measuring evolution of populations

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??