Evolution, Part 2

Natural Selection: Odds and Ends

Mendel and the Modern Synthesis

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On the Origin of SpeciesCalled one long argument for evolution

Hundreds of thousands of observations

Detailed, testable theory

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WeaknessesNo mechanism for HOW variation is passed from parent to offspring

Modern synthesis: combination of Mendels and Darwins ideas

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Fossil RecordComparing fossils presents a pattern of gradual change

Darwin found gaps in the fossil record

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Types of Selection

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Normal Population

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Scenario 1: Leaves towards the bottom of trees are disappearing. Some giraffes can reach leaves higher up, while others necks are not long enough?

Which giraffes will thrive? Which giraffes will die out?

What will future generations look like?

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Scenario 2: A certain species of rabbits in a dense forest come in brown and white fur colors. Hawks, soaring overhead, often eat the rabbits.

Which rabbits will be easier to spot?

Which color trait will more likely be passed on to future generations?

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Directional SelectionIndividuals that display an extreme trait are selected for. Other individuals are selected against.

Population shifts toward this trait, away from the opposite.

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Scenario 3: Lizards range in size from tiny to very large. Small lizards generally run very slowly while large lizards are usually the easiest for predators to see.

What size lizard stands the best chance of survival?

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Scenario 4: Human babies of very low birth weight are often more prone to disease. Babies of very large weights are difficult for mothers to safely deliver.

What size babies stand the best chance of survival?

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Stabilizing SelectionIndividuals that display an average trait are selected for. Individuals that display extreme traits are selected against.

Population shifts towards the average trait, and those with extreme forms die off.

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Scenario 5: Limpets are marine animals that attach to rocks. Their color varies from pure white to dark brown. White-shelled that are on rocks covered with goose barnacles (also white) are at an advantage. On bare, dark-colored rocks, brown limpets are at an advantage.

What advantage is being referred to above?

If you were a dark limpet,where would you live?A white limpet?

What color would have thehardest time surviving?

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Disruptive SelectionIndividuals that display either extreme are selected for. Average individuals are selected against.

Population shifts towards away from the average trait, towards BOTH extremes.

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Assignment:Design three example situations (1 paragraph each) representing the three types of selection we covered. Include the following:

Description of the animals involved and their traitsHow is nature selecting for/against certain animalsWhich trait will be come more common/less common and why?Name the type of selection.

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ExampleA group of crabs live on a sandy beach. The crabs range in color from very light tan to dark brown.

Seagulls, flying overhead, eat the crabs. They can more easily see the dark crabs, since the lighter ones blend in with the color of the sand.Because the seagulls more easily eat the dark crabs, the lighter ones will more likely survive and reproduce, passing that trait onto their offspring.

This is called directional selection.

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Mechanisms for Evolution

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New Definition for EvolutionChange in the allele frequency of a population over time

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Questions About EvolutionDoes chance play a role?

Is evolution predictable?

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Hardy-Weinberg PrinciplePopulations will remain at equilibrium (and therefore not evolve) unless some outside factor affects them

Equilibrium: frequency of alleles is unchanging

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5 Mechanisms for EvolutionNatural Selection

Sexual Selection (mate choice)

Mutations

Gene flow (migration)

Genetic Drift and population bottlenecks

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Gene FlowMovement of individuals (and alleles) from one population to another

What are some barriers?

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Genetic DriftChange in allele frequency caused by chance events

More common in small populations

Can be beneficial, harmful, or neutral

Examples:Founder EffectPopulation Bottleneck

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Speciation and Terminology

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Covergent EvolutionEnvironmental factors allow the development of similar traits in very different organisms

Ex: bats, beetles, birds

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Divergent EvolutionDescendants of a single ancestor diversify into species that fit into different parts of the environment.Adaptive Radiation-when a new population is in a new environment it will undergo divergent evolution until the environment is full.

Co-EvolutionSpecies that live near one another evolve to benefit one another

Ex. Yucca moth and Yucca plant

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Important TerminologyAdaptive Radiation one species, subjected to different environmental pressures, can evolve into multiple species

Speciationformation of a new species

Often, one species can branch from another

Extinction: the death of all members of a single species

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Two TheoriesGradualism tiny changes become large-scale changes over a long period of time

Punctuated Equilibrium sudden environmental changes lead to new species developing quickly

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SpeciesMorphological Concept: uses similarities (physical and physiological) between organisms

Biological Concept: group of organisms that can breed and have fertile offspring

Isolation/SpeciationGeographic Isolation (Allopatric speciation)

Reproductive Isolation (Sympatric speciation)