Populations

Mechanisms

Evolution

KEY CONCEPT A population shares a common gene pool.

Genetic variation leads to phenotypic variation.the physical appearance of a trait based on alleles.

• Alternative forms of genes Ex. Green/Brown skin

Phenotypic variation is necessary for natural selection.Genetic variation is stored in a population’s gene

pool.made up of all alleles in a populationallele combinations form when organisms have offspring

Genetic variation in a population increases the chance that some individuals will survive.

Allele frequencies measure genetic variation.– measures how common the allele is in the population– can be calculated for each allele in gene pool

Organisms of the same species living at the same place at the same time.

Populations evolveNOT individuals

Populations

A normal distribution graphs as a bell-shaped curve.highest frequency near

mean value frequencies decrease

toward each extreme value

Traits not undergoing natural selection have a normal distribution.

Natural selection acts on distributions of traits.

Microevolution is evolution within a population.Can result from natural selectionNatural selection can take one of

three paths.

Directional selectionStabilizing selectionDisruptive selection

Natural selection can change the distribution of a trait in one of 3 ways.

Directional selection favors phenotypes at one extreme.

Stabilizing selection favors the intermediate phenotype.

Disruptive selection favors both extreme phenotypes.

Gene flow – the movement of alleles between populationsWhen organisms join a new population and

reproduce, their alleles become part of that population’s gene pool.

Increases the genetic variation of the receiving population

Natural selection is not the only mechanism through which populations evolve.

Gene flow keeps neighboring populations similar.

Low gene flow increases the chance that two populations will evolve into different species.

Genetic drift causes a loss of genetic diversity.

It is most common in small populations.A population bottleneck event can lead to

genetic drift. It occurs when an event

drastically reducespopulation size.

The bottleneck effect isgenetic drift that occursafter a bottleneck event.

Genetic drift is a change in allele frequencies due to chance.

It occurs when a few individuals start a new population.

The founder effect is genetic drift that occurs after the start of a new population.

The founding of a small population can lead to genetic drift.

Less likely to have some individuals that can adapt

Harmful alleles can become more common due to chance

Genetic drift has negative effects on a population.

Sexual selection occurs due to higher cost of reproduction for females.males produce many

sperm continuouslyfemales are more

limited in potential offspring each cycle

Sexual selection occurs when certain traits increase mating success.

intrasexual selection: competition among males

intersexual selection: males display certain traits to females

There are two types of sexual selection.

Male Irish elks, now extinct, had a 12-foot antler span. Describe how sexual selection could have caused such an exaggerated trait to evolve.

It may have been viewedby females as a “quality” trait in selecting mates

Application

The isolation of populations can lead to speciation.Populations become isolated when there is no

gene flow. Isolated populations adapt to their own

environments.Genetic differences can add up over generations.

Speciation – the rise of two or more species from one species

KEY CONCEPT New species can arise when populations are isolated.

When members of different populations can no longer mate successfully with one another.

Usually final step to becoming separate species

Reproductive Isolation

Isolation caused by differences in mating or courtship behaviors

Example: Fire FliesSome species flash every 2 secondsSome species flash every 5 seconds

These will not interbreed because they are not attracted to different flashing sequences

Behavioral Isolation

Isolation due to physical barriers that divide a populationBarriers

OceansCanyonsRiversMountains

Geographic Isolation

Exists when timing prevents reproduction between populationsEastern spotted skunkWestern spotted skunkAre considered different species because of

mating times

Temporal Isolation

The Monterey Pine sheds its pollen in February. The Bishop Pine, which lives in the same region, sheds its pollen in April.

Write a paragraph explaining what type of isolation this represents.

Explain why you picked the type of isolation that you did.

Explain why it isn’t any two of the remaining types.

Application