DIVERSITY AND EVOLUTION Chapter 2. Diversity of life Approximately 1.5 million living species...
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Transcript of DIVERSITY AND EVOLUTION Chapter 2. Diversity of life Approximately 1.5 million living species...
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DIVERSITY AND EVOLUTION
Chapter 2
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Diversity of life Approximately 1.5 million living species
described Likely at least 10 million species today May represent only 1% of species ever to
have lived on earth 1 billion species presumed to have lived
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Diversity of body form Tremendous diversity within each group of
plants, animals, fungi, protistans, bacteria Structural complexity - apparently
purposeful adaptation of many characteristics to the environment
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Reason for this diversity? Natural selection
Physical environment acts on various characteristics of organisms (variation among individuals of some species)
Sorts out “harmful” ones, leaving individuals with “beneficial” or “neutral” characteristics to produce next generation
Keeps organisms well-suited for survival in their environment
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Natural selection drives evolution Broad scale Development of various “forms” or species
to best match the environment Can best take advantage of variations
within that environment
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History of concept of evolution by natural selection Lamarck - inheritance of acquired
characteristics Darwin, Wallace - natural selection, but
mechanism really unknown Mendel - genetic understanding of the
acquisition of inherited traits
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Evolution by natural selection - established truths 1) individuals that form a population of a
species are not identical
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Evolution by natural selection - established truths 2) some of the variation between
individuals is heritable
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Evolution by natural selection - established truths 3) all populations are capable of
exponential growth, but most individuals die before reproducing, and most others reproduce at less than their maximum rate
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Evolution by natural selection - established truths 4) different ancestors leave different
numbers of descendents; they do not all contribute equally to subsequent generations
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SPECIATION
Interaction of heritable variation, natural selection, barriers to gene flow
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Allopatric (Geographic) Speciation Separating single, interbreeding population
into two or more spatially isolated populations
Geographic barrier, remains long enough for speciation
Founder effect, genetic drift (random mutations)
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Parapatric Speciation No spatial isolation Portion of population invades new,
adjacent habitat Little to no movement/interbreeding Differing natural selection in differing
habitats
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Sympatric Speciation No spatial isolation Production of new species within a
population Rare Most likely to occur in insect parasites of
plants, animals Requires stable polymorphism and under-
or unused resource
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Polyploidy Abrupt speciation by doubling the number
of chromosomes Most common in plants Agricultural-wheat, alfalfa, potatoes Native-birches, willows
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PATTERNS OF SPECIATION
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Anagenesis One species changes into another species
over time Original species “evolves” out of existence
and is replaced by new species Evolutionary extinction
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Cladogenesis One species gives rise to one or more
additional species while still remaining Clade-set of species descended from a
particular ancestral species (e.g., Darwin’s finches)
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TEMPO OF SPECIATION
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Gradualism Steady change in character(s) resulting in
many intermediate forms exhibiting “gradual” shift
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Punctuated equilibrium Rapid, abrupt changes that produce quick
shifts in character No intermediate forms
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REDUCTION IN VARIATION
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Inbreeding depression Mating among close relatives produces an
increase in expression of recessive traits, many of which are deleterious
Often results from small population size Mortality may be increased “Tighter” inbreeding results in more rapid
loss of genetic variation within population
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But…. Not all populations are harmed by
inbreeding Long-term, small populations (e.g., on
islands) may be adapted to inbreeding and survive well even in face of it
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Outbreeding Some degree of outbreeding usually
beneficial in maintaining genetic diversity But too much can also be harmful Too many differences may lead to
problems
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Smaller populations Genetic variation declines faster in smaller
population because of inbreeding Rule of thumb-50 individuals needed to
prevent inbreeding Problem for saving California condor
Only 26 individuals in 1986
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Genetic drift Larger population not subject to inbreeding
can lose genetic variation at rates similar to small populations via genetic drift
Some individuals do not mate, not represented genetically in next generation
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Genetic drift-cont. Rule of thumb-happens only in populations
<500 in size Genetic drift can be counteracted by
minimal levels of immigration into the population
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Neighborhoods Even big populations may run into problems if
individuals don’t move around much to mate Some also just don’t reproduce Effective population size may then be quite small
E.g., grizzly bear in Yellowstone Actual population ~200 Effective population ~50 (25%) Subject to loss of variation
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Bottlenecks Can also reduce genetic variation Bottlenecks - periodic reductions in
population size can reduce genetic variation greatly even if average population size is much larger
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Founder Effects Can also reduce genetic variation Founder effects - developing gene pool of
growing population is limited by what variation founders had, plus mutation
Pair of founders at most have 4 variations in a gene
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ORIGIN OF VARIATION
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Genetic Increase or decrease variability within a
population DNA - mistakes or mutations during
copying of genetic code Gene or point mutation - most important for
enriching the gene pool Chromosome mutation - most important for
rearranging the gene pool
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Point Mutations Change in nucleotide base at single
location Change in single amino acid within
protein, or entirely different protein Frameshift mutation - insertion or deletion
of single base pair
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Mutagens and mutations Mutations usually produced by mutagens
(e.g., weak cosmic rays) 1 mutation per gene in every 100,000 sex
cells Higher organisms have ~10,000 genes 1 in 10 individuals has newly created
mutation
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Most mutations are harmful, but.. 1 in 1000 mutations may be beneficial 1 in 10,000 individuals per generation has
a useful mutation Most individuals have at least one mutant
gene (original, or passed down from ancestors)
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Mutations and Speciation Estimate - 500 mutations necessary to
produce new species from existing one Rate of new mutations ~1 million times
greater than needed to account for known rate of evolution
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Chromosomal Mutations No change to variability Rearrange what is there Deletions, duplications, inversions,
translocations
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Other changes Polyploidy - e.g., tetraploid Failure of gametes to reduce to haploid
state during meiosis 2N + 2N = 4N
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So… Mutations produce the variation, and
natural selection acts upon the changes Add in: nonrandom mating, changing
environment End product = EVOLUTION
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Amount of Variation Results from protein analyses
(electrophoresis) Within a population - 15-58% of genes
exhibit variation Within individuals - 3-17% of genes
exhibit variation
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Applying this information:1) Separate populations of organisms with
movement of individuals among populations generally exhibit most variation within each population, and very little between or among populations
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Applying this information: 2) Reduced movement of individuals
among populations produces more variation between or among populations Populations diverge genetically
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Applying this information: 3) Conservation of endangered species
which move around very little will require protection of many populations in many different habitats to conserve genetic diversity within the species