Biology, Answering the Big Questions of Life/evolve3

Mutations
A normalizing or stabilizing mutation favors those individuals that are like those that came before. Example: Snakes on an island on lake Erie are grey. If any are born with stripes they get eaten, so most snakes on the island remain grey.

Directional mutations are ones that go in one direction over time. The fossil record shows that the ancestors of horses were shorter. As time went on, larger animals were favored and survived so the species as a whole got larger.

Diversifying selection is when two or more phenotypes are maintained. For example near copper mines some of the soil is heavily contaminated with copper. Some alleles allow growth of plants on the copper. Wild type plants will die on the copper, so both alleles are maintained in the population.

In rare cases such a dimorphism of environment leads the species to divide into groups. This is called a disruptive selection.

Speciation
A species according to the biological species concept is defined as a group of actually or potentially interbreeding individuals reproductively isolated from others.

Factors such as mutation can cause a species to change into another species. This is called speciation. There are many ways that speciation can occur.

Speciation often occurs because individuals of a species cannot breed with each other. It is called reproductive isolation. When individuals are reproductively isolated, they begin to differ from each other.

Methods that occur before fertilization are called prezygotic. Those that occur after fertilization are called postzygotic. Prezygotic methods include ecological isolation, behavioral isolation, mechanical isolation, temporal isolation. Postzygotic isolation includes hybrid inviability. hybrid sterility, hybrid breakdown which occurs when there is reduced viability in later generations.

Methods of Species Change
When some kind of barrier like a mountain rises up between two groups of the same species, it may cause allopatric speciation. Sometimes species will split without geographic isolation for reasons such as chromosome duplication (polyploidy). When new species result this way it is called sympatric speciation.

Sometimes it is hard to tell if two species have descended from a common ancestor ( share a phylogeny). When two organisms who may or may not be distantly related develop a similar appearance that evolved completely independently it is called parallel evolution.

When organisms that are only distantly related develop analogous modifications (similar structures) because of similar selective forces on them, it is called convergent evolution. Examples include how dolphins and penguins have fins because they live in the ocean. divergent evolution happens when a species splits and forms different but related new species. This is also called radiation.

Homology occurs when some biological structure derives from the same ancestral structure. For example what became hands for humans became wings for birds and bats.