Unit 7 - Natural Selection
Sunday, 1 May 2022 | |
5-minute read | |
968 words | |
Population Genetics
- The gene pool is the sum of alleles within a population
- A population is a localized group of organisms of the same species
Impacts on a Population
- Changes in frequency often indicate evolution
Small populations are more susceptible to random impact than large populations
- Small gene pool
- Less variation within gene pool
Genetic Drift
Genetic dift is the random fluctuation in allele frequency between generations.
Founder Effect
- A very small population is isolated from a major population
- The small population may have less genetic variation
Genetic Bottleneck
- In a genetic bottleneck, allele frequency is altered due to a population crash.
- Small bottlenecked populations = big effect
Introduction of New Alleles
Gene Flow
Genetic exchange due to the migration of fertile individuals or gametes between populations
- Reduces differences between populations
Mutations
- Change in an organism's DNA
- Original source of genetic variation
Hardy Weinberg Equillibrium
Where $p$ is the dominant allele and $q$ is the recessive allele:
$$ \left\{\begin{array}{l} p + q = 1 \\ p^2 + 2pq + q^2 = 1 \end{array} $$
- Hypothetical, non-evolving population
- Natural populations are never in Hardy-Weinberg equillibrium
Hardy Weinberg Rules
Large population size
A large population has a lower chance of significant changes in the gene pool
No migration
No mutation
Random mating
No sexual selection or competitive advantages
No natural selection
Everyone is equally fit
Solving
Always try to solve for $q$ first, since recessive phenotypes guarantee homozygous recessive traits.
Examples
Example 1
If the statistics for people who have PKU is 1 in 10000, what percentage of the population carries the gene but not exhibit the disease?
Given $q^2 = 0.0001$,
$q = 0.01$
$p + q = 1$, so $p = 0.99$
$2pq = 2(0.01)(0.99) = 0.0198$
Example 2
If only 6% of the population displays pale eyes (recessive gene e), what is the frequency of genotype Ee in this population?
Given $q^2 = 0.06$,
$q = \sqrt{0.06}$
$p + q = 1$, so $p = 1 - \sqrt{0.06}$
$2pq = 2(1 - \sqrt{0.06})(0.06) = 0.369 = 36.9\%$
Example 3
In Drosophila, the allele for normal length wings is dominant over the allele for vestigial wings (vestigial wings are stubby little curls that cannot be used for flight). In a population of 1,000 individuals, 360 show the recessive phenotype. How many individuals would you expect to be homozygous dominant for this trait?
Given $q^2 = \frac{360}{1000} = 0.36$,
$q = \sqrt{0.36} = 0.6$,
$p + q = 1$, so $p = 1 - 0.6 = 0.4$
Therefore, $p^2 = 0.16 = 16\%$
16% of 1000 = 160
Example 4
For foxes there exists a single gene that controls coat thickness. Allele C confers a thick coat while allele c is a thin coat. In a population of 540 foxes, 49 have thin coats. What are the dominant and recessive allelic frequencies?
Given $q^2 = \frac{49}{540}$
$q = \sqrt{\frac{49}{540}} = 30\%$
$p + q = 1$, so $p = 1 - \sqrt{\frac{49}{540}} = 70\%$
Example 5
The gene for albinism is known to be a recessive allele. In Michigan, 9 people in a sample of 10,000 were found to have albino phenotypes.
- What is the allele frequency for the dominant pigmentation allele in this population? Given $q^2 = \frac{9}{10000}$, $q = \sqrt{\frac{9}{10000}} = 0.03$ $p = 1 - 0.03 = 0.97$
- How many of the 10000 people in the sample above were expected to be heterozygous for pigmentation? Solve for $2pq$ $2pq = 2(0.97)(0.03) = 0.058$ 5.8% of 10000 = 580
Evidence of Evolution
Common ancestry of all life forms
- DNA and RNA
- Universal genetic code
- Conserved metabolic pathways
FAME
Fossil Record
The fossil record can give rise to transitional species.
However, not every organism will leave behind a fossil. Fossils are extremely rare, and sometimes only the partial fossil will be preserved.
Anatomical structure
Molecular homology
Embryological
Homology
- Similar structure
- Different structure
- Implies a common ancestor existed
- Molecular Homology
- Developmental Error
Convergent Evolution
- Analogies are products of convergent evolution
Analogous structures
- Structures in different species that have the same appearance, structure, or function, but evolved separately.
Systematics
Classifying organisms and determining their evolutionary relationships
Taxonomy
Science of classifying and naming organisms (nomenclature)
Phylogenies
Like a family tree, root represents ancestral lineage
- Tips of branches represent descendents
- As you move from root to tip, you are moving forward in time
Each lineage has both a shared and unique history
Each lineage has ancestors that are
- Unique to that lineage
- Shared with other lineages
Divergent Evolution
- Diversification of a single ancestral species into several new forms
- Environmental pressures and changes in habitat often drive speciation
Evidence for Divergent Evolution
- Homologous structures
Convergent Evolution
- Different species of different origin may develop similar structures
Conserved elements in Eukaryotes
- Cytoskeleton
- Membrane-bound organelles
- Linear chromosomes
- Endomembrane systems
- Genes that contain introns
Species
Origin of Species
Speciation - origin of species
Microevolution - changes within a single gene pool
Macroevolution - evolutionary change above the species level
Species - population or group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring
- Reproductively viable
- Reproductive isolation - barriers that prevent members of two species from producing viable, fertile hybrids
Types of Reproductive Barriers
Prezygotic barriers
- Impede mating/fertilization
Habitat isolation
Allopatric speciation
- Greek allo- different
- Latin patria homeland
- Temporal isolation
Behavioral isolation
- Mate selection
Mechanical isolation
- Incompatible reproductive organs
Postzygotic barriers
- Prevent hybrid zygote from developing into a viable adult
- Reduced hybrid viability/fertility
- Hybrid breakdown
Origins of Life
RNA Hypothesis
DNA and proteins only serve one function
- DNA stores genetic information
- Proteins perform maintenance
- RNA served both the functions of DNA and proteins
- Through natural selection, RNA later soon evolved to become DNA and proteins
Miller-Urey Experiment
Tested theh Oparin-Haldane Hypothesis
- Life arose from inorganic molecules to create amino acids
- Suggested that organic molecules could be made from inorganic molecules