Day 46 Populations 2

The smallest unit of evolution is a population

-A population is a group of the same species

-The change of an allele frequency over time within a population is known as microevolution

-Genetic variation is what makes evolution possible. If there were no variation within a population one trait cannot reproduce more than another.

-Although genetic variation is what makes evolution possible it is phenotypic variation that evolution works with. If a gene is not expressed it usually has no determination on reproductive success.

All genetic variation originates with mutation. A mutation is something the organism is born with. It has to happen during the formation of the gametes during meiosis or it has to happen during fertilization. Once an organism is more than one cell a mutated cell is simply a damaged cell (like cancer)

A population can only choose from the alleles that are available in that population. If alleles are brought in from another population or emigrated to another population this is referred to as gene flow.

-Gene flow is a good thing. It allows for variation in a population giving them more opportunities to survive stress.

Natural selection is the only mechanism that consistently causes adaptive evolution

-Sexual selection is not adaptive

-Genetic drift is not adaptive

-and artificial selection is not adaptive

-Relative fitness refers to an individuals contribution to the next generation's gene pool

Natural selection can lead to three modes of selection

-Directional selection

-Disruptive selection

-Stabilizing selection

-The normal distribution has an average bell curve

-The average is in the middle with the majority of the population and to each end are the few extremes

-If the population starts selecting for one of the extremes then the population will go through directional selection

-If the population selects for the two extremes and does not favor the average then it will go through disruptive selection

-If the average is selected for than the extremes will become less sever and the population will go through stabilizing selection

-Directional selection affects the evolution of a population

-Disruptive selection could lead to speciation

-Stabilizing selection can lead to a more uniform population

With sexual selection the mutation needs to be located on the sex chromosome to create a sexual dimorphism

-Sexual dimorphism is common among many animals. Humans do not count we are not different enough

Speciation

How does one species become two?

In order for speciation to happen there has to be some type of reproductive isolation

-The definition of a species is a group or populations whose members have the potential to interbreed in nature and produce viable, fertile offspring- but do not produce viable, fertile offspring with members of other such groups

-Because the definition has to do with mating a population needs to be kept reproductively isolated from another until they have changed so much that they can no longer breed

-There are two types of reproductive isolation

-Prezygotic barriers - Before sex

-Postzygotic barriers- after sex

Now lets label them

Allopatric and sympatric speciation

Allopatric speciation is when one species becomes two while being geographically isolated

Sympatric speciation is when one species becomes two while still living in the same geographical region

Sometimes Hybrid zones are created

A hybrid zone can go through 0ne of three phases

-Reinforcement

-Fusion

-Stability