BI158
Chapter 23: Evolution of Populations
The Smallest Unit of Evolution
A common misconception is that organisms evolve during their lifetimes
Natural selection acts on individuals, but only populations evolve.
For example, a population of medium ground finches
During a drought, large-beaked birds were more likely to crack larger seeds and survive.
The finch population evolved by natural selection, in only a generation.
Microevolution: a change in allele frequencies in a population over generations.
Three mechanisms cause allele frequency change:
natural selection
genetic drift (something that happens when a random effect occurs and the response to it)
Ex: a seed that washes up that is skewed from the parent
gene flow
Only natural selection causes adaptive evolution
Concept 23.1 Genetic variation makes evolution possible.
Variation in heritable traits is a prerequisite for evolution.
Mendel’s work on pear 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 influences
Natural 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 basis (binary, white vs red, or earlobe shape)
Others are determined by the influence of two or more genes and vary along a continuum within a population (horses)
Some do not result from genetic differences, but rather from environmental influences among individuals, but rather from environmental influences
Proves that the environment you develop in determines phenotypes
Only genetically determined variation can have evolutionary consequences
Phenotype variation is based off genetic variation
Non Inherited Variation
These caterpillars 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 the 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
Sources of Genetic Variation
New genes and alleles can arise by mutation or gene duplication
Sexual reproduction can result in genetic variation by recombining 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 gene
Point Mutations
Effects 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
In noncoding regions, it generally results in neutral variation, conferring no selective advantage or disadvantage
Mutations to genes can be neutral because of redundancy in the genetic code
Sickle cell anemia could be a result of point mutations which happens because a person has a youth form of hemoglobin, when the blood cell is in an area with less oxygen and the sickle gene causes the cells to have a crescent shape
If you’re heterozygous for sickle cell anemia and may have some symptoms but the genes help have a higher resistance to malaria
Gives an advantage and disadvantage in some way
Altering Gene number of 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
Ex: an ancestral odor-defecting 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
Average is about 1 mutation in every 100K genes per generation
Mutation rates are often lower in prokaryotes and higher in viruses.
Mutations accumulate quickly in prokaryotes and viruses because of short generation times
Sexual Reproduction
Can shuffle existing alleles into new combinations
Recombination of alleles is more important than mutation in producing the genetic differences that make adaptation possible
Concept 23.2: The Hardy-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
Population is a localized group of individuals capable of interbreeding and producing fertile offspring.
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 is no variation no change is possible
If there are two or more alleles for a locus, diploid individuals may be either homozygous or heterozygous
Allele Frequency
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
Total number of dominant alleles at a locus is two alleles for each homozygous dominant individuals plus one allele for each heterozygous individual; the same logic applies for recessive alleles
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 → p+q = 1
Hardy-Weinberg Equilibrium
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
Describes the constant frequency of alleles in such a gene pool
Conditions for Hardy-Weinberg Equilibrium
Theorem describes a hypothetical population that is not evolving
In real populations, allele and genotype frequencies do change over time.
Natural populations can evolve at some loci, while being in H-W equilibrium at other loci
Applying the H-W Equation
We can assume the locus that causes PKU is in H-W equilibrium given that:
The PKU gene mutation rate is low.
Mate selection is random with respect to whether or not an individual is a carrier for the PKU allele.
Natural selection can only act on rare homozygous individuals who do not follow dietary restrictions.
The population is large.
Migration has no effect as many other populations have similar allele frequencies.
Natural selection, genetic drift, and gene flow can alter allele frequencies in a population.
Natural Selection
Differential success in reproduction results in certain alleles being passed to the next generation in greater proportions
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
Describes how allele frequencies fluctuate unpredictably from one generation to the next
Tends to reduce genetic variation through losses of alleles
Think of it as “sampling error”
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
Occurs when a few individuals become isolated from a larger population
Allele frequencies in the small founder population can be different from those in the larger parent population
Bottleneck Effect
Sudden reduction in population size due to a change in the environment
Resulting gene pool may no longer reflect the original population’s gene pool
If the population remains small, the effects of genetic drift may be more prevalent
Helps us understand how human activity affects other species
Case Study: Impact of Genetic Drift on the Greater Prairie Chicken
Loss of prairie habitat caused severe reduction in population of greater prairie chickens
Surviving birds has low levels of genetic variation and less than 50% of their eggs hatched
Genetic variation before and after the bottleneck were compared and the results showed a loss of alleles at several loci
Effects of Genetic Drift
Significant in small populations
Can cause allele frequencies to change eat random
Can lead to a loss of genetic variation within populations
Can cause harmful alleles to become fixed
Gene Flow
Consists of the movement of alleles among populations
Can be transferred through the movement of fertile individuals or gametes (ex. pollen)
Tends to reduce variation among populations over time
Can decrease the fitness of a population
The Great Tit (Parus major) on a Dutch island
→ Mating causes gene flow between the central & eastern populations
→ Immigration from the mainland introduces alleles that decrease fitness on the island
→ Natural selection removes alleles that decrease fitness
→ Birds born in the central with high immigration have lower fitness; birds born in the eat with low immigration have higher fitness
Can increase the fitness of a population
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 populations 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
Natural selection is the only mechanism that consistently causes adaptive evolution.
Evolution from natural selection involves both chance 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
Brings about adaptive evolution by acting on an organism's phenotype
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 of other individuals
Favors certain genotypes by acting in the phenotypes
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 intermediate variants and acts against extreme phenotypes
The Key Role of Natural Selection in Adaptive Evolution
Striking adaptations have arisen by natural selection
Certani octopuses can change color rapidly for camouflage
Jaws of snakes allow them swallow prey larger than their heads
Natural selection Increase the frequencies of alleles that enhance survival and reproduction
Adaptive evolution occurs as the match between a species and its environment increases
Because 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
Natural selection for mating success
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 individuals 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 for health, both the male trait and female preference for that trait should increase in frequency
Balancing Selection
Diploidy maintains genetic variation in the form of recessive alleles hidden from selection in heterozygotes
Occurs when natural selection maintains stable frequencies of two or phenotypes forms in a population
Includes heterozygote advantage and frequency-dependent selection
Heterozygous advantage occurs when heterozygotes have a higher fitness than do both homozygotes; Can result from stabilizing or directional selection
→ Example: Sickle cell allele
→ Homozygotes for sickle cells usually die early. Heterozygotes are more resistant to malaria than homozygotes for normal hemoglobin
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
Why Natural Selection Cannot Fashion Perfect Organisms
Selection can act only on existing variations
Evolution is limited by historical constraints
Adaptations are often compromises
Chance, natural selection, and the environment interact