Transcript BIOL 360 - General Ecology - Cal State LA

Evolutionary Change in
Populations: Population
Genetics, Selection & Drift
biosphere
region
landscape
ecosystem
community
interaction
population
individual
The characteristics of
populations of individuals
change through time
By the end of class today, you should
be able to:
• describe the basic elements required for evolution
to occur
• use the Hardy-Weinberg equation to predict allele
and genotype frequencies
• explain some of the major factors that can change
allele frequencies in populations
• differentiate between directional, stabilizing and
disruptive selection
• distinguish between sexual selection and natural
selection
“Biological evolution ... is change in the
properties of populations of organisms that
transcend the lifetime of a single individual.”
- Douglas Futuyma, in Evolutionary Biology
Evolution, Selection
• Heritability of traits from parents to offspring
• Naturally occurring variation in traits within
species
• Limited resources (number of offspring exceeds
carrying capacity of environment)
• Differential survival / higher reproductive
success of organisms with a particular trait
• Darwin’s theory of evolution via natural
selection faced one major scientific challenge:
• Mechanism and physical basis of inheritance was
unknown
• Blending theory prevailed at end of 19th century
Mendelian Genetics
Mendel’s experiments
Parental generation (P)
First filial generation (F1)
Second filial generation (F2)
Parental generation (P)
First filial generation (F1)
Second filial generation (F2)
Mendel’s Laws
• Law of segregation – two hereditary factors (two genes)
from parent get separated so that each egg/sperm only
gets one copy
Mendel’s Laws
• Law of independent assortment – heredity factors
(genes) for each trait are transmitted independently to
the gametes
Darwin + Mendel =
Evolutionary Synthesis
• Mendel published his work in 1866 – largely ignored
until its rediscovery in 1900
• Application of Mendelian genetics to Darwin’s theory of
evolution by natural selection laid the foundation of
modern population genetics
• Evolution = change in allelic frequencies in a population
Hardy-Weinberg Equilibrium
• Allele frequencies in a
population will remain
constant from generation
to generation unless
disturbed by specific
factors
Hardy-Weinberg Equilibrium
• At equilibrium,
frequencies of
genotypes will follow a
predictable ratio:
• p+q=1
for diploid genotype:
(p + q)2 = 1
p2 + 2pq + q2 = 1
p = frequency of allele 1
q = frequency of allele 2
Hardy-Weinberg Equilibrium is
disturbed by any of the following:
•
•
•
•
•
non-random mating
mutation
small effective population size
immigration
selection (unequal fitness of different genotypes)
Hardy-Weinberg Equilibrium
The expected ratio of genotype frequencies at
equilibrium, p2 + 2pq + q2 = 1, makes it possible to
estimate genotype frequencies in natural populations
- This is significant, as usually only the frequency of
the homozygous recessive phenotype is easily
observed and documented
Hardy-Weinberg Equilibrium – sample problem
• Albinism is a rare genetically determined
trait that is only expressed in the
phenotype of homozygous recessive
individuals (aa)
• Frequency of albinism (aa genotype) in
North American population is about 1 /
20,000 ( = 0.00005)
• What are the frequencies of the
homozygous dominant (AA) and
heterozygous (Aa) genotypes in
the population?
p2 + 2pq + q2 = 1
How does selection change
genotype / phenotype frequencies
in natural populations?
• Three main patterns are observed:
– Stabilizing selection
– Directional selection
– Disruptive selection
Stabilizing Selection
• Selection acts against extreme phenotypes, organisms
with average phenotype have the greatest fitness
The graph is based on infants born in
London from 1935 to 1946. From Karn and
Penrose (1951).
Directional Selection
• One extreme phenotype has fitness advantage over
more average phenotypes
Fishing selects for reduced size in salmon
populations
Disruptive Selection
• Two or more extreme phenotypes have fitness
advantages over average phenotype
e.g., common goldfields of CA –
serpentine specialized races
Phenotype is determined by
multiple factors
• genetic makeup of organism (genotype)
• environmentally influenced “plastic” development of
organism (environment)
• random developmental quirks (noise)
The effectiveness of selection on a
particular phenotype depends on
the extent to which the phenotype
is controlled by the genotype
• selection can only operate on the heritable component
of the phenotype
Not all evolution occurs through
natural selection
• Genetic Drift
• Sexual Selection
Genetic Drift
• Sampling error leads to the fixation / elimination of
certain alleles
• occurs when population size is small due to factors such
as:
– founder effects (small peripheral population becomes
isolated)
– genetic bottleneck (population size crashes to very low level,
as a consequence genetic diversity is lost)
Sexual Selection
• also proposed by Darwin, but largely criticized / ignored
by Victorian scientists
• a mechanism in which female preference for mates with
particular showy traits leads to higher fitness of males
with those traits
• sexually selected traits are often NOT advantageous to
the males in terms of natural selection