7.3: Evolution may lead to speciation

Why do individuals within a population of a species show a wide variation in phenotype?

Genetic factors

• Mutations (primary factor)

• Crossing over between homologous chromosomes during meiosis

• Independent segregation of homologous chromosomes during meiosis

• Random fertilisation of gametes during sexual reproduction

Environmental factors

• Food availability

• Light intensity

What drives natural selection?

• Predation, disease and competition for the means of survival

• Resulting in differential survival and reproduction

What is the process of natural selection?

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

• Due to (named) selection pressure, the new allele might benefit its possessor (explain)

• So organism has a selective advantage

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

• Advantageous allele is inherited by offspring

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

What is stabilising selection, and name an example?

• Organisms with alleles coding for modal 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 decreases 

• So standard deviation decreased

• E.g, human birth weight

What is directional selection, and name an example?

• 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 increases, and those coding for the other extreme of the trait decreases

• E.g, level of antibiotic ressistance

What is disruptive selection?

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

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

• This can lead to speciation 

What is evolution?

The change in allele frequency over many generations in a population, occurring through the process of natural selection

What is speciation and how does it happen?

• How new species arise from existing species

• Reproductive separation of 2 populations of the same specieies

• Resulting in an accumulation of differences in their gene pools

• Leading to an inability of members of different populations to interbreed and produce fertile offspring

How does allopatric speciation take place?

• Population is split due to geographical isolation

• Leading to reproductive isolation, separate gene pools by preventing interbreeding between populations

• Random mutations cause genetic variation within each population

• Different selection pressures act on each population

• So different advantageous alleles are passed on in each population

• So allele frequencies within each gene pool change over many generations

• Eventually different populations cannot interbreed to produce fertile offspring

How does sympatric speciation take place?

• Population is not geographically isolated

• Mutations lead to reproductive selection, separate gene pools by preventing interbreeding within a population

• This could be through gamete incompatibility, different breeding seasons, different courtship behaviors or different body shape/size changes, preventing mating

• So different selection pressures act on each population

• So different advantageous alleles are passed on in each population

• So allele frequencies within each gene pool change over many generations

• Eventually different populations cannot interbreed to produce fertile offspring

What is genetic drift?

A mechanism of evolution in which allele frequencies in a population change over generations due to chance

How does genetic drift take place?

• Some alleles are passed onto offspring more/less often by chance, regardless of selection pressures and wether an allele gives a selective advantage

• Causing random allele frequencies to change over many generations

What is the importance of genetic drift in small populations?

• Gene pool is small, so chance has a greater influence

• This can reduce genetic diversity, meaning some alleles can become fixed or lost entirely

Name some examples of genetic drift

• The bottleneck effect: a population is sharply reduced in size 

• The founder effect: a small, new colony forms from a main population