Forces of Evolution & the Formation of Species
1-19-10, 1-20-10
•Chapter 5
•Evolution
Requirements of evolution:
1. Heritable genetic variation in traits (result of mutations in gametes)
2. A sorting mechanism of these traits
A.
B.
3. Time or transgenerational transmission
•1. Mutation
•sources of new genetic variation =mutation.
•Point Mutation: change in a base on the DNA molecule.
•Chromosomal Mutation: segments of chromosomes are transposed with one another.
•Must occur in sex cells to impact evolution.
•
- characteristics of the environment
- other traits possessed by the individual organism
- harmful, neutral or advantageous nature of mutations is neither absolute nor
permanent
•Scale of Evolution
•
: Changes in genes over time which do not form
reproductive barriers between members of the species.
•Macroevolution: Changes in genes of populations which eventually result in
reproductive barriers leading to speciation.
•processes which lead to microevolution and macroevolution are the same.
•Microevolution
•Change in a population’s allele proportions from 1 generation to the next.
•
•May be the product of natural selection, genetic drift, or both
•Population: a breeding group or the group that you find your mate in.
•Discrete groups the result of :
–distance
–Range
–Niche
–Social organization
•2. Sorting Mechanisms: Gene Flow and Genetic Drift
•Gene Flow: movement of genes between populations.
•works in conjunction with natural selection by providing alternatives for a trait.
•Example: Sickle cell anemia
•genetic drift: random changes in gene frequency in a population.
•
•Includes the founder effect and bottleneck effect
•founder effect: new populations that become isolated from the parent population carry
only the genetic variation of the founders.
–example:Tristan de Cunha
•3. Sorting Mechanism: Natural Selection
•
•Over time, the population’s allele proportions shift toward the favored type
•Differential reproductive success is referred to as fitness
•directional selection: selecting for greater or lesser frequency of a trait in a population.
•Diversifying selection:
•stabilizing selection: maintains a certain phenotype by selecting against deviations from
it.
•4. Speciation
•
•species: an interbreeding group of individuals that are reproductively isolated through
anatomy, ecology, behavior, or geographic distribution from all other such groups.
•Amount of genetic divergence (change) forms a continuum:
•How are species formed?
•anagenesis: evolution of a species into another over a period of time.
–Through directional selection
•cladogenesis: evolution of a species through the branching of a species.
–Through diversifying selection
•Allopatric speciation: speciation occurring from geographic isolation.
Parapatric speciation:
•The Tempo of Evolution
•Gradualism: slow, incremental evolutionary change.
•Punctuated Equilibrium: evolution characterized by rapid bursts of change followed by
long periods of stasis.
•Hardy-Weinberg equilibrium Model
•Use equation to measure microevolution in a Mendelian population
•Hardy-Weinberg (HW) formula is used to measure changes in allele proportions
•
•Used to document allele changes for one gene with two alleles
•Hardy-Weinberg equilibrium is represented by the following equations:
–p2 + 2pq + q2 = 1
–p + q = 1
– p = the frequency of the dominant allele
– q = the frequency of the recessive allele
–p2 = the frequency of the homozygous dominant individuals
–2pq = the frequency of the heterozygous individuals
–q2 = the frequency of the homozygous recessive individuals
•Example
•In an Amish population, you are studying a gene that influences growth
•This gene has two alleles:
–E = dominant allele, codes for normal growth
–e = recessive allele, codes for stunted growth and multiple fingers and toes (polydactyly)
•Each person inherits two of these alleles:
–EE = homozygous dominant, normal growth and stature
–Ee = heterozygous, normal growth and stature
–ee = homozygous recessive, dwarfism (Ellis van Crevald Syndrome)
•In the HW formula, p = the proportion of E alleles (EE, Ee), and q = the proportion of e
alleles (ee) where p + q=1
•If you know that 0.30 (30%) of the population’s alleles are e, then:
p + .30 = 1.00
p = 1.00- 0.30
p = 0.70 = the population’s proportion of E alleles
p2 + 2pq + q2 = 1.00
•is used to measure changes in proportions of offspring genotypes
p2 = p * p = proportion of homozygous dominant offspring (EE)
2pq = 2 * p * q = proportion of heterozygous offspring (Ee)
q2 = q * q = proportion of homozygous recessive offspring (ee)
•In our example, the E allele has a frequency of 0.70 and the e allele has a frequency of
0.30
•This population would produce
0.70 * 0.70 = 0.49 EE children
2 * 0.70 * 0.30 = 0.42 Ee children
0.30 * 0.30 = 0.09 ee children
•Or EE 49%, Ee 42%, ee 9%
•compare these to the actual values of the population. The differences we find will
document and quantify the nature and amount of genetic drift and selection taking place.