SPECIES- A group of individuals that look similar and whose members are capable of producing fertile offspring.

The definition of species derived from two different “concepts”… 1) Morphological Species Concept: Based on internal and external similarities and differences. MORPHOLOGY

2) Biological Species Concept; Classification based on an organisms ability to reproduce and produce fertile offspring.

The offspring of two morphologically dissimilar organisms HYBRIDHYBRID:

*POPULATION- All members of the same species that live in a particular area at the same time.

Within a population, individuals vary in many observable traits These can be presented on a BELL CURVE.

What causes variations within species… 1) Environmental factors 2) Heredity mutations genetic recombination crossing over

Allele Frequency and Genetic Equilibrium A population is a collection of genes GENE POOL- A collection of genes for all the traits of a population.

ALLELE FREQUENCY- The % of a specific allele of a certain gene in the gene pool GENETIC EQUILIBRIUM- A population in which allele frequency does not change.

The Hardy-Weinberg Principle Weinberg: German physician Hardy: British mathematician * Observed that allele frequencies in populations remain relatively the same unless acted on by outside influences

Assumptions for Equilibrium: 1.No mutations occur 2.Individuals neither enter nor leave through migration 3.The population is large (ideally infinitely large) 4.Individuals mate randomly 5.Natural selection doesn’t occur p 2 + 2pq + q 2 = 1.0 DEMO PROBLEMS:

Disruption of Genetic Equilibrium A. Mutations- Physical change of genes or chromosomes Produce totally new alleles for a trait Can change the frequency of alleles

B. Migration- immigration: movement into a population emigration: movement out of a population *Gene flow- The movement of genes into or out of a population through migration.

C. Genetic Drift- Allele frequency in a population changes as result of a random event

Genetic Drift- Significant only in small and medium sized populations An allele may disappear completely in a small population More stable in larger populations

D. Non-Random Mating Random mating is a requirement for genetic equilibrium Many species DO NOT follow random mating. Mating is influenced by a variety of factors…

Reasons for Non- Random Mating: Geographic proximity Selection based on similar physical characteristics ASSORTIVE MATING PROBLEMS FROM NON-RANDOM MATING: Mating may amplify certain recessive disorders.

“Survival of the fittest” Some members of a population are more likely to contribute genes E. Natural Selection-

Types of Natural Selection

1. Stabilizing Selection- Natural selection in which individuals with the average form of a trait have the advantage.

Natural selection in which individuals with one of the extreme forms of a trait has an advantage. 2. Directional Selection

3. Disruptive Selection Type of natural selection in which individuals with either of extreme forms of a trait have advantage.

4. Sexual Selection- Mate selection based on the presence of a specific trait.

The formation of a new species Speciation-

Isolated Populations… 1. Geographic Isolation- A physical separation of members of a population.

2. Reproductive Isolation- Inability of former interbreeding organism to produce offspring Prezygotic Isolation: Obstacle occurs before fertilization Examples: Incompatible behavior Different mating calls Different mating seasons

Postzygotic Isolation: Obstacle occurs after fertilization Examples: Offspring may not develop completely Offspring may die early Offspring may not be fertile

RATES OF SPECIATION 2 major theories regarding the rate of speciation: Gradualism: The idea that divergence of species occurs smoothly and gradually over time

PUNCTUATED EQUALIBRIUM- The idea that things live in Genetic Equilibrium then changed in short spurts of time. Unicellular organisms may evolve in months or even day Other evidence suggests within thousands of years

EXTINCTION: The point at which no organisms of a particular species are available to reproduce change Disruption of habitat

Natural process, ongoing Once gone, gone forever Causes: environmental