Key points about natural selection Affects the population not the individual Changes in the gene frequency (how often a gene shows up in the population) is the driving force of natural selection ‘Survival of the fittest’ When natural selection is not occurring, we call that genetic equilibrium

What influences natural selection? Mutations Recombination of genes Environmental Changes Selective pressures

Types of Natural Selection: 1. Directional - One phenotype is favored in the environment. (ex. "dark" peppered moths have higher survival rates in soot covered forests)

2. Stabilizing - Organisms with extreme phenotypes are eliminated. (ex. birth weight in humans, low survival rate for extremes)

3. Disruptive - Organisms with common traits are eliminated, extremes are favored. (ex. small female and large male elephant seals)

4.Artificial - A breeder chooses which traits to favor. (ex. seedless grapes)

Mechanisms of Evolution: Speciation Speciation Process by which new biological species arise. Mom, Dad… There’s something you need to know… I’m a MAMMAL!

So…what is a species? A population whose members can interbreed & produce viable, fertile offspring. Being reproductively compatible is a key component Western Meadowlark Sturnella magna Eastern Meadowlark Sturnella neglecta Distinct species: songs & behaviors are different enough to prevent interbreeding

1. Geographic isolation New species occur in different areas physical barrier separated ancestor species Harris’s antelope squirrel inhabits the canyon’s south rim (L). Just a few miles away on the north rim (R) lives the closely related white- tailed antelope squirrel Ammospermophilus spp

2. Ecological isolation Species occur in same region, but occupy different habitats so they rarely encounter each other 2 species of garter snake, Thamnophis, occur in same area, but one lives in water & other is terrestrial lions & tigers could hybridize, but they live in different habitats:  lions in grasslands  tigers in rainforest lions & tigers could hybridize, but they live in different habitats:  lions in grasslands  tigers in rainforest

3. Temporal “time” isolation Species that breed during different times of day, different seasons, or different years cannot mix gametes. Eastern spotted skunk (L) & Western spotted skunk (R) overlap in range but eastern mates in late winter & western mates in late summer

4. Behavioral isolation Unique behavioral patterns & rituals isolate species identifies members of species attract mates of same species courtship rituals, mating calls Blue footed boobies mate only after a courtship display unique to their species That’s hot…

5. Mechanical isolation Morphological (physical) differences can prevent successful mating Even in closely related species of plants, the flowers often have distinct appearances that attract different pollinators. These 2 species of monkey flower differ greatly in shape & color, therefore cross- pollination does not happen. Plants

Other forms of Speciation Remember: Genetic Drift: Change to gene pool of a population that takes place strictly by random chance Two major results can stem from Genetic Drift: Bottleneck Effect Founder Effect

Bottleneck Effect Population bottleneck occur when a population’s size is drastically reduced for at least one generation. Results in reduced genetic variation

Here is what it can look like: Example: Northern Elephant Seals- due excessive hunting in the late 1890’s, the population bottleneck leaving fewer than 20 seals left in the world. Today there are more than 30,000 of these seals BUT still have reduced genetic variation

Founder Effect A founder effect occurs when a new colony is started by a few members of the original population. This small population size means that the colony may have: reduced genetic variation from the original population. may lead to increased chances of interbreeding

Example #2- Birds Which colonies would could result from the Founder Effect?