The Process of Evolution

Population Genetics and

Speciation

Edited by L. Bridge

October 2015

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.1

The Process of Evolution“Population Genetics”

• Evolution occurs at the population level

– Genetic changes occur within a population

– Microevolution: A change in gene frequencies

within a population over time.

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Example: The Peppered Moth, a case of industrial melanism

The Process of Evolution

• Population Genetics– Population- all members of a species occupying a

particular area at the same time– Gene pool- the sum total of all alleles of all genes

in a population– Hardy and Weinberg used the binomial equation

p2+2pq+q2 to calculate the genotype and allele frequencies in a population

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Calculating Gene Pool Frequencies Using the Hardy-Weinberg Equation

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The Process of Evolution

• The data from the previous graphic shows that the next generation will have exactly the same ratio of genotypes as before:

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The Process of EvolutionThe Hardy-Weinberg Principle

– Allele frequencies in a gene pool will remain at equilibrium, thus constant, in each generation of a large sexually reproducing population as long as the following five conditions are met

• No mutations• No genetic drift (meaning: need a LARGE population)• No gene flow• Random mating• No selection

– In real life these conditions are rarely met

– Hardy-Weinberg law gives us a baseline by which to access whether or not evolution has occurred

• Any change in allele frequencies indicates evolution

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Microevolution

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1963, Bernard Kettlewell, “Industrial Melanism”

Microevolution

A change in gene pool frequencies within a population over time.

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Tutorial (uses Flash player) http://www.techapps.net/interactives/pepperMoths.swf

Video of Kettlewell’s study (YouTube) http://www.youtube.com/watch?v=LyRA807djLc

Microevolution (other examples)

• MRSA

• Rats/ Warfarin– Warfarin has its effect by inhibiting an enzyme involved in

the metabolism of Vitamin K (Vitamin K oxide reductase)

• mosquitoes/DDT

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Pesticide resistance, herbicide resistance, and antibiotic resistance are all examples of microevolution by natural selection. The enterococci bacteria, shown here, have evolved a resistance to several kinds of antibiotics.

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Agents of Evolutionary Change(1) Mutations: The raw material for

evolutionary change• Only source of new alleles in a population• Brings about variation that leads to adaptation

– Ex: Daphnia thrives at 20C but there is a mutation that requires Daphnia to live between 25 – 30 C.

20 Celsius 27 Celsius

Do you think mutations will play an important role in global warming?

Mutations

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Quaker Parrot Color Mutation

Agents of Evolutionary Change(2) Genetic Drift

• Change in allele frequencies due to chance• Can cause certain alleles to be lost from a population

13http://highered.mcgraw-hill.com/sites/dl/free/0072835125/126997/animation45.html

Bottleneck Effect:

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Founder Effect

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Founder Effect: a few individuals found a colony and their collective genes represent only a fraction of the original gene pool

However, over time these previously rare alleles become overrepresented

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Agents of evolution cont’d.(3) Gene flow: Sharing of genes between two populations

• Keeps the gene pools of 2 or more populations similar• Prevents close adaptation to a local environment

– Ex: Pollen & Plant populations

(4) Nonrandom mating: Occurs when individuals pair up according to phenotype or genotype– Nonrandom mating can indirectly serve as an agent of

evolution by shifting the frequencies of genotypes in a gene pool• Inbreeding is an example of nonrandom – increases frequency of

recessive abnormalities (“inbreeding depression”)

– Sexual selection– Intersexual –vs- Intrasexual selection

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Process of evolution• Agents of evolutionary change cont’d.

(5) Natural selection• Process by which populations adapt to their environment• Charles Darwin explained evolution through natural selection• Evolution by natural selection requires the following

– Variation-members of a population differ (Mutations increase gene variety)– Inheritance-differences are inheritable– Differential adaptedness-some differences have a survival benefit– Differential reproduction-better adapted individuals survive to reproduce more

offspring

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Process of evolution cont’d.• Maintenance of variation

– Sickle cell disease is good example of how variation is sometimes maintained, despite the detrimental effect of the allele

• People homozygous for sickle cell trait die from sickle-cell disease

• People homozygous for normal RBC’s in malaria endemic areas die from malaria

• People who are heterozygous are protected from both severe sickle cell disease and from malaria

– Since these people have one normal allele and one sickle allele, both are maintained in the gene pool

– The favored heterozygote keeps the two homozygotes equally present in the population. “Heterozygote Advantage”

Why doesn’t natural selection completely eliminate the defective hemoglobin allele?

Maintenance of detrimental alleles in populations

• Phenylketonuria (PKU) is a relatively rare inherited genetic disorder. Both parents must pass on the defective gene in order for a baby to have the condition. This is called an autosomal recessive trait.

• Babies with PKU are missing an enzyme called phenylalanine hydroxylase, which is needed to break down an essential amino acid called phenylalanine. The substance is found in foods that contain protein.

• Without the enzyme, levels of phenylalanine and two closely-related substances build up in the body. These substances are harmful to the central nervous system and cause brain damage

• QUESTION: Why is such a harmful gene maintained in the population? Why hasn’t natural selection eliminated it?

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The Process of Evolution• Natural Selection

– “Fitness”: in Darwinian terms, is measured by the number of fertile offspring produced by an individual

• Variations that can contribute to fitness can arise from– Mutation– Crossing over– Independent assortment

– Most traits on which natural selection acts are controlled by polygenic inheritance

– Range of phenotypes which follows a bell-shaped curve

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The Process of Evolution

• Natural Selection in populations

– Three Main Types of Natural Selection

• Stabilizing Selection

• Directional Selection

• Disruptive Selection

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Normal Distribution CurveAKA The Bell Curve

• Pick a trait that varies among individuals in a population (phenotypic)

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The Process of Evolution

• Stabilizing Selection

– Occurs when an intermediate, or average,

phenotype is favored

– Improves adaptation of population to a stable

environment

– Extreme phenotypes are selected against

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Stabilizing Selection

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27.3 The Process of Evolution

• Directional Selection

– One extreme phenotype is favored

– Distribution curve shifts in that direction

– Can occur when population is adjusting to a

changing environment

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Process of evolution cont’d.• Directional Selection

• Ex: evolution of the horse

Equus, the modern day horse, which is adapted to grassland habitat,Is much larger than its ancestor Hyracotherium, which was adapted to a forest habitat.

Grasslands

Forests

Combat, Speed, Grinding teeth

Hiding, low-crowned teeth

27.3 The Process of Evolution

• Disruptive Selection

– Two or more extreme phenotypes are selected

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Process of evolution cont’d.• Disruptive Selection

– Ex: 2 British land snails each

adapted to a particular habitat

Grass fields birds feed on snails with dark shells that lack light bands. Forest areas birds feed mainly on snails with light-banded shells. More difficult to see by birds in the grass fields and therefore survive and reproduce!

More difficult to see by birds in the forest areas and therefore survive and reproduce!

How Sexual Reproduction promotes genetic variation in offspring:

• Crossing over and recombination during meiosis– Segregation of alleles

– Crossing over during synapsis of meiosishttp://www.execulink.com/~ekimmel/crossing_over.htm

– Independent assortment of chromosomeshttp://www.execulink.com/~ekimmel/independent_assortment.htm

– Result: no two gametes from an individual are likely to carry the exact same combination of alleles for each gene

• Random fusion of Gametes from two parents– Which sperm cell will fuse with which egg cell?

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Questions for review

• How does sexual reproduction maintain variation in populations?

• Diploidy?• What is a polymorphic trait? Example?• What is a neutral variation? Example?• Why are some harmful alleles not eliminated by

natural selection?• What is “heterozygote advantage”? Example?• Why doesn’t Natural Selection produce perfect

organisms?

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SPECIATION

• The evolutionary process by which new biological species arise

• The splitting of lineages

• Campbell text Chapter 24

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27.4 Speciation• How do you define a “species”?

Tutorials: http://evolution.berkeley.edu/evosite/evo101/VADefiningSpecies.shtml

http://www.askabiologist.org.uk/from_the_lab/species-concepts

• Biological Species Concept (BSC): a species is a a group of subpopulations that have the same chromosome # and are therefore capable of interbreeding and are isolated reproductively from other species.

• Morphological Species Concept (MSC): species recognition is based on overall similarity. Individuals do not have to be exactly the same as each other, because there is variation in morphology among most species (think how variable people are).

• Ecological Species Concept (ESC): This is used to describe populations that are adapted to certain ecological niches and because of their adaptations will form discrete morphological clusters. 

• Phylogenetic (Cladistic) Species Concept (PSC): considers the evolutionary relationships among organisms and relies on common ancestry and shared evolutionary history to define species. Think of this as a distinct branch on an evolutionary tree.

• What are some of the problems with each of these?32

Divergent Evolution• Same/related species become more and more dissimilar over time

• This is most often driven by geographic isolation.  Continued cutoff of gene flow between populations can lead to eventual speciation.  Example pic

• Adaptive radiation describes divergence of a single lineage into different groups (species) driven by  natural selection of different traits, as a result of having adapted to widely different ecological niches.

• 1)  Example:  Darwin's finches

• 2)Example: According to both fossil and DNA evidence, the polar bear diverged from the brown bear, Ursus arctos, roughly 150,000 years ago.

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Convergent Evolution

• Unrelated species develop more and more similar features due to adaptation to similar environments. 

• The same "selective pressures" result in similar outcomes. 

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euphorb (from Sahara desert)           cactus (from Mojave desert)Ex:

Ex:

Coevolution (AKA “parallel evolution)

• The joint changes that occur in two or more species in close interaction. 

• As one changes, natural selection forces the other to adapt to it.  Organisms must adapt to "fit" each other.

• Examples: symbiotic organisms (parasites & hosts, flowers & pollinators)

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Each exerts a selective pressure on the other to “keep up” or perish

27.4 Speciation

• The Process of Speciation– Occurs when one species give rise to two species

• Occurs when reproductive isolation develops

– Allopatric Speciation: geographical barriers separate a population into two groups

• Premating and then postmating isolating mechanisms occur

– Sympatric Speciation: occurs without geographical barriers

• Ex: Plants- multiplication of chromosome number in one plant may prevent it from successfully reproducing with others of its kind.

– Self-reproduction can maintain a new species36

27.4 Speciation

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An example of how speciation could occur

• Fig. 27.18

Allopatric Speciation

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Prebles meadow jumping mouseZapus hudsonius preblei

Figure 3. Map of showing distribution and subspecies of Zapus hudsonius (Krutzsch 1954; Hafner et al. 1981). (1) Z. h.preblei, (2) Z. h. campestris, (3) Z. h. pallidus, (4) Z. h. luteus, (5) Z. h. intermedius, (6) Z. h. americanus, (7) Z. h. acadicus, (8) Z. h.ladas, (9) Z. h. canadensis, (10) Z. h. hudsonius, (11) Z. h. tenellus and (12) Z. h. alascensis. (Source: Ramey et al. 2005)

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27.4 Speciation

• Adaptive Radiation– A specific type of (divergent) speciation which

gives rise to many new species– Ex: Galapagos Islands finches- studied by Darwin

• Mainland finches migrated to one of the islands– Reproduced and eventually spread to all the islands– Subjected to different environmental selection pressures

• Gave rise to many species of finches which differ primarily in beak shape

– Adapted to allow use of different food sources

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The Galapagos finches

• Fig. 27.19

27.4 Speciation

• The Pace of Speciation– Phyletic Gradualism

• Change is slow but steady before and after a divergence– Explains why so few transitional fossils are found

– Reproductive isolation cannot be detected in fossils

– Punctuated Equilibrium• Long periods of stasis followed by rapid speciation

– Occurs relatively rapidly

– Also can explain lack of transitional fossils

» Rapid development of changes does not result in recognizable transitional links

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Phyletic Gradualism Compared to Punctuated Equilibrium

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Summary:

What is microevolution?•Microevolution involves evolutionary changes within a population, defined as changes in gene frequency.

What is macroevolution? •The process by which new species are produced from earlier ones. This is the process of evolution at the species level or above.

27.5 Classification

– Assignment of species to a hierarchy of categories

– From general to specific these are: domain, kingdom, phylum, class, order, family, genus, species

• Should reflect phylogeny– Species within a genus are more closely related than those in

different genera, for example

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Five Kingdom System of Classification

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• Six-kingdom system – Placed into a kingdom based on mode of nutrition, type

of cell, level of organization• Previously, Kingdom Monera - prokaryotes

• Has been split into TWO bacterial kingdoms: Eubacteria and Archaebacteria

• Kingdom Protista-eukaryotic single-celled and multi-celled plant-like, animal-like, and fungal-like organisms

• Kingdom Fungi- multicellular heterotrophic saprophytic organisms

• Kingdom Plantae- multicellular photosynthetic organisms

• Kingdom Animalia- multicellular heterotrophic animals

Three Domain SystemBased Upon rRNA

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Classification cont’d.

• Three-domain system– Based on rRNA– Domain Bacteria

• “normal” bacteria

– Domain Archae• Archaebacteria that survive in very harsh

environments

– Domain Eukarya• Eukaryotic organisms