POPULATION GENETICS AND NATURAL SELECTION Chapter 4 5 th edition

POPULATION GENETICS AND NATURAL SELECTION

Chapter 4

5th edition

A Paradox?

• Estimates on the number of species currently living on the earth range from 3 million to perhaps 100 million. Most estimates are in the range of 10-30 million.

• It is generally assumed that at least 99.9% of the all species that have ever lived are now extinct.

• A conservative guess then is that something like 10 billion species have made their appearance on the earth since the Cambrian Epoch some 600 million years ago -- and most of these have failed to persist to the present time.

• Even a cursory examination of the fossil record then, gives one a sense of failed experimentation when it comes to "designing" organisms.

• This whole process of developing new species just doesn't seem to be very efficient and seems to rarely produce a species that survives for very long.

A Paradox?• On the other hand, when one takes a more narrow look at the

diversity of life on the planet today, you can't help but be impressed by the marvelous fit of organisms to their environment -- i.e. by the myriad of clever and unique ways that organisms meet the exigencies of life. When you look only at the organisms around today, you can't help but conclude that these organisms have finally hit on a strategy that works!

• How then, are we to resolve this conflict between apparent colossal failure on the one hand and the apparent stunning success on the other?

• The answer to this question comes from an understanding of the process of evolution and natural selection.

Population

• A collection of individuals of the same species. Two distinguishing attributes: the members of the population are reproductively

compatible with one another (i.e., they have a high probability of mating with one another relative to their probability of mating with members of some other population.

the members of the population appear to be similar to one another but somewhat different from the members of other populations.

• Upon close inspection, we usually find evidence of variation among the individuals comprising the population -- some variation in appearance, physiology or behavior.

• We refer to this as phenotypic variation

Phenotype

• the physical expression of the organism, its appearance, its physiological function and its behavior. The phenotype is what we see when we examine the organism (tall or short, blond or brunette, normal hemoglobin or sickle-cell hemoglobin, etc...). The phenotype represents the interaction of two sources of influences: genotype and; the environment.

• Populations generally exhibit phenotypic variation in most traits.

Phenotypic variation• This curve illustrates both

the variation and central tendency for the population.

• This is shown here as a “normal” or bell-shaped curve, but skewed distributions are also possible.

• This variation is vital for the process of evolution, however, only that portion of the variation that has a genetic basis is relevant for the process of evolution.

Evolution: defined

Generation#1 Generation#2

Pop mean generation#1

• Any persistent, genetically-based change from one generation to the next in the form of the frequency distribution of a trait or traits.

Evolution: a two step process

• 1. Production of heritable variation Gene mutation Recombination

• 2. Differential transmission Genetic drift Non-random mating Natural selection

Darwin – Natural Selection

• Organisms begat like organisms (e.g. offspring tend to look like parents).

• Chance variation between individuals. Some of this variation is heritable.

• More offspring are produced each generation than can survive.

• Some individuals, because of physical or behavioral traits, have a higher chance of surviving than others in the same population.

Gregor Mendel

• Augustinian Monk; a contemporary of Darwin, but neither were aware of the other’s work. Mendel discovered the mechanisms of inheritance that Darwin spent much of his life looking for. Studied garden pea (Pisum sativum). Discovered characteristics pass from parent to

offspring in form of discrete packets called genes. This is sometimes referred to as the concept of “particulate inheritance.”

Exist in alternate forms – alleles. Some prevent expression of others; some

alleles are dominant, others are recessive.

Particulate vs. Blended Inheritance

• Blended inheritance: was viewed as the mechanism of inheritance. Characteristics of offspring a “blend” of parents. With flower color, this would involve

convergence on an intermediate color If true, a novel trait could never gain prominence Major challenge to Darwin’s theory

• Mendel’s results directly addressed and dismissed this challenge…..but Darwin never learned of these results during his lifetime

Conditions Necessary To Maintain Constant Allele Frequencies:

• Random Mating• No Mutations• Large population Size (no genetic drift)• No Immigration• Equitable Fitness Between All Genotypes.

Likely at least one of these will not be met and allele frequencies will change.

Potential for evolutionary change in natural populations is very great.

Hardy Weinberg

• Hardy Weinberg principle states that in a population mating at random in the absence of evolutionary forces, allele frequencies will remain constant and the relative abundance of each genotype will be defined by:

(p+q)2 = p2+2pq+q2

Color Variation in Asian Lady Beetles

• SS (81%) SA (18%) AA (1%)• Frequency of S allele ?

SS + 1/2SA = .81 + ½(.18) = .90

(.90)2 + 2(.9x.1) + (.10)2 = 1.0

But What If?

Phenotype Frequencies

(observed)p2 2pq q2

0.875 0.05 0.075

Allele Frequencies(calculated)p q0.9 0.1

(expected from H-W)

p2 2pq q2

0.81 0.18 0.01

Change Due To Chance

• Random processes such as genetic drift can change gene frequencies in populations, especially in small populations.

• Major concern of fragmentation is reducing habitat availability (and reduce gene flow among habitat patches) to the point where genetic drift will reduce genetic diversity within natural populations.

Natural Selection

• Natural Selection, which changes genotypic and phenotypic frequencies in populations, can result in adaptation to the environment.

A test of natural selection

• Darwin’s work was published in 1858.

• The first widely accepted test of his theory of natural selection took place in 1955. This was Kettlewell’s work on Industrial melanism in the peppered moth (Biston betularia)

Natural Selection: what it IS NOT

• Not goal oriented: it is opportunistic. The only thing that counts is how a trait influences the survival and reproductive success of individuals with that trait under current environmental conditions

• Not unidirectional: with improved air pollution regulations, light colored B. betularia are becoming more common

• Not “progressive”: but it is adaptive. Value-laden terms like “progressive” and “regressive” don’t have any meaning in evolution.

• Does not work on a single trait at a time. The phenotype is a compromise to all selective pressures working on the organism at any given time.

• Adaptation is never “perfected”: traits that increase in abundance from one generation to the next are those that are the “best” available from within the gene pool.

What about the Paradox?

• Colossal “failure” of natural selection (based on evidence of massive extinctions in the fossil record) vs. exquisite “success” of natural selection (based on amazing adaptations displayed by current organisms)

• The environment is constantly changing.• Marvelous adaptation to current conditions does not

guarantee that an organism will be well suited to future conditions

• Sometimes “the best” that a population, or a species, has to offer simply is not well suited to a changing world. When this happens, extinction will follow

Microevolution vs. Macroevolution

• Some authors distinguish between microevolution and macroevolution Microevolution: relatively small changes in

the character of a species through time, e.g. industrial melanism

Macroevolution: speciation; the creation of new species

• Both are the result of natural selection• Speciation simply take more time and

involves larger changes.

Speciation

• We have discussed how natural selection can lead to changes, from one generation to the next, in the frequency distribution of phenotypic traits (microevolution).

• How might this process lead to the development of new species (macroevolution)?

• The most widely accepted hypothesis for the development of new species is allopatric speciation. This is a 4-step process involving:

Allopatric Speciation

• 1. Reproductive isolation occurs because of physical, geographic separation of two populations. Gene flow between the two populations is prevented because of geographic separations.


• 2. The isolated populations undergo independent evolution and become adapted to their respective environments.


• 3. Reproductive barriers must evolve to reduce the likelihood of interbreeding between the two populations.


• 4. The real "test" to see if speciation has occurred comes when and if the geographic isolation is removed. If geographic isolation ends and the two populations come into contact with one another, and if reproductive isolating mechanisms are effective at preventing interbreeding, then speciation has occurred and two separate species now exist.


Other models for speciation• Parapatric speciation can occur when a population of a

widespread species enters a new habitat outside the geographic range of the original species. Although no barriers to movement or gene flow may exist, the population that has entered the new habitat may be effectively isolated from other populations. Parapatric speciation may occur for plants and for animals with very low mobility.

• Sympatric speciation occurs when reproductive isolation occurs within the geographic range of the original species. It occurs when a population enters a previously unused habitat (or new host for a parasite) within the original geographic range of the original species. This type of speciation is thought to be rare in nature. As with parapatric speciation, it is thought to occur for species with very low mobility.

Mechanisms for Reproductive Isolation

• Prezygotic mechanisms (fertilization is prevented): Habitat: populations occur in the same region but occupy

different habitat; Seasonal or temporal: populations co-occur but are

sexually mature at different times of the year; Ethological: incompatible-mating behavior; Mechanical: fertilization is prevented by incompatible

differences in reproductive structures.• Post-zygotic mechanisms: fertilization takes place but

hybrids are inviable, sterile or have low vigor.

Summary

• Populations include genetic and phenotypic variation among individuals.


• Natural selection, which changes genotypic and phenotypic frequencies in populations, can result in adaptation to the environment.

• Allopatric speciation is the most widely accepted model for speciation but other models exist (parapatric, sympatric)

Ignoring Evolution can be bad for your health…

Doonsbury by G. Trudeau

Ignoring Evolution can be bad for your health…

Doonsbury by G. Trudeau

Evolution

• Any persistent, genetically-based change from one generation to the next in the form of the frequency distribution of a trait or traits.

Summary


• The Hardy-Weinberg equilibrium model helps identify evolutionary forces that can change gene frequencies in populations.



Chapter Concepts





Darwin

• 1835 Charles Darwin visited Galapagos Islands and became convinced various populations evolved from ancestral form.

• 1838 After reading essay by Thomas Malthus, he theorized some individuals would have a competitive advantage conferred by favorable characteristics.

Darwin – Natural Selection

• Organisms begat like organisms.• Chance variation between individuals.

Some are heritable.• More offspring are produced each generation

than can survive.• Some individuals, because of physical or

behavioral traits, have a higher chance of surviving than others in the same population.

Gregor Mendel

• Augustinian Monk Studied garden pea (Pisum sativum). Discovered characteristics pass from

parent to offspring in form of discrete packets called genes.

Exist in alternate forms – alleles. Some prevent expression of others.

Variation Within Populations

• Variation In Plant Populations Many plant spp. differ dramatically in form

from one elevation to another.• Bonner studied regional variation in climate

and plant in mountains of Europe. Demonstrated environment alone can

induce substantial morphological variation within plant populations.

Evidence of Genetic Variation Among Populations

• Turreson found evidence that genetic differences among ecotypes result from natural selection exerted by local environments.

• Clausen et.al. found evidence of adaptation by ecotypes to local environmental conditions in Potentilla glandulosa.

Variation in Animal Populations

• Chuckwalla (Sauromalus obesus) Herbivorous lizard in desert SW. Variation in rainfall translates into variation

in food availability. Case found lizards from food - rich higher

elevations were approx 25% longer and 2x body weight of those from lower elevations.

Hardy Weinberg

• Hardy Weinberg principle states that in a population mating at random in the absence of evolutionary forces, allele frequencies will remain constant:

(p+q)2 = p2+2pq+q2

Conditions Necessary To Maintain Constant Allele Frequencies:

• Random Mating• No Mutations• Large population Size• No Immigration• Equitable Fitness Between All Genotypes.

Likely at least one of these will not be met and allele frequencies will change.

Potential for evolutionary change in natural populations is very great.

Change Due To Chance


• Major concern of fragmentation is reducing habitat availability to the point where genetic drift will reduce genetic diversity within natural populations.

Evidence of Genetic Drift in Chihuahua Spruce

• Picea chihuahuana – now restricted to peaks of Sierra Madre Occidental in N. Mexico. Ledig et.al. examined populations to

determine if the species has lost genetic diversity as a consequence of reduced population size.

Found a significant positive correlation between population size and genetic diversity of study populations.

Genetic Variation In Island Populations

• In general, genetic variation is lower in isolated and generally smaller, island populations. Reduced genetic variation indicates a lower

potential for a population to evolve.

Genetic Diversity and Butterfly Extinctions

• Frankham and Ralls point out inbreeding may be a contributor to higher extinction rates in small populations. Reduced fecundity, depressed juvenile

survival, shortened life-span.• Saccheri conducted genetic studies on pops

of Glanville fritillary butterflies (Melitacea cinxia). Populations with highest levels of

inbreeding had highest probabilities of extinction.

Natural Selection

• Natural Selection, which changes genotypic and phenotypic frequencies in populations, can result in adaptation to the environment.

Adaptive Change in Colonizing Lizards

• Losos et.al. Genus Anolis

Great diversity includes large amount of variation in size and body proportions.

Length of hind limbs appears to reflect selection for effective use of vegetation.

– Diameter of perching surfaces.

Soapberry Bug Adaptation

• Carroll and Boyd Soapberry Bug (Jadera haematoloma)

feeds on seeds from family Sapindaceae. Slender beaks to pierce fruit walls.

Distance from outside fruit wall to seeds varies widely – beak length should be under selection.

– Found close relationship between fruit radius and beak length.

Summary





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POPULATION GENETICS AND NATURAL SELECTION Chapter 4 5 th edition