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Reference: Campbell 7th Ed. Chapters 23 & 24

Evolution by natural selection gains wide acceptance

Early 1900’s birth of genetics field Questions resurface about evolution and

natural selection “Population Genetics”: study of evolution

from genetic point of view Involves gradual changes in genetic

material over generations, in groups of organisms

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A Population is the smallest unit in which evolution occurs (“microevolution”)

Individuals may vary in observable traits Studying variation in a single trait – use a large

sample Quantitative traits in a population (height, weight)

show variation in a bell-shaped “normal” curve Ex. Body length in a population of fish

X axis: fish length (cm) Y axis: # of fish

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What causes variation in traits? Environmental factors & Hereditary can

account for different phenotypes within a single family

Genotypes (alleles) come from same parents but in different combinations can account for variations in successive offspring due to formation of gametes & how they fuse (Segregation of Alleles)

Ex: Rr x Rr = ?

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Mutation: flawed copies of individual genes Recombination: reassociation of genes in

diploid individual (occurs during meiosis) Segregation of alleles Independent assortment (nonhomologous) Crossing over (homologous)

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Random fusion of gametes: chance game played by gametes Millions of sperm in mating “Chosen One” fertilizes egg Ensures variation in offspring No exact copies of parents, or other

offspring likely

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Try this game: The Great Sperm Race

“Gene pool”: total genetic information available in a population

“Allele frequency”: percentage of allele in gene pool (expressed as a decimal)

Ex: If there are ten individuals in a population and at a given locus there are two possible alleles, A and a, then if the genotypes of the individuals are:

Population 1: AA, Aa, AA, aa, Aa, AA, AA, Aa, Aa, and AA Then the allele frequencies of allele A and allele a are:

pA = (2+1+2+0+1+2+2+1+1+2)/20 = 0.7

pa = (0+1+0+2+1+0+0+1+1+0)/20 = 0.3 *remember, gametes are haploid, and carry only one form of allele

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Phenotypes are controlled by which alleles are inherited (genotypes)

Phenotype frequency: ratio stating number of times a specific phenotype occurs in a population in a single generation

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example, F2: red 1/6 = 0.17pink = 3/6 = 0.50white = 2/6 = 0.33

British mathematician Godfrey Hardy German physician Wilhelm Weinberg

Independently showed that allele frequencies in a population “tend to remain the same from generation to generation unless acted on by outside influences” when populations are in “genetic equilibrium”.

Hardy-Weinberg Equilibrium Based on set of assumptions about ideal

hypothetical population that is not evolving9

1) No mutations occur Allele frequencies do not change overall

2) Individuals don’t migrate 3) Population is large 4) Individuals mate randomly 5) Natural selection does not occur

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Equation:     p2+2pq+q2=1.0

p2 = homozygous dominant condition;  AAq2 = homozygous recessive;  aa2pq = heterozygous ; Aa

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Theoretical state in which allele frequencies remain the same over generations (P = F1 = F2 = F3, etc)

Showed what forces disrupt genetic equilibrium and led to evolutionary change

Real populations usually violate HW conditions, causing gene frequencies to fluctuate

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Evolution results from the change of population’s allele frequencies (genetics) over generations

Any violation of 5 conditions of Hardy-Weinberg Equilibrium results in evolution

Mutagens can cause increase/decrease in allele frequency

Spontaneous mutations occur constantly Mutations can produce new alleles for trait

Affect genetic equilibrium

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Spontaneously introduces new allele variants into a population

Natural selection is often slow to eliminate harmful recessive mutations Natural selection operates only when genes are

expressed (phenotypes); often not when “carried”

Beneficial mutations are vital to evolution in long run

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Gene flow: process of genes moving from one population to another

ex: Baboons

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Gene frequency changes

Immigration: movement of individuals into a populationEmigration: movement of individuals out of a population

Genetic Drift: allele frequencies in a population change as result of random events or chance.

Example: Small population can be affected by single

organism’s ability to reproduce low or high Small populations are much more

susceptible. Why? Abrupt changes in alleles shows high genetic

drift Large population

Retain stable allele frequencies; low genetic drift

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Small population loses genetic variability and becomes vulnerable to extinction

“Bottlenecking” a population

Northern Elephant Seals

Cheetahs = very little genetic variability left

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Most species do not mate randomly Geographic proximity is a factor Matings of related individuals can

amplify traits & result offspring with disorders Similar recessive genes (carried, masked)

often present in genomes of related individuals

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Physical Characteristics (similar genes) Assortative Mating: selection of mate based

on similarity of characteristics Nonrandom mating can affect genotypes

(combination of alleles) of population May not affect on overall allele frequencies

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Blue and white snow geese

Ongoing process in populations

Single most significant factor that disrupts genetic equilibrium

Individuals reproduce more successfully as result of natural selection Contribution of genes to

next generation Stabilizing, Directional,

Disruptive and Sexual all cause evolution in a population (microevolution) 22

Stabilizing Selection: average form of trait causes organism to have an advantage in reproduction; high fitness

Lizard size Small lizard runs too slow Large lizard easily spotted and

captured Selection reduces size range Most common type of selection

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Directional Selection: individuals that display more extreme form of trait have higher fitness than individuals with average

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Disruptive Selection : individuals with either extreme variation of trait have higher fitness than average form of trait

Limpets Shell color Pure white to dark tan

White on rocks with goose barnacles Dark tan on bare rocks blend in Intermediate color at disadvantage

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Sexual selection: preferential choice of a mate based on specific phenotypic trait

Females tend to choose males they mate with due to certain traits male expresses

Genes of successful reproducers rather than of merely successful survivors are amplified through natural selection

27The Tale of the Peacock

Total # of species today is inaccurate due to numerous undiscovered species

Currently, scientists have named and successfully classified over 1.5 million species. It is estimated that there are as little as 2 million to as many as 50 million more species that have not yet been found and/or have been incorrectly classified.

Remote locations: Rainforests and Oceans New species discovered while others become

extinct at fast rate One species can become two through process of

speciationspeciation Speciation results in many related populations

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Morphology: study of internal

and external structure and

form of an organism

Using the MSC, species are defined by structure and appearance

Aka “Phenetic” species concept: a species is a set of organisms that are phenotypically similar and that look different from other sets of organisms.

Phenotypic differences may exist among individuals in one population.

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Adult & juvenile herring gulls

American Black duck (Anas rubripes)

Mallards (Anas platyrhynchos)

Organisms may appear different enough to belong to different species. How different do they have to be to be considered a unique species?

Biological Species Concept: A species is often defined as a group of individuals that actually or potentially interbreed in nature. In this sense, a species is the biggest gene pool possible under natural conditions.

Defines a species as those organisms that can produce viable offspring together. Same chromosome #

Issues: What about hybrids? What about plants, etc that reproduce asexually? What about extinct species?

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Geographic Isolation: physical separation of members of population

Allopatric Speciation Populations physically isolated by an extrinsic barrier

Gene flow between them stops Natural selection and genetic drift cause divergence

Individuals of two populations can no longer interbreed

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Reproductive Isolation: results from barriers to successful breeding between population groups in same area

Parapatric Speciation Two or more separate gene pools form, and

eventually these diverge into different species Two broad types

Prezygotic: before fertilization Postzygotic: after fertilization

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Types of postzygotic isolation Offspring of interbreeding species are

underdeveloped, die early, or are not fertile If death or infertility occurs parents have

wasted gametes from evolution standpoint Prezygotic

Incompatible behavior Reduce chance of hybrid formation Mating times, calls

Frogs, birds

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Gradualism –vs- Punctuated Gradualism –vs- Punctuated SpeciationSpeciation

Speciation usually takes millions of years, but some species form more rapidly

“Gradualism” - Fossil record indicates many species existed without change for long periods

Fossil evidence seems to indicate that “instant” changes can occurred within few thousand years (Hox genes)

Punctuated Equilibrium: theory that speciation occurs during brief periods of rapid genetic change, interspersed with long equilibrium periods In 1972 paleontologists Niles Eldredge

and Stephen Jay Gould published a landmark paper developing this idea.

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