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Ch. 17
Part 1
Genetic Variation
• Provides raw material for natural
selection to occur
• Variation in population  some
members have features that give
them advantage over other members
• Causes of Genetic Variation:
1.
Processes 1-4 involve RESHUFFLING
of alleles in a population…leads to
PHENOTYPIC VARIATION
(also caused by ENVIRONMENT…NOT
passed off to offspring)
Independent assortment
• Chromosomes and alleles
2.
Crossing over
• Between chromatids of homologous
chromosomes
3.
4.
5.
Random Mating
Random fertilization of gametes
Mutation
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Does NOT reshuffle alleles
Produces completely new alleles
Mistakes during DNA replication
Gene mutations unpredictable
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New alleles, usually recessive
Appearance in population is many
generations after initial mutation
Must occur in GAMETES to affect
offspring (not somatic cells)
• Mutation to somatic cells in
ovaries/testes CAN lead to
mutation to gametes
• Review gametogenesis
and germ layer cells
Continuous vs. Discontinuous Variation
Qualitative Phenotypic Differences
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Blood types
Clearly distinguishable categories
No intermediates
Discontinuous variation
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Different alleles at a single gene locus have large effects on
phenotype
Different genes have different effects on phenotypes
Examples:
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Inheritance of sickle cell anemia or hemophilia
Flower color
Stem color
Feather color
Dominance & Gene interaction  reduce phenotypic variation
Quantitative Phenotypic Differences
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Height
Mass
Differences are hard to distinguish
Rages between extremes are created
Continuous variation
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Different alleles at a single gene locus have small
effects on phenotypes
Different genes have same (sometimes additive) effect
on phenotype
Polygenes (polygenetic traits)
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Large number of genes have combined effect on
phenotypes
Discontinuous Variation
Environmental Effects on Phenotype
• Environment affects full genetic potential
of genes
• Examples
– Two plants can have same genetic make
up but one gets more light intensity than
another, it will grow to its full potential
– Allele that controls dark pigment
formation ONLY at low temperatures
• Selective breeding when humans
purposefully apply selection pressures to
populations
– Important to know how much of
phenotypic variation is genetic and how
much is environmental
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Natural Selection
All organisms trying to survive and reproduce
Not all organisms within a population survive to adulthood
to reproduce
– Would lead to exponential population growth (review logistic and
exponential growth)
– Influenced by environmental factors
– Abiotic vs. Biotic Factors
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Abiotic factors caused by non-living components
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Biotic factors  caused by other living organisms
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Water availability
Nutrients in soil
Climate
Pathogens
Predation
Competition
Carrying Capacity
– The largest number of individuals that a given environment can
support
– The part of the logistic graph after the exponential growth…the
flattening out
– The point at which this flat line reaches the y-axis is the size of the
population when the growth rate reaches zero
– This doesn’t mean the population stops growing
– Many factors slow the growth of plants and animals…
Fitness
• Capacity of an organism to survive and
transmit a genotype to its offspring
• Refers to “extent to which an organism has
adapted to their environment”
• The ability of an organism to survive, and
make copies of its alleles that are represented
in the next generation
– Organisms that produce more surviving offspring
are more fit, those that produce fewer are less fit
Carrying Capacity
• The largest number of individuals that a
given environment can support
• The part of the logistic graph after the
exponential growth…the flattening out
• The point at which this flat line reaches the yaxis is the size of the population when the
growth rate reaches zero
• This doesn’t mean the population stops growing
• Many factors slow the growth of plants and
animals…
Limiting Factor
• Condition that can restrict a population’s
growth..create the CARRYING CAPCITY
• Could be:
– Space
– Disease
– Availability of food
– Abiotic and Biotic
Limiting Factors
• Density-dependent Factors
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Factor that limits population as
population density increases
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Competition
Predation
Disease
Parasitism
Crowding and Stress
• Density-Independent Factors
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Factors that affect population but are
unrelated to population density; affect
population regardless of size
Insects vulnerable to this
– Weather /Natural disasters
– Human activities
– Fires
– deforestation
How Environment Influences
Population
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Pressure of environmental factors is sufficiently great  population size will
decrease
Selection Pressure environmental factors which may reduce
reproductive success in a population and thus contribute to evolutionary
change or extinction through the process of natural selection
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competition
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predation
disease
parasitism
land clearance
climate change
pollutants
Selection Pressure INCREASES the
chances of some alleles being passed
onto the next generation and
DECREASES the chances of others
Natural Selection effects of selection pressures on
the frequency of alleles in a population
• Raises frequency of alleles that lead to an
advantage
• Reduces frequency of alleles leading to a
disadvantage