Evolution

11.1 – Genetic Variation Within Populations  Key Concept:  A population shares a common gene pool.

Genetic variation in a population increases the chance that some individuals will survive.  Genetic variation leads to phenotype variation (different coat color, hair color)  Necessary for natural selection  Genetic variation is stored in a population’s gene pool  Made up of all the alleles of all individuals in a population  Allele frequency: a measure of how common a certain allele is in a population. Can be impacted by natural selection.

Genetic variation comes from several sources.  Mutations  Can form a new allele  Passed to offspring if in a gamete  Recombination  Usually occurs during meiosis  Parents’ alleles rearranged during gamete formation

11.2 – Natural Selection in Populations  Key Concept:  Populations, not individuals, evolve.

Microevolution  Evolution within a population  Observable change in allele frequencies  Can result from natural selection  Types:  Directional selection  Stabilizing selection  Disruptive selection

Directional Selection  Favors phenotypes at one extreme

Stabilizing Selection  Favors the intermediate phenotype

Disruptive Selection  Favors both extreme phenotypes

  (From 7.35 – Sexual Selection)

11.3 – Other mechanisms of Evolution  Key Concept:  Natural selection is not the only mechanism through which populations evolve.

Gene Flow  Movement of alleles between populations  Occurs when individuals join new populations and reproduce  Their alleles become part of gene pool  Keeps neighboring populations similar  Low gene flow increases the chance that two populations will evolve into different species bald eagle migration

Genetic Drift  Change in allele frequencies due to chance  Causes a loss of genetic diversity in a population  Common in small populations  Bottleneck Effect is genetic drift after a bottleneck event  Occurs when an event drastically reduces population size

 Founder Effect is genetic drift that occurs after the start of a new population  Occurs when a few individuals start a new population

Sexual selection occurs when certain traits increase mating success.  Sexual selection  Occurs due to higher cost of reproduction for females  Males produce sperm continuously  Females are more limited in potential offspring each cycle  Two types:  Intrasexual selection: competition among males  Intersexual selection: males display certain traits to females

  (From Sexual Selection to the end)

11.5: Speciation through Isolation  Key Concept: New species can arise when populations are isolated.

 Several kinds of barriers can prevent mating between populations, leading to reproductive isolation. These are all known as pre-zygotic isolation.  Behavioral isolation: differences in courtship or mating behaviors. (Different song)  Geographic isolation: physical barriers that divide a population into two or more groups. (See pic) also called allopatric speciation.  Temporal isolation: timing prevents reproduction between populations. (Different pollination times in trees)

  (10 min Speciation Crash Course Video – Have to click on speciation)   (CP – start at 6:50)  Quick Intro to speciation 

11.6 – Patterns in Evolution  Key Concept:  Evolution occurs in patterns.

Species can become extinct.  Extinction: elimination of a species from Earth  Background extinction  Mass extinction

Background Extinction  Occur randomly, but at a low rate  Usually affect only a few species in a small area  Can by caused by local changes in the environment

Mass Extinction  Rare, but very intense  Can operate at a global level  Caused by a catastrophic event such as an ice age  At least 5 mass extinctions in the last 600 million years

Extinction  Species go extinct because they lack the variation needed to adapt