Part 2

Evolution Notes

Natural Selection and Natural Selection and MacroevolutionMacroevolution

Natural Selection shapes a population, Natural Selection shapes a population, making it adapted to its current making it adapted to its current environment. This happens over a relatively environment. This happens over a relatively short period of time.short period of time.

Most scientists agree that natural selection, Most scientists agree that natural selection, acting over very long periods of time, leads acting over very long periods of time, leads to speciation. (“Adding branches to the to speciation. (“Adding branches to the tree.”)tree.”)

There are two patterns of evolution There are two patterns of evolution described by scientists based on described by scientists based on observations of living and extinct observations of living and extinct organisms.organisms.

Patterns of EvolutionPatterns of Evolution Divergent evolutionDivergent evolution: : the process of two the process of two

or more related species becoming more or more related species becoming more and more dissimilarand more dissimilar These species will share homologous

structures Convergent evolution: the process by

which distantly related species become more similar as they adapt to the same kind of environment These species appear similar on the outside,

but do not share homologous structures. Their similar structures are considered analogous.

Example of Divergent Example of Divergent EvolutionEvolution

Each of these mantid species is exquisitely adapted to a very specific environment. The “body plan” of each species is homologous, but adaptation by natural selection has made each look different.

Examples Examples of of

Divergent Divergent EvolutionEvolution

What do these 2 examples have in common?

Galapagos Finches

Hawaiian Honeycreepers

Divergent Evolution of Divergent Evolution of AnimalsAnimals

Example of Convergent Example of Convergent EvolutionEvolution

The placental mouse has more homologous features with the wolf than with the marsupial mouse.

The similarities between the placental mouse and marsupial mouse are analogous.

Analogous FeaturesAnalogous Features

Homology vs. AnalogyHomology vs. Analogy

http://www.pbs.org/wgbh/evolution/change/family/

More on Patterns of More on Patterns of EvolutionEvolution

Both divergent and convergent evolution Both divergent and convergent evolution involve involve speciation, the formation of speciation, the formation of new speciesnew species..

Isolated populations become more different Isolated populations become more different over time as natural selection acts to adapt over time as natural selection acts to adapt them to a certain environment, or a unique them to a certain environment, or a unique niche within the environment.niche within the environment.

If enough time passes and enough genetic If enough time passes and enough genetic change occurs, the isolated populations change occurs, the isolated populations become different species.become different species.

Species DefinitionsSpecies Definitions Historically, a species has been defined by its structural Historically, a species has been defined by its structural

differences compared to similar species. However, differences compared to similar species. However, structure is not adequate, by itself, in defining a species.structure is not adequate, by itself, in defining a species.

Morphological species concept: Morphological species concept: defines a species as a group of defines a species as a group of structurally unique organisms that structurally unique organisms that differ from other described species.differ from other described species.

Biological species concept: Biological species concept: defines a species as a group of defines a species as a group of organisms able to interbreed to organisms able to interbreed to produce viable offspring in nature.produce viable offspring in nature.

http://www.nytimes.com/2010/04/09/science/09fossil.htmlhttp://www.nytimes.com/2010/04/09/science/09fossil.html

Isolating Mechanisms are Isolating Mechanisms are needed for new species to needed for new species to

arisearise A key of isolating mechanisms is that A key of isolating mechanisms is that

they act as barriers that keep two they act as barriers that keep two populations from interbreeding.populations from interbreeding. Geographic isolationGeographic isolation: : If a population If a population

splits into 2 due to a physical barrier splits into 2 due to a physical barrier (i.e.: mountain range forms, a lake dries (i.e.: mountain range forms, a lake dries up into small ponds, a subpopulation up into small ponds, a subpopulation crosses to a different island, etc.),crosses to a different island, etc.), the the two populations become different due two populations become different due to natural selection.to natural selection. Over time the Over time the populations maypopulations may become different become different enough that they can no longer enough that they can no longer interbreed.interbreed.

Geographic Isolation & Geographic Isolation & SpeciationSpeciation

Examples of Geographic Examples of Geographic IsolationIsolation

Isolating Mechanisms, Isolating Mechanisms, continuedcontinued Reproductive Isolation: Reproductive Isolation: type of isolation type of isolation

that prevents interbreeding, even when two that prevents interbreeding, even when two populations live in the same geographic populations live in the same geographic region.region. Prezygotic barriers: these barriers prevent Prezygotic barriers: these barriers prevent

fertilization between gametes of two different fertilization between gametes of two different species.species. (ex: males of one population use a (ex: males of one population use a different mating call to attract females than a different mating call to attract females than a related species.)related species.)

Postzygotic barriers: these barriers prevent Postzygotic barriers: these barriers prevent hybrid offspring from either surviving or hybrid offspring from either surviving or producing offspring.producing offspring. (ex: an embryo fails to (ex: an embryo fails to develop and dies or the resulting offspring are develop and dies or the resulting offspring are sterile like the mule that results from the mating of sterile like the mule that results from the mating of a donkey and a horse)a donkey and a horse)

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Natural Selection is Natural Selection is notnot the the only mechanism of only mechanism of

evolutionevolution If a population If a population isis changing changing

genetically, then a biologist can genetically, then a biologist can investigate if it may be due to one of investigate if it may be due to one of the following:the following: Natural selectionNatural selection Nonrandom mating (selective Nonrandom mating (selective

breeding)breeding) MutationMutation genetic drift genetic drift gene flowgene flow

Closer Look: Nonrandom Closer Look: Nonrandom MatingMating

Farmers and animal breeders have long Farmers and animal breeders have long known that the traits of a population known that the traits of a population could be changed by nonrandom mating.could be changed by nonrandom mating. Example: Oranges with smaller and smaller Example: Oranges with smaller and smaller

seeds were bred until “seedless” oranges seeds were bred until “seedless” oranges were createdwere created

In this case, farmers did not allow “nature to In this case, farmers did not allow “nature to take its course”. They selected a trait they take its course”. They selected a trait they wanted and only bred plants that had that wanted and only bred plants that had that form of the trait.form of the trait.

Closer Look: Genetic Closer Look: Genetic DriftDrift

Genetic drift: Genetic drift: the process by which the process by which random evolutionary changes occur random evolutionary changes occur in small populations due to chance in small populations due to chance or random events.or random events.

The evolutionary changes are The evolutionary changes are notnot necessarily adaptive, unlike necessarily adaptive, unlike evolution by natural selection.evolution by natural selection.

Genetic Drift can decrease Genetic Drift can decrease genetic variationgenetic variation

Bottleneck effect: occurs when a Bottleneck effect: occurs when a population decreases rapidly due to natural population decreases rapidly due to natural disaster, disease, etc.disaster, disease, etc. The small remaining The small remaining population may not have allele frequencies that population may not have allele frequencies that are representative of the original population.are representative of the original population.

Founder effect: occurs when one or a few Founder effect: occurs when one or a few individuals leave a large population and individuals leave a large population and colonize a new location.colonize a new location. The founders only The founders only bring with them a small sample of the alleles of bring with them a small sample of the alleles of the original population, so the new population is the original population, so the new population is likely to be significantly different genetically.likely to be significantly different genetically.