AP Biology Unit 7- Evolution

Evolution is the change in a population of organisms over time (NOT IN AN INDIVIDUAL)

Vocabulary

• Species – a group of organisms that can mate and produce fertile offspring (reproductive isolation)

• Allele – form or variation of a gene (usually dominant & recessive)

• Gene – a piece of DNA that codes for a trait

  • Example: flower color - gene purple or white – allele

• Genotype – a gene combination for a trait (e.g. PP, Pp, pp)

• Phenotype - the physical feature resulting from a genotype (e.g. purple, white)

• Fitness – the reproductive success of an organism

• Selective pressure – anything (biotic or abiotic) that changes the fitness of living organisms within a given environment. It is the driving force of natural selection.

• Selective pressure can push a population into a new direction

• Directional selection shifts the frequency curve for a phenotypic character in one direction by favoring what had been rare in individuals (most common during periods of environmental change)

• Diversifying (disruptive) selection occurs when environmental conditions favor individuals at both extremes of the phenotypic range over intermediate phenotypes. Stabilizing selection favors intermediate forms and acts against extreme phenotypes.

Balanced Polymorphism

Two different versions of a gene are maintained in a population of organisms because individuals carrying both versions are better able to survive than those who have two copies of either version alone.

Darwin’s Principles of Natural Selection

• Left unchecked, the number of organisms of each species will increase exponentially, from generation to generation

• In nature, populations tend to remain stable in size

• Environmental resources are limited

• There is natural variation in a population, and the variation is inherited.

• Production of more individuals than can be supported by the environment leads to a struggle for survival.

What is the ultimate source of all variation in any population?

• Mutations.

• Chance creates new genetic variations by mutation and sexual recombination.

• Natural selection favors some variations over others (depending on the particular environment).

  • This produces organisms that are better fit for their environments.

• Mutations are RANDOM, but natural selection is NOT RANDOM!!!

Structures- Vestigial, homologous, analogous

• Analogous structure - The same in function or form but NOT present in a common ancestor and NOT derived from the same embryonic tissue.

  • Convergent evolution

• Vestigial structure - once served a purpose, but no longer advantageous or can become harmful.

  • Example: appendix, wisdom teeth.

• Homologous structure - similar physical features in organisms that share a common ancestor, but the features serve completely different functions

  • Divergent evolution

Speciation / Reproductive barriers

• Allopatric speciation - Species are separated by a geographical barrier

• Parapatric speciation - Partial separation of 2 separating species

• Sympatric speciation - Reproductively isolated subpopulation in the midst of its parent population (change in niche)

• Geographical isolation

  • Mates are separated by geographical barriers (i.e., mountains)

• Prezygotic isolation

  • Organisms do not make a zygote

• Temporal isolation

  • Potential mates live in the same area but reproduce at different times

• Phylogeny

  • The study of the evolutionary relationships between organisms/groups, calibrated by fossils and/or DNA/proteins.

• Behavioral isolation

  • Potential mates meet but do not behave the right way (e.g., wrong dance)

• Mechanical isolation

  • Potential mates attempt to mate, but the sperm cannot be successfully transferred (parts do not fit)

• Gametic mortality

  • Sperm is transferred, but the egg is not fertilized. (Correct proteins need to match from sperm to egg)

    • Prezygotic

• Ecological isolation

  • Potential mates occupy different local habitats within the same area

    • I.E., Rana Aurora and Rana Catesbiana (different metamorphosis times of tadpoles)

• Postzygotic isolation

  • Made, does not make babies

• Zygote mortality

  • Made/ fertilized but dies

• Hybrid inviability - not strong

• Hybrid sterility - robust but cannot make babies

• Bacteria evolve much faster than humans/ animals! (Binary fission is very quick, about 30 minutes)

Types of evolution

• Coevolution - Two species evolve together, each exerting selective pressure on the other.

• Divergent evolution - Share a common ancestor but separate, becoming different species due to growing differences/ speciation (I.E. Chimpanzee and Human)

• Convergent evolution - Not closely related, but have been shaped/ evolved in similar environments or selective pressures

• Adaptive radiation - The evolution of many diversely adapted species from a common ancestor

  Evidence for evolution can be found in:

  1. The fossil record

  2. Convergent evolution (analogous

  structures)

  3. Divergent evolution (homologous

  structures)

  4. Conserved early development

  5. Molecular biology (DNA/RNA/Proteins)

Natural Selection

• Altruism - An organism’s behavior benefits other organisms at a cost to itself (measured in terms of reproductive fitness).

• Kin selection - Favors reproductive success of an organism’s relatives

  • i.e. Queen bees and worker bees

Hardy Weinburg Equilibrium Equations

Hardy- Weinburg Assumptions

• No mutation

• Random mating

• No gene flow

• Infinite or large population size

• No selection (i.e. sexual).