BI158 Chapter 23

The Smallest Unit of Evolution

• A common misconception is that organisms evolve during their lifetimes 

• Natural selection acts on individuals, but only populations evolve

• Consider, for example, a population of medium ground finches on Daphne Major Island

  • During a drought, large-beaked birds were ore likely to crack large seeds and survive

  • The finch population evolved by natural selection in only a generation 

• Microevolution is a change in allele frequencies in a population over generations 

• Three mechanisms cause allele frequency change 

  • Natural selection 

  • Genetic drift 

  • Gene flow 

• Only natural selection causes adaptive evolution 

Concept 23.1 Genetic variation makes evolution possible

• Variation in herbitable traits is a prerequisite for evolution 

• Mendel’s work on pea plants provided evidence of discrete heritable units (genes)  

Genetic Variation

• Genetic variation among individuals is caused by differences in genes or other DNA segments 

• Phenotype is the product of inherited genotype and environmental influence s

• Nautral selection can only act on phenotype variation with a genetic component 

Phenotypic Variation 

• Some phenotypic differences are determined by a single gene and can be classified on an either-or bases 

• Other phenotypic differences are determined by the influence fo two or more genes and vary along a continuum within a population 

• Some phenotypic variation does not result frog genetic difference among individuals, but rather from environmental influences 

• Only genetically determined variation can have evolutionary consequences 

Variable Morphology 

Non-heritable Variation 

• These catepillars have a different appearance due to chemicals in their diets. Those fed oak flowers look like oak flowers; those fed oak leaves look like twigs

Measuring Genetic Variation 

• Genetic variation can be measured as gene variability or nucleotide variability 

  • For gene variability, average heterozygosity measures that average percent of loci that are heterozygous in a population 

  • Nucleotide variability is measured by comparing the DNA sequences of pairs of individuals 

  • Nucleotide variation rarely results in phenotypic variation - why?

Sources of Genetic Variation

• New genes and alleles can arise by mutation gene duplication 

• Sexual reproduction can result in genetic variation by recombing existing alleles 

Formation of New Alleles 

• A mutation is a random change in nucleotide sequence of DNA 

• Only mutations in cells that produce gametes can be passed to offspring 

• A point mutation is a change in one base in a genes 

Point Mutations 

• The effects of point mutations can vary 

  • Mutations that result in a change in protein production are often harmful 

  • Harmful mutations can be hidden from selection in recessive alleles 

  • Mutations that result in a change in protein production can sometimes be beneficial 

  • Point mutations in noncoding regions generally result in neutral variation, conferring no selective advantage or disadvantage 

  • Mutations to genes can be neutral because of redundancy in the genetic code 

Altering Gene Number or Position 

• Chromosomal mutations that delete, disrupt, or rearrange many loci are typically harmful 

• Duplication of small pieces of DNA increases genome size and is usually less harmful 

• Duplicated genes can take on new function by further mutation 

  • E.g., An ancestral odor=detecting gene has been duplicated many times; humans have 350 copies of the gene, mice have 1,000

Rapid Reproduction

• Mutation rates are low in animals and plants 

  • The average is about one mutation in every 100,000 genes per generation 

• Mutation rates are often lower in prokaryotes and higher in viruses 

  • However, mutations accumulate quickly in prokaryotes and viruses because they have short generation times 

Sexual Reproduction

• Sexual reproduction can shuffle existing alleles into new combinations 

• In organisms that reproduce sexually, recombination of alleles is more important than mutation in producing the genetic differences that make adaptation possible 

Concept 23.2: The Hard-Weinberg equation can be used to test whether a population is evolving 

• The first step in testing whether evolution is occurring in a population is to clarify what we mean by a population

Gene Pools and Allele Frequencies 

• A population is a localized group of individuals capable of interbreeding and producing fertile offspring 

• A gene pool consists of all the alleles for all loci in a population 

• A locus is considered “fixed” if all individuals in a population are homozygous for the same allele 

• If there are two or more alleles for a locus, diploid individuals may be either homozygous or heterozygous 

• Two populations of caribou show partial isolation. Although their home ranges overlap, they rarely interbreed

Allele Frequencies 

• The frequency of an allele in a population can be calculated 

  • For diploid organisms, the total number of alleles at a locus is the total number of individuals times 2 

  • The total number of dominant alleles at a locus is two alleles for each homozygous dominant individual plus one allele for each heterozygous individual; the same logic applies for recessive alleles 

• By convention, if there are two alleles at a locus, p and q are used to represent their frequencies 

• The frequency of all alleles in a population will add up to 1 

  • For example, p + q = 1

• For example, consider a population of wildflowers that is incompletely dominant for color 

  • 320 red flowers (CR CR) 

  • 160 pink flowers (CR CW) 

  • 20 white flowers (CW CW) 

• Calculate the number of copies of each allele 

  • CR = (320 x 2) + 160 = 800 

  • CW = (20 x 2) + 160 = 200 

• To calculate the frequency of each allele 

  • P = freq CR = 800 / (800+200)

• Sjdf

• kf

• Think of allele frequencies as two sides of a coin 

  • If each is 0.5, then there is a 50% chance of getting one or the other when you flip that coin 

  • If you are flipping two coins 

    • The chance of getting two heads (or two tails) calculated by multiplying 0.5 x 0.5 or 0.25 (45%) 

    • The chance of getting one head, then one tail is also 0.5 x 0.5. However you could also have one tail, then one head (different sequence). So, it would be 2 x (0.5 x 0.5) 

  • So, this sets up the Hardy Weinberg Equation: 

    • p^2 + 2pq + q^2 = 1 

    • p=.8, q=.2 

The Hardy- Weinberg Equation 

• The Hardy-Weinberg equation describes the genetic makeup we expect for a population that is not evolving at a particular locus

• If the observed genetic makeup of the population differs from expectations under Hardy-Weingber, it suggests that the population may be evolving 

Hardy-Weinberg Equalibrium 

• In a population where gametes contribute to the next generation randomly and Mendelian inheritance occurs, allele and genotype frequencies remain constant from generation to generation 

• Such a population is in Hardy-Weinberg equilibrium 

• The frequency of genotypes can be calculated 

  • CRCR = p^2 = (0.8)^2 = 0.64 

  • CRCW = 2pq = 2(0.8)(0.2) = 0.32 

  • CWCW = q^2 = (0.2) ^2 = 0.4

• The frequency of genotypes can be confirmed using a Punnet Sqaure

• If p and q represent the relative frequencies of the only two possible alleles in a pollution at a particular locus, then 

  • p^2 + 2pq + q^2 = 1 

  • Where p^2 and q^2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the herteroygous genotype 

Conditions for Hardy-Weinberg Equalibrium 

• The Hardy-Weinberg theorem describes a hypothetical population that is not evolving 

• In real populations, allele and genotype frequencies do not change over time 

• Natural populations can evolve at some loci, while being in Hardy-Weinberg equilibrium at other loci 

• The five conditions for nonevolving populations are rarely met in nature 

1. NO mutations 

2. Random mating

3. No natural selection 

4. Extremely large population size 

5. No gene flow 

Applying the Hardy-Weinberg Equation

• We can assume the locus that causes phenylketonuria (PKU) is in Hardy-Weinberg equilibrium given that 

1. The PKU gene mutation rate is low 

2. Mate selection is random with respect fo whether or not an individual is a carrier for the PKU allele

3. Natural selection can only act on rare homozygous individuals who do not follow dietary restrictions 

4. The population is large 

5. Migration has no effect as many other population have similar allele frequencies 

• The occurrence of PKU is 1 per 10,000 births 

  • q^2 = 0.0001 

  • q= 0.01 

• The frequency of normal alleles is 

  • p = 1 - q = 1 - 0.01 = 0.99 

• The frequency of carriers is 

  • 2pq = 2 x 0.99 x 0.01 = 0.0198 

  • Or approximately 2% of the U.S. population 

Concept 23.3 

• Three major factors alter allele frequencies and bring about most evolutionary change 

  • Natural selection 

  • Genetric drift

  • Gene flow 

Natural Selection 

• Differential success in reproduction results in certain alleles being passed to the next generation in greater proportions 

  • For example, an allele that confers resistance to DDT in fruit flies increased in frequency after DDT was used widely in agriculture 

• Natural selection can cause adaptive evolution, an improvement in the match between organisms and their environment 

Genetic Drift 

• The smaller a sample, the greater the chance of random deviation from a predicted result 

• Genetic drift describes how allele frequencies fluctuate unpredictably from one generation to the next 

  • Genetic drift tends to reduce genetic variation through losses of alleles

  • You can think of it as ‘sampling error’

Genetic Drift Example

• Suppose if by chance only 5 plants survive to reproduce for the first generation, and only two red ones the second generation 

• That leaves only red flower alleles in that population

The Founder Effect 

• The founder effect occurs when a few individuals become isolated from a larger population 

  • (e.g., a few get moved to an island) 

• Allele frequencies in the small founder population can be different from those in the larger parent population 

The Bottleneck Effect 

• The bottleneck effect is a sudden reduction in population size due to a change in the environment 

• The resulting gene pool may no longer be reflective of the original population’s gene pool

• If the population remains small, it be further affected by genetic drift 

• Understanding the bottleneck effect can increase understanding of how human activity affects other species 

Case Study: Impact ofGenetic Drift on the Greater Prairie Chicken 

• Loss of prairie habitat caused a severe reduction in the population of greater prairie chickens in Illinois 

• The surviving birds had low levels of genetic variation, and only 50% of their eggs hatched 

• Researchers used DNA from museum specimens to compare genetic variation in the population before and after the bottleneck 

• The results showed a loss of alleles at several loci 

• Researchers introduced greater prairie chickens from populations in other states and were successful in introducing new alleles and increasing the egg hatch rate to 90%

Effects of Genetic Drift: A Summary

1. Genetic drift is significant in small populations

2. Genetic drift can cause allele frequencies to change at random 

3. Genetic drift can lead to a loss of genetic variation within populations 

4. Genetic drift can cause harmful alleles to become fixed 

Gene flow 

• Gene flow consists of the movement of alleles among populations 

• Alleles can be transferred through the movement of fertile individuals or gametes (for example, pollen) 

• Gene flow tends to reduce variation among populations over time 

• Gene flow can decrease the fitness of a population 

• Consider, for example, the great tit (Parus major) on the Dutch island of Vlieland 

  • Mating causes gene flower between the central and eastern populations 

  • Immigration from the mainland introduces alleles that decrease fitness on the island 

  • Natural selection removes alleles that decrease fitness 

  • Birfds born in the central region with high immigration have lower fitness; birds bron in the east with low immigration have a higher fitness 

• Gene flow can increase the fitness of a population 

• Consider, for example, the spread of alleles for resistance to insecticides 

  • Insecticides have been used to target mosquitoes that carry West Nile virus and malaria 

  • Alleles have evolved in some population that confer insecticide resistance to these mosquitoes 

  • The flow of insecticide resistance alleles into a population can cause an increase in fitness 

  • Gene flow is an important agent of evolutionary change in modern human populations 

Concept 23.4 Natural selection is 

• Evolution by natural selection involves both change and “sorting” 

  • New genetic variations arise by chance 

  • Beneficial alleles are “sorted” and favored by natural selection 

• Only natural selection consistently increases the frequencies of alleles that provide reproductive advantage 

Natural Selection: A Closer Look 

Relative Fitness 

• Natural selection brings about adaptive evolution by acting on an organism’s phenotype 

• The phrases “struggle for existence” and “survival of the fittest” are misleading as they imply direct competition among individuals 

• Reproductive success is generally more subtle and depends on many factors 

• Relative fitness is the contribution an individual makes to the gene pool of the next generation, relative to the contributions fo other individuals

• Selection favors certain genotypes by acting on the phenotypes of individuals

Directional, Disruptive, and Stabilizing Selection 

• There are three modes of selection 

  • Directional selection favors individuals at one extreme end of the phenotypic range 

  • Disruptive selection favors individuals at both extremes of the phenotypic range 

  • Stabilizing selection favors intermediates variants and acts against extreme phenotypes 

The Key Role of Natural Selection in Adaptive Evolution

• Striking adaptations have arisen by natural selection 

  • For example, certain octopuses can change color rapidly for camouflage 

  • For example, the jaws of snakes allow them to swallow prey larger than their heads 

• Natural Selection increases the frequencies of alleles that enhance survival and reproduction 

• Adaptive evolution occurs as the match between a species and its environment increases

• Beause the environment can change, adaptive evolution is a continuous process 

• Genetic drift and gene flow do not consistently lead to adaptive evolution as they can increase or decrease the match between an organism and its environment 

Sexual Selection 

• Sexual selection is the natural selection for mating success 

• It can result in sexual dimorphism, marked differences between the sexes in secondary sexual characteristics 

• Intrasexual selection is direct competition among individuals of one sex (often males) for mates of the opposite sex 

• Intersexual selection, often called mate choice, occurs when inidivudals of one sex (usually females) are choosy in selecting their mates 

• Male showiness due to mate choice can increase a male’s chances of attracting a female, while decreasing his chances of survival  

• How do female preferences evolve? 

• The “good genes” hypothesis suggests that if a trait is related to male genetic quality of health, both the male trait and female preference for that trait should increase in frequency.

Sexual Selection Example

• Female gray tree frogs prefer to mate with males that make long calls over shrot calls 

• One experiment artificially fertilized clutches of egg with half LC male’s sperm and half SC male’s sperm, then raised offspring in a common environment 

• Their results (two consecutive years) showed that LC male offspring overall did better than SC male offspring 

• This supports the idea that female choice for long calls is based upon ‘good genes’ 

Balancing Selection 

• Diploidy maintains genetics variation in the form of recessive alleles hidden from selection in heterozygotes 

• Balancing selection occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population 

• Balancing selection includes 

  • Heterozygote advantes 

  • Frequency-dependent selection 

Heterozygote Advantage 

• Heterozygote advantage occurs when heterozygotes have a higher fitness than do both homozygotes 

• Natural selection will tend to maintain two ro more alleles at that locus 

• Heterozygote advantage can result from stabilizing or directional selection 

• Example: Heterozygous carriers of the sickle cell allele

  • Homozygotes for sickle cell (Hb^SHb^S) usually die early 

  • Heterozygotes (Hb^AHb^S) are more resistant to malaria than homozygotes for normal hemoglobin (Hb^AHb^A) 

• In regions where the malaria parasite is common, selection favors individuals heterozygous for the sickle-cell allele 

Frequency-Dependent Selection

• In frequency-dependent selection, the fitness of a phenotype declines if it becomes too common in the population

• Selection favors whichever phenotype is less common in a population 

• For example, frequency-dependent selectijon results in approximately equal numbers of right moutheed, and left mouthed scale-eating fish