E8 Summary

Stage 2 Biology – Ecosystems Notes

 

E8. Natural selection acts on variation in a population

 

·         Gene pool = sum of all of the alleles of all the individuals in a population

·         Changes in the gene pool

o   Mutations

§  Changes in DNA and can give rise to new allelic forms of genes

§  Gene mutations are any locus are rare – but they are the only source of new alleles

o   Natural selection

§  Most powerful force likely to bring about changes in the gene pool

o   Gene flow

§  Exchange of alleles between populations

§  Can occur by movement of organisms with different genotypes in and out of populations

§  Consequent change in frequency of genes in the gene pool

§  If flow of genes is reduced = greater diversity between the two populations

§  Observed differences between three sub-species can be due to little flow of genes between the populations, meanwhile they have evolved and developed in separate environments

·         Genetic variability in populations

o   Variation is essential for evolution and makes it possible for populations to change with environmental changes

o   Variation is the ‘raw material’ on which natural selection can act

o   Extent of variation depends on whether a species reproduces sexually or asexually

§  In asexual species, mutations are the only source of variation

§  In sexual species, mutations are the only way new alleles can arise, but as a result of meiosis and fertilisation, new combinations are found in the offspring compared to parents

o   Effects of the environment can cause some variation, but these will not be inherited

o   In humans, phenotypic differences may be those characteristics that are distinguishable (ie. hair/skin), but behaviour and intelligence have an inherited component; genetic diseases

·         Dominant and recessive characteristics

o   Colour blindness is simple and due to one gene pair

o   Height cannot be modelled using a single pair of genes

§  Polygenic

§  Two or more separate genes in the genome have an additive effect to determine a single characteristic

·         Genetic variation may not be apparent

o   Some are bigger than others

o   Some have slightly different colours, shape and therefore resistance to environmental factors

·         Variation exists between individuals in nearly all populations (structural, functional or behavioural)

Theory of evolution

·         The oldest life form on Earth existed over 4.5 billion years ago

·         All organisms have evolved from a simple common ancestor

·         Species change through time

·         Evolution can be recognised by the changing frequencies of genes in populations

 

·         Charles Darwin

o   Hypothesised that for all populations there were more individuals born than ever survive to reproduce

o   Factors in the environment act to hold all populations in check

o   Suggested natural selection – nature select those individuals who were best adapted – fittest

o   Variation between individuals gave some organisms a reproductive advantage which would enable them to leave more offspring surviving in the next generation

o   Proposed that life evolved from a common ancestor and that due to natural selection, populations were modified over time according to the different selective pressures

o   He did not have any knowledge about the gene as the unit of inheritance

·         Gene frequency

o   Characteristics are a result of expression of one or more genes

o   When nature selects characteristics, certain gene types are being favoured or selected

o   Leads to changes in frequency of particular alleles in the gene pool

o   Therefore changes in the gene pool are evidence of evolution occurring

o   Natural selection acts on characteristics expressed in the phenotype

o   Selection can be by biotic or abiotic factors

§  Predators, disease-causing organisms, organisms that co-exist

§  Rainfall, temperature, nutrient levels, soil characteristics, light availability, wind

o   Evolution is extremely slow, although can be observed over shorter periods if selective pressures are strong (example of the moth Biston betularia)

§  Most individuals were light coloured and found on lichen-covered trees/rocks – well camouflaged

§  There was a black mutant form that was rare

§  A change occurred due to the production of soot from industry and many lichens had died out so the tree trunks were darker – then the black moth camouflaged better

§  These changes occurred over 80-100 years (quickly)

o   Rabbit population and myxoma virus

§  Rabbits were introduced – became a major pest destroying pastureland

§  Virus was introduced to control numbers – spread by a mosquito host

§  Virus initially killed rabbits in 6-10 days after infection, but started taking 3-4 weeks

§  Natural selection

·         Milder form lived in rabbits for longer so spread more easily

·         Rabbits that were naturally resistant were surviving an dpassing on genes

§  Now only has 25% kill rate

·         Bacterial resistance

o   Natural selection by an abiotic factor

o   Some variation between bacterial populations

o   Variation arisen by mutations in DNA or transfer of genes by plasmids

§  Sections of DNA in bacteria and yeast

§  Replicate independently of the main DNA

§  Contain genes that confer resistance to antibiotics

§  Can transfer these sections to other bacteria in a process called conjugation

o   Antibiotics kill or inhibit growth of microorganisms – natural antibiotics produced by fungi provide protection against other microbes that may be competing for resources

§  Interfere with cell wall production; block protein synthesis; affect function of cell membranes

 

o   Penicillin

§  Was effective against a range of bacteria

§  With continued use, individual bacteria with resistance have survived and reproduced and passed on genes

§  Over generations there have been changes in the gene pool – increase in frequency of genes coding for resistance

o   Other antibiotics have been developed, but resistant strains continue to appear

o   Overuse and misuse – leads to increased resistance

§  Taking antibiotics when they are not required

§  Not finishing a dose

§  Taking more than one antibiotic for a single infection

§  Pressure from patients to get doctors to prescribe antibiotics when the infection is viral (antibiotics won’t work)

§  Taking excess can lead to increase in resistant strains in the intestine – could spread to the bloodstream

§  Antibiotics used in Australia with livestock to be used as growth promoters

§  Ingestion of antibiotics from eating meat/fruit/vegetables treated in this manner could increase risk of spreading resistant strains

·         It is the population that evolves, not a single individual