Download Evolution of Populations

9/1/2014
Basic Vocabulary
• Population: group of a single species living together,
that interbreed and produce fertile offspring
Evolution of Populations
Chapter 23
Pg. 469 – 487
Evolution Basics
• Individuals cannot evolve! Populations do.
• Evolution is based on genetic variation.
• Genes affect fitness.
– Fitness: reproductive success
– How do genes affect fitness? Ability to survive, find a mate, reproduce,
raise offspring capable of reproducing, help family members raise
offspring.
– Natural selection alters frequency distribution of heritable traits
(EVOLUTION):
• Directional selection: one extreme phenotype is favored; ex: horses have
grown larger over time (fossil evidence)
• Disruptive/diversifying selection: both extreme phenotypes are favored; ex:
peppered moths
• Stabilizing selection: intermediate phenotype is favored; ex: birth weight
• There is no goal to natural selection.
Microevolution vs. Macroevolution
• Microevolution: change in allele frequencies of
a population over generations (wolf  different
dog breeds)
• Macroevolution: broad patterns of change in
groups of related species that have occurred
over long time spans
– Includes origins of new groups of organisms and
changes in the gene pool of a population.
– These patterns determine phylogeny – evolutionary
relationships among species/groups of species
– Species: a group of populations whose members have the potential to
interbreed in nature and produce viable, fertile offspring
• Alleles: variations of a single gene
• Diploid: two alleles for a gene
(homozygous/heterozygous)
• Gene pool: all of the alleles in the population
• Population genetics: study of how populations
change genetically over time
Things that Prevent Evolutionary
Perfection:
• Selection only acts on existing variations.
• Changing environments may cause new traits to become
advantageous and old traits to become harmful.
• Historical constraints limit adaptations. Our ancestors’
eyes were in the front of their head, so it’s unlikely we’ll
develop them on the backs of our heads
• Adaptations are often compromises, and traits may not
be helpful in all situations. Example – seal flippers
• Not all evolution is adaptive. Evolution can happen by
chance, and mass extinctions occur, often due to natural
disaster.
Contrasting Theories of Macroevolution
• Phyletic gradualism: argues evolution occurs by gradual
accumulation of small changes over long periods of geologic
time (hundreds to millions of years)
– Fossil evidence provides snapshots of evolution, revealing only major
changes
– Missing intermediate stages means an incomplete fossil record
• Punctuated equilibrium: geologically long periods of stasis with
little or no evolution – it is interrupted (“punctuated”) by short
periods of rapid evolution ranging over tens of thousands of
years
– Fossil history should have fossils mostly from extended periods of stasis
with few, if any, fossils from short bursts of evolution
– Missing intermediate stages is considered data that confirm rapid
evolutionary events
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Patterns of Evolution
• Divergent evolution: 2+ species originate from a
common ancestor and become increasingly different
– Ex: Darwin’s finches (speciation, adaptive radiation)
• Convergent evolution: 2 unrelated species share
similar traits as a result of independent evolution to
similar environments (analogous traits)
– Ex: sharks, porpoises, and penguin fins; vertebrate and
squid eyes
• Parallel evolution: 2+ related species or 2 related
lineages that have made similar evolutionary changes
after their divergence from a common ancestor
• Coevolution: one species evolves in response to
new adaptations that appear in another species
– Ex: prey species gains an adaptation allowing it to
escape its predator – though most predators will fail,
some variants will be successful. Selection favors
these variants and subsequent evolution results in
new adaptations in the predator species
– Coevolution occurs between predator and prey,
plants and plant-eating insects, pollinators and
flowering plants, and pathogens and animal immune
systems
– Ex: marsupial and placental mammals
5 Agents of Evolutionary Change
(Causes of Changes in Allele Frequencies)
Natural Selection
Mutation
Causes of Changes in Allele
Frequencies
• Natural selection: increase or decrease in allele
frequencies due to environmental impact
Gene Flow
Genetic Drift
Non-random mating
Variation
• Variation originates with or is maintained by
– Mutations: random changes to DNA caused by errors in mitosis and
meiosis, environmental damage, and constantly changing DNA
– Sexual reproduction: mixing, or recombination, of alleles in offspring
leading to new phenotypes
– Diploidy: presence of two copies of each chromosome in a cell
– Heterozygotes have a hidden recessive allele – variation can be “stored” for
future generations
– Outbreeding: mating with unrelated partners; increases possibility of
creating new allele combinations
– Balanced polymorphism: maintains diversity of phenotypes in a
population
• Heterozygote advantage: greater fitness than homozygotes, as in sickle-cell
disease
• Hybrid vigor (heterosis): heterozygotes may have superior qualities –
beneficial traits are both expressed
• Frequency-dependent selection (minority advantage): least common
phenotypes have a selective advantage
– Advantageous genes survive and become more
common.
– Individuals with variations better suited to the
environment pass more alleles to the next
generation (predation, physiological, sexual
selection).
– Variation must be present for natural selection to
operate.
Neutral Variation
• Not all variation has selective value
• Neutral variation can be seen
especially at the molecular level in
DNA and proteins – many genes
don’t affect an organism’s ability to
survive or reproduce
• Environment typically determines
whether a variation is neutral or has
selective value
• Example: differences in fingerprint
patterns among humans, hair and
eye color, freckles, polydactyly,
dwarfism, webbed toes
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Causes of Changes in Allele
Frequencies
• Mutations: random errors and changes in DNA during
replication that introduces new alleles, and thus genetic
variation, that may provide a selective advantage
– Mutations may invent alleles that never before existed in a gene –
these are original new traits
– In most cases, mutations are deleterious (harmful), but may rarely
be beneficial
– Example: antibiotic and pesticide resistance alleles
• These can arise via mutation or they may already exist as part of genetic
variation.
• Application of antibiotics or pesticides eliminates susceptible individuals,
allowing nonsusceptible individuals to reproduce rapidly without competition.
– Most organisms are well-adapted to their environments, but change
can quickly make a mutation beneficial
Causes of Changes in Allele
Frequencies
• Genetic drift: random increase or decrease of alleles (NOT
attributed to natural selection); much stronger in smaller
populations
– Founder effect: allele frequencies in a group of migrating individuals
are, by chance, the same as that of their population of origin
• A small group from a larger population are isolated and start a new colony; small
population may not be representative of the original population, and certain alleles
may be under or over represented – this skews the gene pool
– Bottleneck effect: population undergoes a dramatic decrease in size,
possibly because of a destructive geological or meteorological event
• Genes of the remaining small population become common because they happened
to survive; this narrows the gene pool
• Important concept in conservation biology of endangered species – loss of alleles
from gene pool, reduces variation, reduces adaptability
Causes of Changes in Allele
Frequencies
• Gene flow: movement of fertile
individuals between populations
resulting in removal of alleles from a
population when they leave
(emigration) or the introduction of
alleles when they enter (immigration)
– May reduce genetic differences between
populations and make them more similar
– Gene flow in human populations is
increasing today
Causes of Changes in Allele
Frequencies
• Nonrandom mating: individuals choose mates based on their
particular traits, either similar or different from their own, or
just those that happen to be close by
– Inbreeding: individuals mate with relatives
– Sexual selection: females choose males based on appearance
(intersexual), behavior (intersexual), or ability to defeat other
males in contests (intrasexual)
• Traits that help an individual attract mates may evolve, even if they decrease
survival
• Creates individuals with new allele combinations in the following ways:
– Crossing over – chromosomes trait equal segments of genes
– Independent assortment – random combinations of maternal and paternal chromosomes
– Random joining of gametes during fertilization (SEX INCREASES DIVERSITY)
• Often leads to sexual dimorphism – difference between 2 sexes (size, color,
ornamentation, behavior)
Human Impact
• Humans impact evolution of many species by reducing
population size and decreasing genetic variation
• When variation decreases, populations lack variation
necessary to respond to selection pressures imposed by
changing environments
– Monocultures: agriculture; reduces genetic variation because only a
few varieties (maybe only one) of many wild varieties of a plant are
used
• Monocultures, by definition, have no
variation, and are extremely susceptible
to changing environmental conditions
– Overuse of antibiotics
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