1) Individuals vary within populations, variation is heritable (phenotypic variation)

2) Exponential increase of organisms and limited resources leads to a 'struggle for existence' (females are usually capable of having more offspring than necessary)

3) Differential reproductive fitness/survival at the individual level

4) Favorable characteristics passed on, gradual increase in frequency leads to adaptation

1) Directional - Phenotypic frequency shifts in one direction (towards recessive or dominant homozygote)

2) Disruptive: Frequency shifts towards more 'extreme' phenotypes from intermediate phenotypes (shift towards both homozygotes)

3) Stabilizing aka balancing: Frequency shifts towards heterozygotes

Each fitness divided by the highest fitness

Ex: fitnesses of 0.1,0.2,0.6

w = 0.16, 0.33, 1

Leads to balanced polymorphism (neither p or q are 1 or 0)

Heterozygote fitness: A1A2 is the most fit

q^ = q at equilibrium = s1/(s1+s2)

p^ = s2/(s1+s2)

Example: Sickle-cell heterozygosity

Frequency-dependent: Fitness varies with frequency of given genotypes

Example: Scale-eating fish; fitness varies with frequency of right vs. left-mouthed fish

A group of individuals that can interbreed to produce viable, fertile offspring (biological concept)

A group of individuals with a great number of morphological similarities (morphological concept)

Pre-zygotic: Habitat isolation, temporal isolation (organisms active at different times/seasons), behavioral isolation, mechanical/gametic isolation

Post-zygotic: F1 hybrid inviability, F1 hybrid infertility, F2 or backcross hybrid breakdown (F1 hybrid is fertile but subsequent generations are not or hybrid cannot interbreed with parent species to produce more hybrids)

Biological: Asexual species, hybrids (e.g. zorse, mule)

Morphological: Sexual dimorphism, life-cycle (frog and tadpole), cryptic species (very hard to tell apart e.g. different species of Drosophila), convergent evolution

A barrier to gene flow/migration (rise in sea level, mtn formation, etc) within a species' geographic range isolates subgroups. If the 2 new 'sub-ranges' are different enough, the two subspecies will undergo local adaptation and diverge from one another; loss of reproduction between subspecies heralds speciation

Post-zygotic isolation occurs first, pre-zygotic occurs later (reinforcement); interbreeding is impossible even if subspecies come back together

Special case: Founder effect

Continued disruptive selection splits a species in two; also requires assortative (like x like) mating to minimize heterozygosity

Thought to be common among herbivorous insects

Ex: Apple Maggot Fly

Pre-1850s: Apple maggot fed only on Hawthorn Fruit (native to US)

Late 1800s: Apples added to diet; mating on host plant leads to reproductive isolation (Hawthorn and Apple fruits mature at different times)

Variation of sympatric speciation

Autoployploidy: A species (usu. a plant) undergoes polyploidy and becomes tetraploid (instantly reproductively isolated from diploid ancestor [mating produces sterile triploid]); two tetraploids can mate to produce tetraploid offspring

Allopolyploidy: Two different diploid species hybridize; the hybrid undergoes polyploidy to become tetraploid and is reproductively isolated from both parent species, but can mate with itself

Examples: wheat (diploid species hybridized w/ another to produce a tetraploid (alloploidy); hybrid is hybridized with another diploid to produce a hexoid hybrid (baking wheat)), tragopogon (3 diploid species in US during early 1800s; they hybridized with each other to produce 2 new tetraploid species)