Lec 6 - Mechanisms of Evolution (BIOL 208)

A complete set of vocabulary flashcards covering evolutionary mechanics, population genetics, selection types, species concepts, and speciation mechanisms.

Evolution

species & populations of organisms change over generations (ecology wouldn’t exist without evolution)

Trait

physical, physiological, or behavioural character that influences ability of an organism to survive & reproduce

Alleles

Different forms of a gene (such as $M$ or $m$).
gene = unit of hereditary

Gene Pool

sum of all the alleles in a pop

higher # of alleles =

greater diversity

Individual Fitness

individual's relative genetic contribution to the next gens gene pool

Microevolution

frequencies of alleles changing gene pool within a population over gens
changes = natural selection or random

What level of ecology does evolution start at?

Pop. (not individual)

Genotype & phenotype

set of genes an organism has that determines phenotype: observed traits (ex. behavior, morphology, or physiology)
-each genotype can have multiple phenotypes

Phenotypic Variation (VP = VG +VE)

phenotypic variation = genes + enviro
Differences in observable traits among individuals in a pop.

Phenotypic Plasticity

(individual lvl) ability of a genotype to produce different phenotypes in response to distinct enviro conditions

Describe example of how hydrangea have phenotypic plasticity

Hydrangeas change color based on soil pH + conc. of Al
-high pH & high aluminum = blue
-low pH & low aluminum = pink
(even flowers sharing flowerbed & soil can have diff colours)

Phenotypic differences we see are due to differences in ______ ________.
Variation in flower shape & colour is due to differences in _______.
Evolution requires phenotypic variation among individuals that is _____.

• genetic material

• genome

• heritable

Heritability ($H^2$ )

total phenotypic variation in a trait - attributed to genetic variation (random assortment, recombination, mutation)
$H^2 = \frac{V_G}{V_P}$. (VP = VG + VE)
-heritability increases with increased VG & decreases with increased VE

H2 = _____

0 = _____

1 = ______

• index of heritability

• environmentally determined (won’t be passed to next gen)

• genetically determined

Hardy-Weinberg Principle

The principle stating that if no evolution is occurring, genotype frequencies can be predicted from allele frequencies using $p^2 + 2 \times p \times q + q^2 = 1.0$.

p= 1 allele

q = other form of allele
= 1.0 = no evolution

A pop at Hardy-Weinberg equilibrium will maintain a constant allele frequency gen after gen (no evolution). What are the 5 rules for Hardy-Weinberg equilibrium?

1. random mating

2. no mutations

3. large pop

4. no immigration

5. all genotypes have equal fitness (equal likelihood of survival)

What are the 3 mechanisms of evolution?

1. gene flow (migration)

2. genetic drift

3. natural selection (non-random)

Gene Flow (mechanism of evolution)

random - migration between populations

Genetic Drift (mechanism of evolution)

random - genetic variation is lost due to chance or random events (variation in mating, fecundity, and mortality)

due to founders effect or population bottleneck

Population Bottleneck

reduction in pop genetic diversity due to a large reduction in pop size
(ex. pop killed by humans)

Founder Effect

small subset of individuals leaves a large pop to colonize a new area, bringing limited genetic variation.

Natural Selection (mechanism of evolution)

non-random - individuals with certain phenotypes survive and reproduce more successfully = changes in trait frequencies over time

What are the 4 rules of natural selection?

1. more offspring produced than can survive

2. traits vary within a pop + are heritable (=genetic variation)

3. some traits give advantage

4. traits with higher fitness become more common (preferred phenotype)

Adaptation

inherited trait evolved through natural selection - provides an advantage (higher relative fitness) in a particular enviro = increases survival & reproductive success

ex. finches in the Galápagos Islands - variation in beak shapes (non-random mating)

A major drought on the galapagos islands was associated with a decline in seed abundance & increase in seed hardiness. What effect did this have on beak shape?

Finch pop declined - selection pressure on beak depth distribution. Finch’s that survived tended to have larger beaks that could crack seeds

What are the 3 types of natural selection?

stabilizing, directional, disruptive

Stabilizing Selection

intermediate (average) phenotypes become more frequent, while extreme phenotypes become less frequent (bell-shaped curve)

ex. stabilizing selection for egg size
- how much of the variation in egg size in their study population is the result of genetic differences among females (heritability) and how much is the result of environmental influences (phenotypic plasticity)

• egg size is highly heritable in Ural owls, h2 = 0.60 (evolution through natural selection)
  - very small and very large eggs hatch at a lower rate compared to intermediate-sized eggs

Directional Selection

extreme phenotype becomes more frequent over time

ex. larger organisms = higher rate of survival

ex. Directional Selection: Adaptation by Soapberry Bugs to New Host Plants

-differences in beak length observed in the field among bugs feeding on the various native and introduced host plants were retained in bugs that developed on alternative hosts = evidence for a genetic basis

Disruptive Selection

2+ extreme phenotypes become more frequent, while intermediate phenotypes become less frequent = pop become phenotypically diverse (2 peaks)

ex. small and large males have higher reproductive success than males of intermediate body size

What is an example of disruptive selection? (Darwin's Finches)

birds with bigger beaks only mate with other big beaks & birds with small beaks only mate with other small beaks (nonrandom patterns of mate choice)

-beak length, depth, and width are highly heritable, with heritability values of 0.62, 0.82, and 0.95, respectively

Speciation

2+ species arise from a common ancestor as populations become

• genetically divergent

• reproductively isolated

Speciation is an ______ of evolution

outcome

Adaptive Radiation (type of speciation)

rapid diversification of a common ancestor into multiple species adapted to different ecological niches

ex. finch’s beak shape
niche partitioning leading to speciation 

Morphological Species Concept (Carl Linnaeus)

species grouped by morphological similarities or structural traits

What are some problems with the Morphological Species Concept?

• sexually dimorphic species - ex. male duck looks very colourful and female looks plain

• polymorphic species in insects - same species of butterflies have lots of different wing patterns - caterpillar & butterfly at diff life stages

Biological Species Concept

groups of interbreeding pops, reproductively isolated from other such groups = hybrid

Isolating Mechanism

Something that stops two species from producing viable offspring (prezygotic or postzygotic)

Prezygotic Isolating Mechanism

prevents a zygote from being formed
- ecological (must be in same place)
- temporal (must be fertile at same time)
- behavioural (mating cues - ex. olfactory cues (pheromones), visual cues (colour patterns), sound cues ( bird calls)

-mechanical isolation - genitals must be compatible

Postzygotic Isolating Mechanism

zygote is formed, but resulting offspring cannot reproduce
Hybrid Inviability - zygotes do not make it to maturity
Hybrid Sterility - offspring reach maturity but cannot produce their own offspring

Phylogenetic Species Concept

species = smallest distinct lineage sharing a common ancestor (branch on the tree of life)

What are 3 advantages of the Phylogenetic Species Concept?

• sexual & asexual organisms (ex. many plants/conifers can create hybrids that can produce viable offspring (that can survive and reproduce)

• genetic, morphological, & behavioural evidence stable hybrid (hybrids that reproduce with same hybrid) can turn into their own species over time 

Allopatric Speciation

single pop becomes spatially subdivided into geographically isolated sub-populations

ex. sea lvls rises -  genetic drift (founders effect) 1 species becomes 2 genetically separate species

Parapatric Speciation

continuously distributed population where spatial distances and non-random mating lead to reduced gene flow

ex. species with reduced mobility - snails

-ex. flowers in contaminated vs uncontaminated soil over time can start flowering at different times = over time become diff species

Sympatric Speciation

in ancestral population without spatial subdivision, driven by non-spatial isolating mechanisms - positive assortative mating
-ex. green beetles selectively mate with other green beetles  - only mate with specific phenotype
-ex.flies that eat red apples only mate with only flies that like red apples

Connect natural selection → adaptation → speciation → adaptive radiation.

Natural selection - enviro pressures favours advantageous traits —> leads to adaptation, organisms become better suited to their enviro (increases fitness). —> Over time, adaptations cause species to become isolated = new species arise —>

adaptive radiation, rapid speciation, multiple species evolve from a common ancestor to fill various ecological niches

ex. drought causes less seeds & seeds harden - birds with large beaks survive - birds only mate with birds with same beak - become different species - over gen’s fich’s evolved into many distinct species with beaks evolved to eat diff kinds of bird seeds

polygenic traits

Characteristics that are controlled by multiple genes

quantitative genetics.

mathematical treatment of continuously varying traits and how they respond to natural selection ex. milk production in cows and sugar content in fruit