Natural selection
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. |
According to the theory of natural selection posed by Charles Darwin and Alfred Wallace, the organism which is most responsive to change is most likely to survive. |
Key components of natural selection |
Inherited variation |
There is genetic variation within a population that can also be inherited. |
Competition |
There is a struggle for survival as species tend to produce more offspring than the environment can support. |
Selection |
Environmental pressures lead to differential reproduction within a population. |
Adaptations |
Individuals with beneficial traits will be more likely to survive and pass these traits on to their offspring. |
Evolution |
Over time, there is a change in allele frequency within the population gene pool. |
Competition
Species tend to produce more offspring than the environment can support |
The Malthusian dilemma, proposed by Thomas Malthus, states that population multiply exponentially, while food resources multiple linearly. This means that a stable population will eventually outgrow its reoousrce base, leading to competition for survival. |
When there is an abundance of resources, a population will grow according to its biotic potential (exponential J-curve). With more offspring, there are less resources available to other members of the population (environmental resistance). This will lead to a struggle for survival and an increase in the mortality rate (causing population growth to slow and plateau). |
Allele frequency
Individuals that are better adapted tend to survive and produce more offspring while the less well adapted tend to die or produce fewer offspring |
The variation that exists within a population is genetic and determined by alleles. |
Alleles encode for the phenotypic polymorphisms of a particular trait and may be beneficial, detrimental or neutral. |
Due to natural selection, the proportion of different alleles will change across generations (evolution). |
As beneficial alleles improve reproduction prospects, they are more likely to be inherited by future generations. |
Detrimental allies result in fewer offspring and are less likely to be present in future generations. |
If environmental conditions change, what constitutes a beneficial or detrimental trait may change, and thus allele frequencies in the population are constantly evolving. |
Natural Selection on Daphne Major
Antibiotic resistance
Antibiotics are chemicals produced by microbes that either kill or inhibit the growth of bacteria (bactericidal and bacteriostatic respectively). |
In a bacterial colony, over many generations, a small proportion of bacteria may develop antibiotic resistance via gene mutation.
If antibiotics are used to treat these infections, it acts as a selective pressure and causes the antibiotic resistance gene to become more prevalent. |
An example of antibiotic resistance is the evolution of Staphylococcus aureus, which evolved to be methicillin resistant, and infections cannot be treated by that antibiotic. |
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Variation
Natural selection can only occur if there is variation among members of the same species |
Mutation, meiosis and sexual reproduction causes variation between individuals in a species |
Natural selection needs variation, as it allows for differentiation for survival. |
The three main mechanisms for genetic variation in a species are mutation, meiosis and sexual reproduction. |
Mutations |
A gene mutation is a change in the nucleotide sequence of a section of DNA coding for a specific trait. |
Meiosis |
Meiosis promotes variation by creating new gene combinations via either crossing over or independent assortment. |
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Crossing over |
It involves. the exchange of segments of DNA between homologous chromosomes during Prophase I. |
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Independent assortment |
The orientation of each bivalent during Metaphase I occurs independently, meaning different combinations of chromosomes can be inherited when the bivalents separate in Anaphase I. |
Sexual reproduction |
As meiosis results in genetically distinct gametes, random fertilisation by egg and sperm will always result in different zygotes. |
For mutations and meiosis, refer to Unit 3: Genetics
For sexual reproduction, refer to Unit 11.4: Sexual reproduction
Adaptations
Adaptations are characteristics that make an individual suited to its environment and way of life |
Individuals that reproduce pass on characteristics to their offspring |
Adaptations are features of organisms that aid their survival by allowing them to be better suited to their environment. |
Classifications of adaptations |
Structural |
Physical differences in biological structure. |
Behavioural |
Differences in patterns of of activity. |
Physiological |
Variations in detection and response by vital organs. |
Biochemical |
Differences in molecular composition of cells and enzyme functions. |
Development |
Variable changes that occur across the lifespan of an organism. |
Biological adaptations have a genetic basis and may be passed to offspring when the parents reproduce. |
Adaptive radiation
Changes in beaks of finches on Daphne Major. |
Adaptive radiation is the rapid evolutionary diversification of a single ancestral line and occurs when members of a single species occupy a variety of distinct niches with different environmental conditions. |
Daphne Major is a volcanic island that forms part of the Galápagos Islands and is the habitat of a variety of bird species, known as Darwin's finches, subfamily Geospizinae. |
These finches shoed adaptive radiation and marked variation is beak size and shape according to diet. (Smaller beaks - smaller seeds, larger beaks - larger seeds). |
In 1977, an extended drought changed the frequency of larger beak sizes within the population by natural selection. |
The dry conditions result in plants producing larger seeds with tougher seed casings. |
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Between 1976 and 1978 there was a change in average beak depth within the finch population. |
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Finches with larger beaks were better equipped to feed on the seeds and this produced more offspring with larger beaks. |
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