Changing population characteristics

Darwin’s 4 Observations Starter Darwin’s 4 Observations Offspring generally appear similar to their parents No two individuals are identical (variation) Organisms have the ability to produce large numbers of offspring Populations in nature tend to remain fairly stable in size This lead to the theory of natural selection Can you explain it?

Learning Objectives Success criteria To understand natural selection and modern day examples Success criteria Learners should be able to demonstrate and apply their knowledge and understanding of: (h) the mechanism by which natural selection can affect the characteristics of a population over time To include an appreciation that genetic variation, selection pressure and reproductive success (or failure) results in an increased proportion of the population possessing the advantageous characteristic(s). (i) how evolution in some species has implications for human populations. To include the evolution of pesticide resistance in insects and drug resistance in microorganisms.

Natural selection Within any species there is variation, due to different alleles Organisms produce far more young than will survive There will be competition for limited resources Only those best adapted will survive, this is called survival of the fittest Those that survive pass on their ‘successful’ genes to the next generation Over generations this leads to evolution as the alleles that cause the advantageous adaptations become more common in the population These changes may result in new species being formed.

Key Term Definition Speciation Species The formation of a new species A group of organisms, with similar morphological, physiological, biochemical and behavioural features, which can interbreed to produce fertile offspring and are reproductively isolated from other species.

Natural selection

Evolution in action Drug resistance in microorganisms Peppered moths Pesticide resistance in insects

Antibiotic resistance Staphylococcus aureus is a bacterium that can be found on the skin and in the throat. It causes a range of illnesses from minor skin infections to life-threatening diseases such as meningitis and septicaemia.

Antibiotic resistance and MRSA Some strains of Staphylococcus aureus have evolved become resistant to one or more of the commonly used antibiotics including methicillin. These are termed methicillin-resistant Staphylococcus aureus (MRSA). This resistance is then inherited by offspring or it can be passed on via plasmids between bacteria. The resistance gene increases in frequency in the population. Applying an antibiotic to a population of bacteria applies a very strong selection pressure in favour of mutants able to resist the antibiotic.

Antibiotic resistance

Natural selection in action Your turn now, using textbook p294 review pesticide resistance in insects

Plenary Questions Explain why evolution occurs in a shorter time in populations of microorganisms than in populations of mammals Explain why evolution tends to happen in short bursts Microorganisms have short life cycle Every generation can have mutations arise Selection pressure can change species in just a few generations In mammals life cycle is longer and mutations less frequent When environment changes this applies new selective pressures. If variations are advantageous these are selected, species changes. Once species is well adapted to new environment changes slow or stop.