5.2 Natural Selection Flashcards
Who posed the theory of natural selection?
Charles Darwin and Alfred Wallace
The ones most responsive to change survive!
What causes variation?
- Mutations (a change in the nucleotide sequence of a section of DNA coding for a specific trait): forms new alleles
- Meiosis (because of crossing over and independent assortment): produces gametes with unique combinations of alleles, thus increasing the genetic variation of individuals within the species.
- Sexual reproduction and random fertilisation: the combination of gametes is random and results in a zygote that has genes from both of its parents.
What happens if there is no variation among members of the same species?
Natural selection can’t occur.
Why does natural selection occur. Key points.
- Overpopulation
- Competition
- Adaptation
- Inheritance
- Time scale
Overpopulation
Key points explained
Species tend to produce more offspring the environment can support
Competition
Key points explained
Because of overpopulation, there is competition between individuals of the same species
Adaptation
Key points explained
Some individuals will be better adapted to their
environment due to variation in the species, and as a result will obtain more resources, survive better and breed more successfully
Inheritance
Key points explained
The better adapted individuals survive and reproduce and pass on their genes to their offspring. Therefore their offspring will also be be well adapted for the environment and survive better.
Timescale
Key points explained
Over time the allele frequency in a population’s gene pool will change (more alleles for successful traits)
Case study: Galapagos finches
Adaptive radiation and natural selection
- On the Galapagos Islands, Darwin observed several species of finches with unique beak shapes.
- The finches were scattered all around the island, therefore they had evolved to adapt to particular food sources available on the part of the island they were inhabiting.
- This is why some of the finches had larger and thicker beaks (for big seeds), while others had thin and narrower beaks (for small seeds).
Case study: antibiotic resistant bacteria
Natural selection
Bacteria has started to develop antibiotic resistance (e.g. MRSA)
- Within a bacterial population, there is variation caused by mutations
- This is a chance a chance mutation might cause bacteria to become resistant to an antibiotic
- Therefore when the population is treated with this antibiotic, the resistant bacteria do not die
- This means the resistant bacteria will survive and reproduce to produce and pass on the genes for antibiotic resistance to the next generation.
- Over time, the whole population of bacteria becomes antibiotic-resistant because the antibiotic-resistant bacteria are best suited to their environment
What does natural selection lead to?
Natural selection increases the frequency of characteristics that make individuals better adapted and decreases the frequency of other characteristics leading to changes within the species.
