Download Populations evolution

Evolution of Populations
Chapter 11
11-1: Genes and Variation
Population: group of individuals in the
same species that interbreed; share a
common gene pool
 Gene Pool: all of the alleles of all the
individuals in a population

11-1: Genes and Variation

Allele Frequency: proportion of one allele,
compared with all the alleles for that trait,
in the gene pool; given as a decimal or a
percentage
◦ Evolution occurs when there is a change in the
allele frequency of a population
11-1: Genes and Variation

Where does genetic variation come from?
◦ Mutations: change in DNA
 Occur during replication or due to chemicals,
radiation, etc.
 Don’t always change the phenotype
◦ Gene Shuffling/Recombination
 Mostly occurs during meiosis (independent
assortment, crossing-over)
11-2: Natural Selection in Populations

Natural Selection acts on organisms that
survive and reproduce, NOT the genes
11-2: Natural Selection in Populations

Single Gene Traits: one
gene controls the trait

Polygenic Traits: more
than one gene controls
the trait; this produces a
range of phenotypes
11-2: Natural Selection in Populations

Normal Distribution: graph of numerical
data that forms a bell-shaped curve and is
symmetrical; typical for a polygenic trait
◦ Natural selection leads to changes in allele
frequency, which can shift the curve
 Remember: changes in allele frequency = evolution!
11-2: Natural Selection in Populations

Stabilizing Selection: favors the average
phenotype; the curve narrows
11-2: Natural Selection in Populations

Directional Selection: one extreme form is
favored (one end OR the other)
11-2: Natural Selection in Populations

Disruptive Selection: both extremes are
favored and the intermediate forms are
eliminated
11-3: Other Mechanisms of Evolution

Gene Flow: movement of alleles from one
population to another
◦ Causes allele frequency to increase or decrease
by chance
◦ Occurs often in fungi or plants who have seeds,
spores, etc. that are easy for dispersal
11-3: Other Mechanisms of Evolution

Genetic Drift: the random change in allele
frequency in a population
◦ Causes a loss of genetic diversity
◦ More prevalent in smaller populations (easier
for change to have an affect)
11-3: Other Mechanisms of Evolution

Types of Genetic Drift:
◦ Bottleneck Effect – results from an event that
drastically reduces the size of a population
 Greatly reduces genetic variation
11-3: Other Mechanisms of Evolution

Types of Genetic Drift:
◦ Founder Effect – occurs after a small number
of individuals colonize a new area
11-3: Other Mechanisms of Evolution
11-3: Other Mechanisms of Evolution

Sexual Selection: traits that increase the
ability of individuals to attract or acquire
mates appear with increasing frequency in
a population
◦ Types: Competition between males for female
attention; Physical traits attract females
FORMATION OF A NEW SPECIES
SPECIATION
FOR NEW SPECIES TO FORM,
ISOLATION MUST OCCUR
CAUSING THEM TO FAIL TO
INTERBREED AND PRODUCE
FERTILE OFFSPRING.
TYPES OF REPRODUCTIVE ISOLATION
BEHAVIORAL ISOLATION - BEHAVIOR
INTERFERES
GEOGRAPHIC ISOLATIONBARRIER SEPARATES
TEMPORAL ISOLATION –
GROUPS REPRODUCE AT
DIFFERENT TIMES OF THE DAY OR
YEAR, SO THEY ARE NOT COMPATIBLE
11-4: Hardy-Weinberg Equilibrium
If no change takes place, the population
doesn’t evolve and genetic equilibrium has
been reached.
 Hardy-Weinberg Equilibrium: condition in
which a population’s allele frequencies for
a given trait do not change

11-4: Hardy-Weinberg Equilibrium

Requirements for Hardy-Weinberg
◦ Very large populations
 So genetic drift doesn’t have much effect
◦ No emigration or immigration
 Gene pool must be kept separate
◦ No mutations
 Would cause allele frequency to change
◦ Random mating
 Atypical because some traits are favored over others
◦ No natural selection
 Equal opportunity for survival and reproduction of
all genotypes
11-4: Hardy-Weinberg Equilibrium
Populations rarely meet all of the HardyWeinberg conditions, and the idea is more
often used to compare real-life models to
predicted data.
 Hardy-Weinberg Equation used to predict
genotype frequencies in populations

11-4: Hardy-Weinberg Equilibrium
p2 + 2pq + q2 = 1 and p + q = 1
p = frequency of the dominant allele
 q = frequency of the recessive allele
 p2 = % of homozygous dominant individuals
 q2 = % of homozygous recessive individuals
 2pq = % of heterozygous individuals
