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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.
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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.
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Key components of natural selection
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Inherited variation
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There is genetic variation within a population that can also be inherited.
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Competition
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There is a struggle for survival as species tend to produce more offspring than the environment can support.
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Selection
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Environmental pressures lead to differential reproduction within a population.
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Adaptations
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Individuals with beneficial traits will be more likely to survive and pass these traits on to their offspring.
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Evolution
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Over time, there is a change in allele frequency within the population gene pool.
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Species tend to produce more offspring than the environment can support
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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.
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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).
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Individuals that are better adapted tend to survive and produce more offspring while the less well adapted tend to die or produce fewer offspring
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The variation that exists within a population is genetic and determined by alleles.
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Alleles encode for the phenotypic polymorphisms of a particular trait and may be beneficial, detrimental or neutral.
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Due to natural selection, the proportion of different alleles will change across generations (evolution).
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As beneficial alleles improve reproduction prospects, they are more likely to be inherited by future generations.
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Detrimental allies result in fewer offspring and are less likely to be present in future generations.
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If environmental conditions change, what constitutes a beneficial or detrimental trait may change, and thus allele frequencies in the population are constantly evolving.
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Antibiotics are chemicals produced by microbes that either kill or inhibit the growth of bacteria (bactericidal and bacteriostatic respectively).
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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. |
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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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Natural selection can only occur if there is variation among members of the same species
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Mutation, meiosis and sexual reproduction causes variation between individuals in a species
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Natural selection needs variation, as it allows for differentiation for survival.
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The three main mechanisms for genetic variation in a species are mutation, meiosis and sexual reproduction.
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Mutations
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A gene mutation is a change in the nucleotide sequence of a section of DNA coding for a specific trait.
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Meiosis
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Meiosis promotes variation by creating new gene combinations via either crossing over or independent assortment.
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Crossing over
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It involves. the exchange of segments of DNA between homologous chromosomes during Prophase I.
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Independent assortment
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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.
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Sexual reproduction
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As meiosis results in genetically distinct gametes, random fertilisation by egg and sperm will always result in different zygotes.
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Adaptations are characteristics that make an individual suited to its environment and way of life
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Individuals that reproduce pass on characteristics to their offspring
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Adaptations are features of organisms that aid their survival by allowing them to be better suited to their environment.
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Classifications of adaptations
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Structural
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Physical differences in biological structure.
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Behavioural
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Differences in patterns of of activity.
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Physiological
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Variations in detection and response by vital organs.
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Biochemical
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Differences in molecular composition of cells and enzyme functions.
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Development
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Variable changes that occur across the lifespan of an organism.
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Biological adaptations have a genetic basis and may be passed to offspring when the parents reproduce.
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Changes in beaks of finches on Daphne Major.
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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.
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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.
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These finches shoed adaptive radiation and marked variation is beak size and shape according to diet. (Smaller beaks - smaller seeds, larger beaks - larger seeds).
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In 1977, an extended drought changed the frequency of larger beak sizes within the population by natural selection.
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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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