evolutionbirds by 83I72YwX

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									                                            Evolution

•   Variety is the spice of life
•   As Darwin realized, evolution depends on variation
•   Individuals with adaptive variations are more likely to survive and reproduce
•   They tend to pass on those adaptive variations to their offspring

•   Darwin realized that evolution must be tied to variation
•   Species were really just local groups of individuals, all of whom varied from one another in
    certain ways

•   Just as farmers selected the best varieties to breed, nature must somehow select those
    individuals best fit to survive
•   In every natural population, some varieties must be better equipped to prevail in the struggle
    for existence

•   Darwin’s theory of evolution by natural selection forms the basis of our modern theory
•   Synthetic theory (molecular/popn. perspectives)

•   Species usually begin when small parts of a larger population become geographically isolated
    from the parent population
•   Differences in local conditions favor different varieties in the isolated population, whose gene
    pool is a subset of the original population

•   These differences could eventually become isolating mechanisms
•   These mechanisms would keep populations from interbreeding if/when geographic barriers
    were removed, and they were reunited with one another (secondary contact)

•   Because they are geographically isolated, members of this small populations can only
    interbreed with one another
•   Mutations and recombinations within this isolated population can lead to entirely new
    varieties, and new combinations of existing traits

•   The population may experience genetic drift, a random fluctuation in the proportion of a
    particular allele in a small inbred population
•   Natural selection favors some combinations of alleles over other combinations

•   Over time, the effect of natural selection, genetic drift, mutation, and recombination can
    become isolating mechanisms
•   When the isolated population is reunited with the parent population, they can no longer
    interbreed
•   They have become reproductively isolated, and can now be considered a new species

•   Local environmental conditions are often very different for two distantly separated
    populations
•   Conditions of temperature, humidity, rainfall, vegetation and predators can be very different at
    either extreme end of the range

•   Variation in any trait often changes very gradually as we go from one end of a species' range
    to the other
•   This change in variation maps out changes in local environmental conditions, and we call this
    gradient of variation a cline

•   Body size in House Sparrows, for example, shows a gradual change from Mexico to Canada
•   Mexican sparrows are smaller on average than Canadian sparrows

•   This is a result of the simple mathematics of how the volume of an organism changes with
    respect to its surface area
•   Imagine an ideal sparrow, a perfect little sphere

•   The formula for the surface area of a sphere is pi*r2
•   The formula for the volume of a sphere, on the other hand, is 4/3 pi*r3
•   So for increasing radius, volume increases much faster than surface area

•   This means that larger animals have less surface area than smaller animals with respect to
    their internal volume
•   There are many implications of this simple allometric relationship, in which two traits change
    at different rates from one another

•   Small animals, for example, with relatively more surface area than larger animals, will radiate
    more of their internal heat to the environment
•   The larger you are, on the other hand, the more heat you retain simply because there is
    relatively less surface area over which body heat can be lost

•   So higher temperatures favor smaller body sizes, and lower temperatures favors larger body
    sizes
•   Mexican sparrows are relatively small, and Canadian sparrows are relatively large

•   The two local populations of Canadian sparrows and Mexican sparrows live very far apart
•   They don't interbreed because they never meet in the wild

•   When we bring these two populations together in the laboratory, the difference in body size is
    so pronounced that they cannot interbreed
•   Doesn't that mean that they are separate species?

•   In the strictest sense of our definition of species, they would be considered different species
•   But we still consider them to be a single species
•   Why?

•   Each local population of sparrows can and does interbreed with other adjacent local
    populations
•   There is an exchange of genes, or gene flow between the populations

•   So the populations at either end of the range are still tied together as a single species because
    genes are exchanged from one end of the range to another, through many local populations

•   We call variant groups of local populations races
•   Nearly every species has diversified into several races
•   If local races are so distinct that we can easily tell one from another, we call these races
    subspecies (botanists prefer variety)

•   "Subspecies" is a convenient way to identify local races that have diverged enough from the
    main population that they have the potential to someday form a new species
•   Subspecies are incipient species

•   Ornithologists as a group are especially fond of subspecies, so there are many recognized
    subspecies of birds
•   They use the rule of thumb that when 75% of the individuals in a local population are easily
    distinguished from other populations, they are recognized as subspecies

•   If geographic isolation is removed before this process is complete, the subspecies is often
    reabsorbed into the parent population
•   Sometimes species form a series of very distinct geographic races, because they are relatively
    isolated, and there is very little gene flow between them

•   Because they are not yet fully separated species, these local races or subspecies can interbreed
    along the edges of their respective ranges, forming broad hybrid zones
•   The eastern Myrtle Warbler and the western Audubon’s Warbler were originally separated
    during the Pleistocene by advancing glaciers
•   Made secondary contact with one another about 7,500 years ago when the glaciers retreated

•   They freely interbreed along the borders of their ranges, and were recently recognized as a
    single species, the Yellow-rumped Warbler
•   It is a polytypic species, a species with two or more subspecies or geographical varieties

•   The Nightingale of eastern Europe, and the Nightingale Thrush of western Europe, were also
    formed by a glacial separation of their ancestral species
•   But they no longer interbreed, even though their ranges now overlap

•   They are vicariant species
•   Vicariance is the evolution of closely related taxa in different geographical areas, from
    populations that were separated by the formation of a natural barrier

      Isolating mechanisms are barriers to gene flow between populations - they are potential
                                     pathways for speciation

•   Geographic isolation results in reproductive isolation
•   In relative geographic isolation, reproductive isolating mechanisms can serve to separate
    populations, so that when or if they are reunited, they can no longer interbreed

•   Isolating mechanism - any factor that acts to reduce or block the flow of genes between two
    populations
     > Geographic isolating mechanisms
     > Reproductive isolating mechanisms
         – Temporal           — Mechanical
         – Behavioral         — Ecological

•   Reproductive isolating mechanisms can be pre-zygotic, occurring before fertilization, or post-
    zygotic, occurring after fertilization
•   Post-zygotic isolating mechanisms all involve some type of genetic weakness in the hybrid
    offspring
•   Zygotes form, but are weak or sterile
•   The Eastern Meadowlark and Western Meadowlark can interbreed, but the resulting hybrids
    are all sterile
•   We see the same effect in mules, the sterile hybrids of a female horse and a male donkey

•   Most speciation is allopatric – geographic ranges of the organisms do not overlap
•   Sympatric speciation is relatively rare – sympatric species have ranges that overlap

•   There are two races of White-crowned Sparrows that share winter quarters in California
•   Never mate with one another because their gonads mature at slightly different times

•   The Western Grebe has two color morphs, a light form and a dark form
•   Each form has slightly different mating call

•   In mixed wild flocks, the two forms rarely interbreed
•   Many ornithologists now recognize them as separate species

•   A morph is a very distinctive variety, with no intermediate forms
•   Like the two color morphs of the Blue Goose and Snow Goose

•   Line between species is sometimes blurred
•   Hybrids of Blue-winged Warblers and Golden-winged Warblers show a mixture of the two
    species courtship behavior
•   These hybrids are always the last to mate

•   Mating systems are a good example of how variation at the population level affects
    evolutionary processes
•   Can be monogamous or polygamous
     > serial monogamy
     > polygyny (one male, many females)
     > polyandry (one female, many males)

•   Most birds are monogamous (~90%), serial monogamy is common
•   Polygamy is relatively rare in birds (~10%), with polygyny more common than polyandry

•   Consider the first few finches that reached the Galápagos Islands
•   Isolated from the parent population
•   Spread from island to island, forming several isolated island populations
•   Each island population had slightly different gene pool (total variation of population)

•   Shaped by different conditions on each island (ex. food supply)
•   Change in frequency of certain genes over time (ex. larger or smaller beaks)
•   Differences would accumulate to the point where the new population could no longer
    interbreed with the parent population

•   When the isolated population is reunited with the parent population, they can no longer
    interbreed
•   They have become reproductively isolated, and can now be considered a new species

•   Biological species concept - species are populations of similar organisms that can interbreed
    with one another, but are reproductively isolated from other such populations by one or more
    isolating mechanisms

•   We classify organisms by their similarities
•   These can be physical, physiological, even behavioral

•   Taxonomy is the description, naming, and classification of living organisms
•   About 1.4 million species now named, maybe as many as 10 million or more

•   A taxon (taxa) is any rank in classification, a collection of related organisms
•   Genus, species, subspecies, race - are the lowest ranks, Kingdom, Domain are the highest rank

•   Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species (D K P C O F G S)
•   Taxa are often split or lumped into smaller or larger groups, like sub-orders or infra-orders

•   Similar species are grouped together into genera
•   Each species in a genus shares many characteristics with its allied species

•   We always refer to species by their genus and species name, either underlined or italicized
•   Binomial nomenclature invented by Linnaeus, as a convenient way to refer to organisms that
    often had many common names

•   Before Linnaeus, scientific names were lengthy descriptions (in Latin) of the organism
•   Plantago media, for example (plantain) was: Plantago foliis ovato-lanceolatis pubescentibus,
    spica cylindrica, scapo tereti (plantain with hairy ovate-lanceolate leaves, an ovate naked
    spike and a terete scape)

•   Earlier classifications are based exclusively on external or phenetic traits (phenotypic)
•   In recent years, molecular evolution has become increasingly important

•   Molecules evolve in the same way that wings and beaks evolve
•   Molecular evolution is often very slow
•   Molecule may be critical to the survival of the individual

•   By looking at the differences in amino acid sequence, we can often determine the distance of
    the relationship between two organisms

•   The molecular subunits of ribosomal RNA are often used to establish an evolutionary
    yardstick
•   All organisms share the same basic system of protein synthesis
•   We can use r-RNA and similar molecules to look at evolution between broad groups of
    organisms

•   We have a rough idea of how often changes occur in these molecular structures
•   We can develop a crude yardstick for the timing of critical events (molecular clock)

•   We classify organisms in an attempt to understand their phylogeny, their evolutionary history
•   Phylogeny is the evolutionary history of organisms (their lineage)
•   Every scheme of classification is arbitrary

•   Early attempts at classification (evolutionary taxonomy) relied on phenotypic traits
•   Certain traits were presumed to be more important than others
•   Taxonomists gave them higher importance values or weights

•   Many biologists didn’t like this system
•   Weighted characters was a hopelessly subjective idea
•   Determined not by nature, but by what the taxonomist felt was right

•   More conservative numerical or phenetic taxonomists also saw the whole thing as a circular
    argument
•   Evolutionary taxonomy assumes that evolution has taken place

•   Uses these evolved traits to construct a phylogenetic pattern
•   Pattern is used to prove that evolution has taken place…

•   Phenetic taxonomy emerged as a reaction against this subjectivity and circularity
•   Start by measuring numerous physical characters, like beak length, leg length, and body size

•   Or you can choose some physiological character
•   Concentration of certain chemicals in the blood
•   Get average values for different species

•   Subtract these values to get the distance values of that trait for each pair of species
•   Choose a cluster statistic to look for possible evolutionary trees, branching patterns that best
    fit the data

•   In their attempt to remain objective, by weighting all characters equally, numerical
    taxonomists don’t try to differentiate between homologous and analogous traits
•   Homologous traits represent divergence from a common ancestor
•   Traits like the pattern of bones in the forelimbs of vertebrates

•   Analogous traits result from convergent evolution, in which evolution shapes similar traits in
    unrelated groups of species
•    Both sharks and dolphins have paired fins and a streamlined shape, but one is a fish and the
    other is a mammal

•   Fins and streamlining are just common solutions hit upon by two groups of animals that need
    to move quickly through the water
•   They are not a good basis for classifying animals

•   So numerical taxonomy is also subjective
•   You have to assume which statistic is the best one to use, and you have to ignore analogous
    traits

•   Our current scheme of classification is called cladistic analysis or cladism
•   Each taxon is a clade, a branch on the tree of life (cladogram)
•   Clades are determined by traits they share, traits that are different from their ancestors

•   Consider three species of birds which share some (but not all) of three traits
     > Hooked beak (h)
     > Crest on head (c)
     > Webbed feet (w)
•   How many ways can we group them?

•   Each possible grouping makes certain assumptions
•   We choose the cladogram that makes the fewest assumptions (most parsimonious)
•   We call these traits synapomorphies, shared derived characteristics

•   Monophyletic - taxon contains the common ancestor and all of its descendants
•   Cladists only recognize monophyletic groups
•   Cladists try to avoid paraphyletic and polyphyletic groups

•   Monophyletic - contains the common ancestor and all of its descendants


•   Paraphyletic - contains common ancestor but only some descendants (most similar)

•   Polyphyletic - contains some descendant species but no common ancestor (may even come
    from different ancestors)

•   Most paleontologists assume that dinosaurs all evolved from one of the many species of
    thecodonts, a primitive reptile
•   This would make dinosaurs a monophyletic group

•   If the minority is correct, and dinosaurs evolved from more than one group of thecodonts, then
    dinosaurs would be a polyphyletic group
•   We will examine the evolution and classification of birds to determine whether birds are
    descendants of dinosaurs

								
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