3.7.3 EVOLUTION

• Individuals within a population of a species may show a wide range of variation in phenotype due to genetic and environmental factors. 

  • Primary source of genetic variation is mutations, which can produce different alleles of genes.
  • Further sources of genetic variation include meiosis (independent assortment and crossing over) and the random fertilisation of gametes during sexual reproduction to create new allele combinations.
• • The environment can influence the way an organism’s genes are expressed (not reach full genetic potential). Can be due to biological factors like predators or non-biological factors like sunlight.
• A species needs variation to survive changing selection pressures and so natural selection/evolution can occur.
• Characteristics that show continuous variation are normally polygenic (determined by many genes).

  Characteristics that show discontinuous variation are usually monogenic (determined by a single gene).
• Predation, disease and competition (selection pressures) result in differential survival and reproduction (process by which natural selection occurs).

  • Organisms with phenotypes providing selective advantages (due to variation from mutation) more likely to reproduce and pass on their favourable alleles to the next generation. 
  • Means the proportion of individuals with the favourable allele will increase in the next generation within the population. (Favourable) allele frequency within gene pool increases and frequency of unfavourable allele decreases. Population evolves.
• Evolution - a change in the allele frequencies in a population.
• Stabilising Selection

  • Selects for average phenotype 
  • Occurs when environmental conditions stay the same. 
  • Results in low diversity - reduces variation.
  • Mean stays the same
• Directional Selection

  • Selects for extreme phenotype
  • Occurs when environmental conditions change (new/change in selection pressure).
  • Individuals with phenotypes suited to the new conditions survive and pass on genes. 
  • Results in the increase of a favoured allele over time (mean changes in that direction)
• Disruptive Selection

  • 2 extreme phenotypes are selected for (2 modal values)
  • Opposite of stabilising
  • Over time, the population becomes phenotypically divided.
  • Can lead to development of 2 new species - accounts for biodiversity
• Speciation - the process by which new species arise from an existing one after a population becomes separated and cannot interbreed.

  • Reproductive separation of two populations can result in the accumulation of differences in their gene pools (through natural selection). 
  • New species arise from existing ones, when these genetic differences lead to an inability of members of the populations to interbreed and produce fertile offspring. 
• Allopatric speciation - the formation of two species from an original one due to geographical isolation. The environments occupied by the two groups are different (different selection pressures), so different alleles are favoured in natural selection.
• Sympatric speciation - the formation of two species from one original species due to reproductive isolation whilst occupying the same geographical location. This can be by:

  • Temporal variation- breeding seasons at different times.
  • Behavioural variation- mutations affecting courtship.
  • Mechanical variation- anatomical differences preventing mating.
  • Gametic variation- results in genetic or biochemical incompatibility.
  • Hybrid sterility- cannot produce viable gametes.
• For both sympatric + allopatric speciation:

  • Variation exists in each population due to random mutations.
  • Different selection pressures for the 2 groups causes different phenotypes to be selected for in natural selection.
  • So the 2 groups evolve differently (allele frequencies change) to the point where the genetic differences are so great, they cannot interbreed to produce fertile offspring (different species). 
• Genetic Drift - Causes changes in allele frequency in small populations. 

  • Genetic drift describes change in allele frequencies in the gene pool of a population (evolution) due to chance (not selection pressures).
  • Due to the random nature of gamete production and fertilisation, certain alleles may increase in the population due to chance.
  • Effect of genetic drift more prominent in smaller populations - The gene pool is smaller, so there are fewer alleles and any change in frequency spreads more quickly. More likely to develop into new species.