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5.2 – Natural Selection

5.2 – Natural Selection 

Occurrence of natural selection:

  • Within a species, different individuals of that species show genetic variation.
  • Individuals that are best suited for their environment will survive and reproduce.
  • If there was no variation within a species, then all individuals would be the same and no individual would be favoured over the other and natural selection would not take place

Genetic variation:

  • Sexual reproduction can produce variation in a species through fertilization and meiosis.
  • Sexual reproduction occurs when two different members of a species create offspring that have a combination of genetic material contributed from both parents.
  • During meiosis 50% of the females chromosomes will end up in the egg(haploid gamete) and 50% of the male’s chromosomes will end up in the sperm (haploid gamete).
  • During meiosis chromosomes will line up or assort independently of each other creating (2n ) possible variations of chromosomes in the sex cells.
  • During meiosis, specifically prophase I, crossing over might occur in homologous chromosomes where parts of each chromosome are exchanged.
  • Random fertilization through sexual reproduction gives millions of sperms a chance at fertilizing the egg. This allows mutations that have occurred in different individuals to come together in their offspring.
  • Lastly, genetic mutations might occur where new alleles are produced. Genetic mutations are the original source of variation within a species.
  • Mutations that give an advantage are selected for.
  • Mutations that give a disadvantage are selected against.

Struggle for survival:

  • Populations tend to produce more offspring than the environment can support or that could survive in a particular community or ecosystem.
  • For example, fish produce thousands of eggs but only few make it to adulthood.
  • Plants also can produce hundreds or thousands of seeds to be released into the environment.
  • When parents don’t spend a lot or even any time caring for their young, they produce many offspring. This is a reproductive method used to make sure some offspring make it to the next generation.
  • Parents that put a lot of time and energy protecting and raising their young tend to have far smaller litters, i.e. most mammals.
  • If there are too many organisms, the demand for resources increases.
  • However, there is a limited supply of resources in an ecosystem.
  • Overpopulation and a limited amount of resources create competition within a population.
  • They have to compete for mates, food, space, predation, and disease.
  • Within a population, there is genetic variation between the individuals in the population.
  • The organisms with the beneficial characteristics will be able to out-compete the other individuals with the less beneficial or harmful genetic traits for limited resources and mates.
  • Therefore, these individuals will survive and reproduce and pass these genetic traits onto the next generation of offspring.
  • Organisms with less desirable traits will die or produce less offspring
  • Over many generations the accumulation of these beneficial genetic traits may result in a change in the population known as evolution.
  • For another species to develop, these genetically different individuals eventually have to become reproductively isolated (separated from the general population) where they will only reproduce with individuals with similar genetic traits.
  • Acquired characteristics of an individual such as large muscles are not passed on to an organism’s offspring

Antibiotic resistance:

  • Antibiotics kill bacteria directly or weaken the bacteria so your immune system can fight and destroy the invading pathogen.
  • If a patient has a bacterial infection, when antibiotics are given to fight the infection the majority of the original population of bacteria will be destroyed.
  • However, some of these bacteria might not die because of changes within their DNA. These changes could be caused by mutations within their genome or the transfer of an antibiotic resistant gene from another bacterium.
  • Resistance is more likely to occur if the proper amounts of antibiotics aren’t taken or if a patient doesn’t finish the prescription.
  • These resistant bacteria will survive and reproduce, creating more identical resistant bacteria.
  • These resistant bacteria will make the person sick again in the future.
  • However if given the same antibiotic, these bacteria will no longer be destroyed.
  • Another antibiotic can be prescribed to kill these new resistant bacteria.
  • Resistance can be passed onto other pathogenic bacteria, creating more species of resistant bacteria.

Finches on Daphne Major:

  • Beak shape changes according to the food – environmental change causes change in available food.