       • Definition: occurs when there is a dramatic reduction in the population size.
       • Effect on Gene pool: as a result there is often loss of alleles from the population, decreasing genetic diversity. When when the population increases there will still be a lack in diversity.
     • Founder Effect
       • Definition: when a small number of individuals leaves the original population and starts a new population somwhere else.
       • Effect on gene pool: the isolated founder population is likely to have different allele frequencies to the original population. here will be no gene flow between the original and founder populations due to geographical isolation. Selections pressures acting on the two populations may differ significantly and this will cause the populations to diverge.
   • Natural Selection
     • 1. All organisms have a high reprodutive rate, but food supply and other resources in the environment are limited. This leads to a continual struggle for existence among the offspring.
     • 2. Among the offspring there is variation in all the characteristics, some will be better adapted to the environment than others.
     • 3. Those organisms possessing favourable variations will be able to survive longer in a given environment and so will have more chance to survive and reproduce, and therefore will pass on these favourable characteristics to their offspring
     • 4. Over a period of time, each successive generation will be better adapted to the environment.
     • Types of natural selection
       • Stabilizing; selection selection against individuals that deviate from the mean i.e. selection against the extremes of a normal bell curve, results in a larger number around the mean and smaller range/variation of the characteristic
       • Directional: selection against individuals to the right or left of the mean i.e. one extreme only, results in the shift of the mean towards the other extreme only, results in the shift of the mean towards the other extreme, associated with gradually changing conditions. eg. peppered moths industrial revolution
       • Disruptive: selection in which 2 or more variants are at an advantage and the intermediate types(mean) are selected against, the results in the mean moving in 2 directions and a curve with 2 peaks, could be a start of speciation.

2. Process of Speciation

   • Allopatric Speciation
     • Definition: The formation of two different species( from and anestral species) which involves a geographic barrier.
     • Process:
       • common ancestor
         • geographic barrier
           • no gene flow
             • the two populations are in different environments with different selection pressures
               • allele frequency changes
                 • geographic barrier removed
                   • reproductive isolating mechanism prevents mating
                     • =speciation has occured
   • Speciation Definition: is the formation of a new species. For it to occur there must be no gene flow, be isolated for a long period of time and have different selection pressures from different environments.
     • Isolating Mechanisms
       • Geographical Barrier: continental drift, mountain ranges, rivers, lakes, seas/islands, desserts
       • Climate Change: temp. changes that can cause ice ages and vice versa, results in sea levels changing, can produce geographic barriers.
       • Ecological Barriers: different adaptations have been selected as the environment has changed
       • Biological/Reproductive Barriers: prevents species producing fertile offspring
         • Pre-zygotic Isolation:occur before fertilisation
           • Habitat differences=live in same area, occupy different habitats
           • Temporal= mate different seasons active at different times of day,tides
           • Structural/Mechanical= genitalia different shape so can't mate
           • Gamete Incompatibility= mating occurs but gametes do not fuse, chemicals incompatible
         • Post-zygotic Isolation: prevents successful reproduction after fertilization
           • Hybrid Inviability= zygote forms but dies due to unmatched chromosomes
           • Hybrid Sterility= hybrid forms but is sterile due to unmatched chromosomes so meiosis cannot occur and no gametes are produced. e.g. mule
           • Hybrid Breakdown= F1 are fertile but F2 their offspring are not
   • Sympatric Speciation
     • Definition: The formation of 2 different species(from an ancestral species) not involving a geographic barrier
     • From reasons such as:
       • Polyploidy: occurs when an organism(almost always plants) aquires an extra set of chromosomes i.e. >3n is an instant form of speciation, as it occurs in one generation.
         • 2. Allopolyploidy
           • Occurs when two closely related species contributes sets of chromosomes. Look for 4n or above. Look for same genus different species.
             • AA x BB= AB sterile hybrid because chromosomes can't form homologous pairs during meiosis and therefore gametes cannot form
             • AB=AABB chromosome number doubles by non-disjunction in mitosis, now fertile 4n tetraploid
         • Normal: haploid(2n) diploid(2n) Polyploids: triploid(3n) tetraploid(4n)
           • Advantages of Polyploidy: is that they often show "hybrid vigour" eg. bigger, grow faster, produce more offspring and are disease and frost resistant.
         • 1. Autopolyploidy
           • Occurs when only one species contributes the extra set(s) of chromosomes.Look for triploid or tetraploid, can occur by: 2 sperm fusing with one egg(rare) or by non-disjunction. This occurs due to the complete failure or the spindle fibres to separate the chromosomes/chromatids during cell division.
             • Triploid Autopolyploidy
               • Gametes A(n) x AA (2n)= AAA sterile 3n hybrid, can grow vegetatively
             • Tetraploid Autoplyploidy
               • Gametes AA(2n) x AA(2n) =AAAA fertile 4n tetraploid
       • Niche Isolation
         • Can arise from a change in niche or lifestyle where mating can only be between those who have adopted the new lifestyle
       • Sexual selection

3. Patterns of Evolution

   • Punctuated Equilibrium
     • Definition: When species stay the same for a long period of time called stasis, these are then punctuated by short bursts of evolution which produce a new species. This is catalysed by a crucial change in the environment.
   • Gradualism
     • Definition: When two populations slowly diverge from one another by accumulating fifferent adaptations when different selection pressures are acting upon them.
   • Divergent Evolution
     • Definition: occurs when ancestral species evolves into two or more species that have become specialised to occupy different ecological niches. If there is a large number of species from an ancestral species due to availability of ecological niches, this is called adaptive radiation.
       • Convergent Evolution
         • Definition: occurs when two or more species evolve from different ancestors but develop similar features (analogous structure) due to similar selection pressures

Imágenes y archivos del mapa