Biology - 7.3 Evolution may lead to speciation

Explain why individuals within a population of a species may show a wide range of variation if phenotype

• Genetic factors

  • Mutations (primary source of genetic variation)

  • Crossing over between homologous chromosomes during meiosis

  • Independent segregation of homologous chromosomes during meiosis

  • Random fertilisation of gametes during sexual reproduction

• Environmental factors (depends on factors - food availability, light intensity)

What is evolution?

• Change in alle frequency over time / many generations in a population

• Occuring through the process of natural selection

Describe factors that may drive natural selection

• Predation, disease and competition for the means of survival

• These result in differential survival and reproduction

  • E.g. natural selection

Name the principles of natural selection in the evolution of populations

MARIA

Mutations

Advantage

Reproductive success

Inheritance

Allele frequency

Explain the principles of natural selection in the evolution of populations

• Random gene mutations can result in (named) new alleles of a gene

• Due to (named) selection pressure, the new allele might benefit its possesor (explain) → organism has a selective advantage

• Possesors are more likely to survive and have increased reproductive success

• Advantageous allele is inherited by members of the next generation (offspring)

• Over many generations, (named) allele increases in frequency in the gene pool

Explain the effects of stabilising selection

• Organisms with alleles coding for average variations of a trait have a selective advantage

• So frequency of alleles coding for average variations of a trait increase and those coding for extreme variations of a trait decrease

• So range / standard deviation is reduced

E.g. babies with an average weight

Explain the effects of directional selection

• Organisms with alleles coding for one extreme variation of a trait have a selective advantage

• So frequency of alleles coding for this extreme variation of the trait increase and those coding for the other extreme variation of the trait decrease

Explain the effects of disruptive selection

• Organsisms with alleles coding for either extreme variation of a gene have a selctive advantage

• So frequency of alleles coding for both extreme variations of the trait increase and those coding for the average variation of the trait decrease

• This can lead to speciation

Define and describe speciation

How new species arise arise from existing species

1. Reproductive separation of two populations (of the same species)

2. This can result in accumulation of differences in their gene pools

3. New species arise when these genetic differences lead to an inability of members of these populations to interbreed and produce fertile offspring

Describe allopatric speciation

1. Population is split due to geographical isolation

2. This leads to reproductive isolation, seperating gene pools by preventing interbreeding between populations

3. Random mutations cause genetic variation within each population

4. Different seelection pressures / environments act on each population

5. Do different advantageous alleles are selected for in each population

6. So allele frequencies within each gene pool change over many generation

7. Eventually different populations cannot interbreed to produce fertile offspring

Describe sympatric speciation

1. Poplation is not geographically isolated

2. Muations lead to reproductive isolation, seperating gene pools by previnting interbreeding

   • Gamete incomaptibility

   • Diffferent breeding seasons

   • Different courtship behaviour preventing mating

   • Body shape / size changes preventing mating

3. Different selection pressures act on each population

4. So different advantageous alleles are selected for in each population

5. So allele frequencies within each gene pool change over many generations

6. Eventually different populations cannot interbreed to produce fertile offspring

Explain genetic drift and its importance in small populations

Genetic drift: mechanismms of evolution in which allele frequencies in a population change oer generations due to chance

As some alleles are passed on to offspring more/ less often due to chance

• Regardless of selection pressures and whether alleles give a selective advantage

So strongest effects in small populations as gene pool is small and chancce has a greater influence

• Population sharply reduced in size → bottleneck effect

• Small new colony forms from main pop. → founder effect

This can reduce genetic diversity → some alleles lost entirely