1. Ch. 17
Part 1

2. Genetic Variation PHENOTYPIC 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:
Independent assortment
Chromosomes and alleles
Crossing over
Between chromatids of homologous chromosomes
Random Mating
Random fertilization of gametes
Mutation
Does NOT reshuffle alleles
Produces completely new alleles
Mistakes during DNA replication
Gene mutations unpredictable
New alleles, usually recessive
Appearance in population is many generations after initial mutation
Must occur in GAMETES to affect offspring (not somatic cells)
Processes 1-4 involve RESHUFFLING of alleles in a population…leads to
PHENOTYPIC VARIATION
(also caused by ENVIRONMENT…NOT passed off to offspring)
Mutation to somatic cells in ovaries/testes CAN lead to mutation to gametes
Review gametogenesis and germ layer cells

6. Continuous vs. Discontinuous Variation
Qualitative Phenotypic Differences
Continuous vs. Discontinuous Variation
Quantitative Phenotypic Differences
Blood types
Clearly distinguishable categories
No intermediates
Discontinuous variation
Different alleles at a single gene locus have large effects on phenotype
Different genes have different effects on phenotypes
Examples:
Inheritance of sickle cell anemia or hemophilia
Flower color
Stem color
Feather color
Dominance & Gene interaction  reduce phenotypic variation
Height
Mass
Differences are hard to distinguish
Rages between extremes are created
Continuous variation
Different alleles at a single gene locus have small effects on phenotypes
Different genes have same (sometimes additive) effect on phenotype
Polygenes (polygenetic traits)
Large number of genes have combined effect on phenotypes

8. Discontinuous Variation

10. 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

12. 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
Abiotic factors caused by non-living components
Water availability
Nutrients in soil
Climate
Biotic factors  caused by other living organisms
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…

13. 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

14. 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…

16. Limiting Factor
Condition that can restrict a population’s growth..create the CARRYING CAPCITY
Could be:
Space
Disease
Availability of food
Abiotic and Biotic

17. Limiting Factors Density-dependent Factors Density-Independent Factors
Factor that limits population as population density increases
Competition
Predation
Disease
Parasitism
Crowding and Stress
Density-Independent Factors
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

19. How Environment Influences Population
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
competition
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