measuring evolution of populations
DESCRIPTION
Measuring Evolution of Populations. Gene Variation is Raw Material. Evolution - change over time Evolution is descent with modification Darwin Through time, species accumulate differences such that ancestral and descendent species are not identical. Gene Variation is Raw Material. - PowerPoint PPT PresentationTRANSCRIPT
MeasuringEvolution of Populations
Gene Variation is Raw MaterialEvolution - change over timeEvolution is descent with modification
DarwinThrough time, species accumulate differences such
that ancestral and descendent species are not identical.
Gene Variation is Raw MaterialNatural selection and evolutionary change
Some individuals in a population possess certain inherited characteristics that play a role in producing more surviving offspring than individuals without those characteristics.The population gradually includes more individuals
with advantageous characteristics.
Darwin versus Lamarck
Macroevolution
It is large evolutionary change
Evolution of new species from a common ancestor
Evolution of one species into two or more species
Microevolution Evolution on a small scale
Is the change in gene frequencies within a population over time
As the changes in populations accumulate, they can lead to the formation
of new species.
Populations & gene poolsConcepts
a population is a localized group of interbreeding individuals
gene pool is collection of alleles in the populationremember difference between alleles & genes!
allele frequency is how common is that allele in the population how many A vs. a in whole population
Hardy-Weinberg PrincipleHardy-Weinberg - original proportions of
genotypes in a population will remain constant from generation to generationSexual reproduction (meiosis and
fertilization) alone will not change allelic (genotypic) proportions.
Hardy-Weinberg PrincipleNecessary assumptions
Allelic frequencies would remain constant if…population size is very largerandom matingno mutationno gene input from external sourcesno selection occurring
Hardy-Weinberg theoremCounting Alleles
assume 2 alleles = B, bfrequency of dominant allele (B) = p frequency of recessive allele (b) = q
frequencies must add to 1 (100%), so:
p + q = 1
bbBbBB
Hardy-Weinberg theoremCounting Individuals
frequency of homozygous dominant: p x p = p2 frequency of homozygous recessive: q x q = q2 frequency of heterozygotes: (p x q) + (q x p) =
2pqfrequencies of all individuals must add to 1 (100%), so:
p2 + 2pq + q2 = 1
bbBbBB
H-W formulasAlleles: p + q = 1
Individuals: p2 + 2pq + q2 = 1
bbBbBB
BB
B b
Bb bb
What are the genotype frequencies?What are the genotype frequencies?
Using Hardy-Weinberg equation
q2 (bb): 16/100 = .16
q (b): √.16 = 0.40.4
p (B): 1 - 0.4 = 0.60.6
q2 (bb): 16/100 = .16
q (b): √.16 = 0.40.4
p (B): 1 - 0.4 = 0.60.6
population: 100 cats84 black, 16 whiteHow many of each genotype?
population: 100 cats84 black, 16 whiteHow many of each genotype?
bbBbBB
p2=.36p2=.36 2pq=.482pq=.48 q2=.16q2=.16
Must assume population is in H-W equilibrium!Must assume population is in H-W equilibrium!
Using Hardy-Weinberg equation
bbBbBB
p2=.36p2=.36 2pq=.482pq=.48 q2=.16q2=.16
Assuming H-W equilibriumAssuming H-W equilibrium
Sampled data Sampled data bbBbBB
p2=.74p2=.74 2pq=.102pq=.10 q2=.16q2=.16
How do you explain the data? How do you explain the data?
p2=.20p2=.20 2pq=.642pq=.64 q2=.16q2=.16
How do you explain the data? How do you explain the data?
Null hypothesis Null hypothesis
Application of H-W principleSickle cell anemia
inherit a mutation in gene coding for hemoglobinoxygen-carrying blood proteinrecessive allele = HsHs
normal allele = Hb
low oxygen levels causes RBC to sicklebreakdown of RBCclogging small blood vesselsdamage to organs
often lethal
Sickle cell frequencyHigh frequency of heterozygotes
1 in 5 in Central Africans = HbHs
unusual for allele with severe detrimental effects in homozygotes1 in 100 = HsHs
usually die before reproductive age
Why is the Hs allele maintained at such high levels in African populations?Why is the Hs allele maintained at such high levels in African populations?
Suggests some selective advantage of being heterozygous…Suggests some selective advantage of being heterozygous…
Malaria Single-celled eukaryote parasite (Plasmodium) spends part of its life cycle in red blood cells
Single-celled eukaryote parasite (Plasmodium) spends part of its life cycle in red blood cells
1
2
3
Heterozygote AdvantageIn tropical Africa, where malaria is common:
homozygous dominant (normal) die of malaria: HbHb
homozygous recessive die of sickle cell anemia: HsHs
heterozygote carriers are relatively free of both: HbHs
survive more, more common in population
Hypothesis:In malaria-infected cells, the O2 level is lowered enough to cause sickling which kills the cell & destroys the parasite.
Hypothesis:In malaria-infected cells, the O2 level is lowered enough to cause sickling which kills the cell & destroys the parasite.
Frequency of sickle cell allele & distribution of malaria
Hardy-Weinberg equilibriumHypothetical, non-evolving population
preserves allele frequenciesServes as a model (null hypothesis)
natural populations rarely in H-W equilibriumuseful model to measure if forces are acting
on a populationmeasuring evolutionary change
W. Weinbergphysician
G.H. Hardymathematician
Evolution of populations Evolution = change in allele frequencies in a
population hypothetical: what conditions would cause
allele frequencies to not change? non-evolving population
REMOVE all agents of evolutionary change1. very large population size (no genetic drift)2. no migration (no gene flow in or out)3. no mutation (no genetic change)4. random mating (no sexual selection)5. no natural selection (everyone is equally fit)
http://nhscience.lonestar.edu/biol/hwe.html
5 Agents of evolutionary changeMutation Gene Flow
Genetic Drift Selection
Non-random mating
2005-2006
5 Agents of evolutionary change
Five Agents of Evolutionary ChangeMutation
Mutation rates are generally so low they have little effect on Hardy-Weinberg proportions of common alleles.ultimate source of genetic variation
Gene flowmovement of alleles from one population to
anothertend to homogenize allele frequencies
Five Agents of Evolutionary ChangeNonrandom mating
assortative mating - phenotypically similar individuals mateCauses frequencies of particular genotypes to differ
from those predicted by Hardy-Weinberg.
Five Agents of Evolutionary Change
Genetic drift – statistical accidents.Frequencies of particular alleles may change by
chance alone.important in small populations
founder effect - few individuals found new population (small allelic pool)
bottleneck effect - drastic reduction in population, and gene pool size
Genetic Drift - Bottleneck Effect
Five Agents of Evolutionary Change
Selection – Only agent that produces adaptiveevolutionary changeartificial - breeders exert selection natural - nature exerts selection
variation must exist among individualsvariation must result in differences in numbers of viable
offspring producedvariation must be genetically inherited
natural selection is a process, and evolution is an outcome
Five Agents of Evolutionary Change
Selection pressures:avoiding predatorsmatching climatic conditionpesticide resistance
Measuring FitnessFitness is defined by evolutionary biologists
as the number of surviving offspring left in the next generation.relative measure
Selection favors phenotypes with the greatest fitness.
Interactions Among Evolutionary ForcesLevels of variation retained in a population
may be determined by the relative strength of different evolutionary processes.
Gene flow versus natural selectionGene flow can be either a constructive or a
constraining force.Allelic frequencies reflect a balance between gene
flow and natural selection.
Natural Selection Can Maintain Variation
Frequency-dependent selectionPhenotype fitness depends on its frequency within
the population.Negative frequency-dependent selection favors rare
phenotypes.Positive frequency-dependent selection eliminates variation.
Oscillating selectionSelection favors different phenotypes at different
times.
Forms of SelectionDisruptive selection
Selection eliminates intermediate types.Directional selection
Selection eliminates one extreme from a phenotypic array.
Stabilizing selectionSelection acts to eliminate both
extremes from an array of phenotypes.
Sexual Selection
Kinds of Selection
Sexual selection Favours the selection of any trait
that confers an advantage in terms of the mating success of the individual
This is associated with sexual dimorphism: which is the physical (often extreme) differences in the appearance of males and females
The most common forms of sexual selection are the results of female mate choice and male to male competition
Sexual selectionFemales can chose based on physical
traits, colouration, or behavioural traits such as courtship displays and songs
Sometimes males develop features that enable them to establish and defend a territory from other males=sometimes detaining the females
How would you be able to tell these are not env’tal selective pressures?
Both sexes would possess the features.
Sexual selectionSome features are a compromise
between mating and remaining conspicuous to predators==bright colours and song.
What about plants?Sexual diversity is not limited to just
animalsPlants do not select mates but they do
need to attract suitors to assist in pollination
Flowers and scents are the most obvious examples of sexual features that have evolved==maximize pollination
Selection on Color in GuppiesGuppies are found in small northeastern
streams in South America and in nearby mountainous streams in Trinidad.Due to dispersal barriers, guppies can
be found in pools below waterfalls with high predation risk, or pools above waterfalls with low predation risk.
Evolution of Coloration in Guppies
Selection on Color in GuppiesHigh predation environment - Males
exhibit drab coloration and tend to be relatively small and reproduce at a younger age.
Low predation environment - Males display bright coloration, a larger number of spots, and tend to be more successful at defending territories.In the absence of predators, larger,
more colorful fish may produce more offspring.
Evolutionary Change in Spot Number
Limits to SelectionGenes have multiple effects
Pleiotropy - ex. PKUEvolution requires genetic variation
Intense selection may remove variation from a population at a rate greater than mutation can replenish.thoroughbred horses
Gene interactions affect allelic fitnessepistatic interactions –ex. You may have a
widows peak but if you have the baldness gene you will not see your widows peak.
2005-2006
Any Questions??Any Questions??Any Questions??