17. Selection and Evolution
17.1 Variation
Variation refers to the differences that exist between individuals of the same species.
Types of Variation
1. Discontinuous Variation (Qualitative Differences)
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Definition: Individuals fall into distinct categories with no intermediates.
Examples: Blood groups, inheritance of hemophilia, and sickle-cell anemia.
Genetic Basis: Usually controlled by a single gene (monogenic) with alleles that have
a large effect on the phenotype.
2. Continuous Variation (Quantitative Differences)
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Definition: Individuals show a range of phenotypes between two extremes, with
many intermediates.
Examples: Height and weight.
Genetic Basis: Controlled by many genes (polygenes) at different loci, each having a
small, additive effect on the phenotype.
Sources of Variation
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Gametic Variation: This is caused by independent assortment, crossing over, random
mating between organisms, random fertilization, and mutations.
Mutations: These are changes in the DNA sequence that can produce new alleles.
o Somatic Mutations: Occur in body cells and are not inherited by offspring.
o Germ-line Mutations: Occur in cells containing gametes and can be passed to
offspring, providing material for natural selection.
Environmental Variation: Differences caused by the environment (e.g., light
intensity, food availability). While it allows for the full genetic potential to be
realized, it is not passed to offspring.
17.2 Natural and Artificial Selection
Natural Selection
Natural selection is the process where individuals best adapted to their environment are more
likely to survive and reproduce.
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Why it Occurs:
Organisms have the reproductive potential to increase their populations.
However, natural populations tend to remain stable in size over long periods.
This leads to competition for resources (struggle for existence) among
offspring.
Selection Pressures: Biotic factors (predation, competition, infection) and abiotic
factors (non-living components) act as selection pressures that determine which
variations provide an increased chance of survival.
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Types of Selection
1. Stabilizing Selection: Favors intermediate phenotypes and eliminates extreme
variants. It maintains organisms that are already well-adapted to a stable environment.
2. Directional Selection: Favors one extreme phenotype, leading to a change in the
features of the population over time.
o Example: The evolution of the peppered moth.
3. Disruptive Selection: Favors both extreme phenotypes over the intermediates,
maintaining polymorphism within the population.
o Example: Galapagos finches.
17.3 Population Genetics
Factors Affecting Allele Frequency
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Genetic Drift: Random changes in allele frequency due to chance, which are most
noticeable in small populations.
Founder Effect: Occurs when a small group breaks off from a larger population to
establish a new colony. The founding population may have different allele frequencies
or miss some alleles entirely compared to the parent population.
The Hardy-Weinberg Principle
This principle provides a formula to calculate allele and genotype frequencies in a population.
It assumes equilibrium is maintained under certain criteria:
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No mutations.
No natural selection (no allele is favored over another).
An infinitely large population.
Random mating.
No migration (no change in population size or gene flow).
17.4 Artificial Selection
Humans purposefully apply selection pressures to populations to increase the frequency of
desirable features.
Improving Milk Yield in Dairy Cattle
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Desirable Features: Docility, fast growth rate, and high milk yield.
Process: Individuals with desirable features are chosen to interbreed, and the best
offspring are chosen for future generations.
Progeny Testing: Since milk production is a sex-limited trait, a bull's genetic value is
assessed by measuring the performance of its female offspring.
Crop Improvement
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Disease Resistance: Selection for varieties of wheat and rice resistant to infections.
Dwarf Varieties: Incorporating mutant alleles that affect gibberellin synthesis or
sensitivity (DELLA proteins). This puts more energy into grain production rather than
stem growth.
Maize Improvement:
o Inbreeding Depression: Repeated inbreeding leads to smaller and weaker
plants due to homozygosity.
o Hybridization: Crossing two inbred varieties to produce vigorous, uniform,
and high-yielding heterozygous plants.
17.5 Evolution
General Theory of Evolution
The theory that organisms have changed over time, beginning with simple life forms. It relies
on the principles of variation, overproduction of offspring, and natural selection.
Molecular Evidence
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Amino Acid Sequences: Comparing sequences in proteins like cytochrome c reveals
similarities between species. Fewer differences indicate a more recent common
ancestor.
Mitochondrial DNA (mtDNA): Comparing nucleotide sequences in mtDNA helps
track evolutionary timing and species divergence. mtDNA is inherited through the
female line and undergoes a constant rate of mutation (the Molecular Clock
hypothesis).