The Evolution of Populations BIO evolution

Chapter 23: The Evolution of Populations

Key Concepts

  • Genetic variation makes evolution possible

  • The Hardy-Weinberg equation can be used to test whether a population is evolving

  • Natural selection, genetic drift, and gene flow can alter allele frequencies in a population

  • Natural selection is the only mechanism that consistently causes adaptive evolution

The Smallest Unit of Evolution

  • Microevolution is defined as a change in allele frequencies in a population over generations.

  • Three primary mechanisms that cause allele frequency changes:

    • Natural selection

    • Genetic drift

    • Gene flow

  • Notably, only natural selection consistently initiates adaptive evolution.

Concept 23.1: Genetic Variation Makes Evolution Possible

  • The presence of variation in heritable traits is essential for evolution to occur.

  • Gregor Mendel’s research on pea plants provided foundational evidence for the existence of discrete heritable units known as genes.

Genetic Variation (1 of 2)

  • Genetic variation between individuals is attributed to differences in genes or DNA segments.

  • The phenotype, which is the observable characteristics, results from the interplay of inherited genotype and environmental influences.

  • Only variations with a genetic basis can be acted upon by natural selection.

Genetic Variation (2 of 2)

  • Some phenotypic differences are determined by a single gene and can be categorized distinctly (either-or).

  • Other phenotypic differences are influenced by multiple genes and exhibit variation along a continuum within a population.

Variation Within a Population (1 of 2)

  • Genetic variation can be quantified as either gene variability or nucleotide variability.

  • Nucleotide variability is assessed by comparing DNA sequences between pairs of individuals and typically does not lead to observable phenotypic variation.

Variation Within a Population (2 of 2)

  • The chapter includes a figure (Figure 23.4) illustrating extensive genetic variation at the molecular level.

Nonheritable Variation

  • Certain phenotypic variations arise from environmental influences, not genetic differences.

    • Example: A caterpillar’s appearance differing due to dietary chemical exposure rather than its genotype.

  • Only genetically determined variations have evolutionary implications.

Sources of Genetic Variation

  • New genes and alleles can originate through mutations and gene duplications.

  • Sexual reproduction contributes to genetic variation by recombining existing alleles.

Formation of New Alleles (1 of 2)

  • A mutation is described as a random change in nucleotide sequences in DNA.

  • A point mutation pertains to alteration in a single base of a gene.

  • Crucially, only mutations occurring in gamete-producing cells can be inherited by offspring.

Formation of New Alleles (2 of 2)

  • The effects of point mutations can vary:

    • Mutations in noncoding DNA regions are often harmless.

    • Mutations affecting genes may be neutral due to genetic redundancy.

    • Altered protein production may typically be harmful, although beneficial mutations can occur.

    • Harmful mutations can remain concealed from selection if they are recessive.

Altering Gene Number or Position

  • Chromosomal mutations can result in deletions, disruptions, or rearrangements in DNA sequences, frequently leading to detrimental outcomes.

  • Duplication of small DNA sequences generally increases genome size and is less harmful.

  • Duplicated genes may develop new functions through additional mutations.

  • Example: An ancient olfactory receptor gene that has undergone multiple duplications in mammals, with humans possessing 380 copies and mice having 1200.

Rapid Reproduction

  • Mutation rates in animals and plants are relatively low, approximately one mutation per 100,000 genes per generation.

  • Prokaryotes exhibit lower mutation rates compared to eukaryotes, while viruses typically have higher mutation rates due to shorter generation times.

Sexual Reproduction

  • Sexual reproduction can shuffle alleles into new combinations, where recombination is crucial for producing genetic diversity that facilitates adaptation.

Concept 23.2: The Hardy-Weinberg Equation to Test Population Evolution

  • The initial step in assessing the ongoing evolution of a population is defining what constitutes a population.

  • A population is a localized assembly of individuals capable of interbreeding and generating fertile offspring.

  • A gene pool encompasses all alleles at all loci within a population.

  • A locus is considered fixed when all individuals in a population are homozygous for the same allele.

Allele Frequencies (1 of 4)

  • A diploid organism can either be homozygous or heterozygous for a locus with two or more alleles.

  • Allele frequency in a population is calculated by:

    • Total alleles = Total individuals x 2

    • For a dominant allele in a diploid, alleles are summed as:

    • 2 alleles for homozygous dominant + 1 allele for each heterozygous individual.

Allele Frequencies (2 of 4)

  • In populations with two alleles, frequencies are represented as p and q.

  • The frequencies must add up to 1:
    p + q = 1

Allele Frequencies (3 of 4)

  • Example: Calculating allele frequencies in wildflowers with incomplete dominance:

    • 320 red flowers (CRCR)

    • 160 pink flowers (CRCW)

    • 20 white flowers (CWCW)

    • Number of CR alleles:
      CR = (320 imes 2) + 160 = 800

    • Number of CW alleles:
      CW = (20 imes 2) + 160 = 200

Allele Frequencies (4 of 4)

  • Frequency calculations for each allele:

    • p = ext{freq}_{CR} = rac{800}{800 + 200} = 0.8

    • q = ext{freq}_{CW} = rac{200}{800 + 200} = 0.2

    • Verification of frequency sum:
      0.8 + 0.2 = 1

The Hardy-Weinberg Equation

  • The Hardy-Weinberg equation delineates the anticipated genetic constitution for a population that is not evolving at a specific locus.

  • Discrepancies between observed genetic composition and Hardy-Weinberg expectations indicate potential evolution within the population.

Hardy-Weinberg Equilibrium (1 of 3)

  • Confirms that in a population where gametes contribute randomly to the next generation with Mendelian inheritance, allele and genotype frequencies remain constant over generations.

  • Such a population is termed to be in Hardy-Weinberg equilibrium.

Hardy-Weinberg Equilibrium (2 of 3)

  • The Hardy-Weinberg equilibrium dictates constant allele frequencies in a gene pool, as illustrated in populations, e.g., 500 wildflowers with 1000 alleles, where:

    • p = ext{freq}_{CR} = 0.8

    • q = ext{freq}_{CW} = 0.2

Hardy-Weinberg Equilibrium (3 of 3)

  • Genotype frequency calculations yield:

    • For homozygous dominant:
      CRCR = p^2 = (0.8)^2 = 0.64

    • For heterozygous:
      CRCW = 2pq = 2(0.8)(0.2) = 0.32

    • For homozygous recessive:
      CWCW = q^2 = (0.2)^2 = 0.04

  • Verification using a Punnett square is possible.

The Hardy-Weinberg Principle (1 of 2)

  • If p and q represent the relative frequencies of two alleles in a allele population, then:
    p^2 + 2pq + q^2 = 1

  • Here, p^2 and q^2 denote homozygous genotype frequencies, while 2pq indicates the frequency of heterozygous genotype.

Conditions for Hardy-Weinberg Equilibrium (1 of 2)

  • The Hardy-Weinberg theorem depicts a hypothetical, static population.

  • Actual populations experience changes in allele and genotype frequencies over time due to:

    1. No mutations: Modifications to the gene pool when mutations occur or whole genes are deleted or duplicated.

    2. Random mating: Dominance in gametic mixing if individuals mate within a subset (inbreeding).

    3. No natural selection: Allele frequency changes attributed to differential survival or reproductive success.

    4. Extremely large population size: Genetic drift affects small populations where allele frequencies fluctuate randomly.

    5. No gene flow: The migration of alleles can modify allele frequencies between populations.

Hardy-Weinberg Equilibrium

  • It is feasible for natural populations to evolve at certain loci while retaining Hardy-Weinberg equilibrium at others.

Applying the Hardy-Weinberg Principle (1 of 2)

  • Analysis of a locus causing phenylketonuria (PKU) presumes Hardy-Weinberg equilibrium if:

    1. No new mutations of PKU allele are introduced.

    2. Mating choice is random concerning being a carrier of PKU.

    3. Selection occurs solely against rare homozygous individuals not adhering to dietary measures.

    4. The population is sufficiently large enough to mitigate sampling errors.

    5. Migration rates are negligible as similar allele frequencies exist in other populations.

Applying the Hardy-Weinberg Principle (2 of 2)

  • The prevalence of PKU is noted as 1 per 10,000 births:

    • q^2 = 0.0001
      ightarrow q = 0.01

  • Consequently, the frequency of normal alleles is calculated as:

    • p = 1 - q = 1 - 0.01 = 0.99

  • Expected frequency of carriers is:

    • 2pq = 2 imes 0.99 imes 0.01 = 0.0198

    • Approximately 2% of the Canadian population.

Concept 23.3: Natural Selection, Genetic Drift, and Gene Flow Alter Allele Frequencies

  • Three major mechanisms modifying allele frequencies and instigating significant evolutionary change:

    • Natural selection

    • Genetic drift

    • Gene flow

Natural Selection

  • Natural selection leads to differential reproductive success, fostering increased representation of certain alleles in subsequent generations.

  • It facilitates adaptive evolution, enhancing the organism's match with its habitat.

    • Example: An allele granting DDT resistance in fruit flies rose in prevalence following widespread agricultural use of the insecticide.

Genetic Drift

  • Smaller samples exhibit heightened chances of random deviation from predicted outcomes.

  • Genetic drift entails fluctuating allele frequencies unpredictably across generations.

  • It generally diminishes genetic variation through allele loss.

Genetic Drift in Small Populations

  • Figure 23.9 illustrates the impact of genetic drift across generations.

The Founder Effect

  • The founder effect arises when a subset of individuals separates from a larger population.

  • The resultant allele frequencies within the foundational population may diverge from those in the larger parent population.

The Bottleneck Effect (1 of 3)

  • The bottleneck effect refers to a dramatic reduction in population size due to environmental changes.

  • Following a bottleneck, the gene pool may differ from that of the original population.

The Bottleneck Effect (2 of 3)

  • Illustrative figure (Figure 23.10) details the bottleneck effect's impact on population genetics.

The Bottleneck Effect (3 of 3)

  • Insights into the bottleneck effect can enhance understanding of the impact of human-driven ecological change on other species.

Case Study: Impact of Genetic Drift on the Greater Prairie Chicken

  • Historical distribution of the Greater Prairie Chicken across Canadian and US prairies.

  • Habitat destruction resulted in significant population decline, leading to a reduction in genetic variation; with only 50% of offspring hatching successfully.

Genetic Drift and Loss of Genetic Variation (1 of 2)

  • The Greater Prairie Chicken faced extinction in Canada, with remaining populations surviving in the US.

Genetic Drift and Loss of Genetic Variation (2 of 2)

  • Utilizing preserved DNA from museum specimens, researchers compared genetic variation pre- and post-bottleneck, revealing allelic loss at multiple loci.

  • Introductions of individuals from other states resulted in new alleles and improved hatch rates exceeding 90%.

Effects of Genetic Drift: A Summary

  1. Genetic drift significantly impacts small populations.

  2. Genetic drift can cause non-systematic changes in allele frequencies.

  3. Genetic drift contributes to the loss of genetic variation.

  4. It has the potential to fix harmful alleles over time.

Gene Flow (1 of 2)

  • Gene flow involves the transfer of alleles among populations.

  • Movement of fertile individuals or gametes, such as pollen, accomplishes allele transfer.

  • Gene flow typically diminishes inter-population variability over time.

  • It may adversely affect population fitness by introducing non-adaptive alleles.

Gene Flow and Local Adaptation (1 of 3)

  • Variance in Lake Erie water snakes banding represents adaptations to mainland and island environments.

  • Non-banded snakes achieve a superior survival rate due to better camouflage than banded counterparts, demonstrating local adaptation insights.

Gene Flow and Local Adaptation (2 of 3)

  • Figure 23.12 depicts the interaction between gene flow and local adaptations exemplified in the Lake Erie water snake (Nerodia sipedon).

Gene Flow and Local Adaptation (3 of 3)

  • Gene flow may enhance fitness within populations; for instance, alleles conferring insecticide resistance have emerged in some mosquito populations, influencing fitness positively.

Gene Flow (2 of 2)

  • A key driver of evolutionary change in human populations is gene flow.

Concept 23.4: Natural Selection as the Sole Consistent Mechanism for Adaptive Evolution

  • Evolution through natural selection is governed by both chance and systematic selection:

    • Emergence of new genetic variations occurs randomly.

    • Beneficial alleles are systematically favor.

  • Natural selection reliably elevates the frequencies of alleles conferring reproductive advantages, catalyzing adaptive evolution.

Natural Selection: A Closer Look

  • Natural selection drives adaptive evolution by targeting the phenotypes of organisms.

Relative Fitness (1 of 2)

  • Terms like “struggle for existence” and “survival of the fittest” can be misleading due to their implication of direct competition.

  • Reproductive success has various subtleties influenced by numerous factors.

Relative Fitness (2 of 2)

  • Relative fitness signifies an individual's contribution to the next generation’s gene pool compared to others.

  • Natural selection favors specific genotypes through phenotypic expression.

Directional, Disruptive, and Stabilizing Selection

  • There exist three primary modes of selection:

    • Directional selection favors organisms on one phenotypic extrema.

    • Disruptive selection promotes phenotypes at both extremes of a range.

    • Stabilizing selection preserves intermediate phenotypes, culling extremes.

Modes of Selection

  • Illustrative examples illustrate how directional, disruptive, and stabilizing selections modify phenotypic distributions in varied contexts, emphasizing adaptation.

The Key Role of Natural Selection in Adaptive Evolution (1 of 3)

  • Distinct adaptations like the rapid color change in some octopuses signify successful natural selection processes.

Adaptation: Movable Jaw Bones in Snakes

  • Figure 23.14 highlights the evolutionary adaptation of snake jaws allowing for the ingestion of large prey.

The Key Role of Natural Selection in Adaptive Evolution (2 of 3)

  • Natural selection amplifies frequencies of alleles that improve survival and reproductive success.

  • Adaptive evolution intensifies the fit between a species and its environment, marking it as a continuous process in response to environmental changes.

The Key Role of Natural Selection in Adaptive Evolution (3 of 3)

  • Unlike natural selection, genetic drift and gene flow do not consistently engender adaptive evolution, but can induce variabilities in organism-environment fit.

The Evolution of Populations (3 of 3)

  • A contextual discussion point regarding peppered moths illustrates phenotypical responses to industrial habitat alterations, questioning the selective pressures at play.

Sexual Selection (1 of 3)

  • Sexual selection identifies the process wherein individuals with specific inherited traits are more apt to secure mates compared to peers of the same sex.

  • This can culminate in sexual dimorphism, showcasing marked differences between sexes in secondary sexual traits.

Sexual Selection (2 of 3)

  • Intrasexual selection represents direct competition among same-sex individuals for access to mates.

  • Intersexual selection, also known as mate choice, arises as one sex (typically females) exhibits preferences that can influence traits within the opposing sex.

  • For instance, male ornamentation may amplify attractiveness, yet can also elevate predation risks.

Sexual Selection (3 of 3)

  • Evolutionary basis of female preferences leading to trait selection is probed, with the good genes hypothesis positing that traits of male health are linked to increased reproductive success for both parties, reinforcing trait propagation.

Inquiry: Do Females Select Mates Based on Traits Indicative of “Good Genes”?

  • Female grey tree frogs exhibit a preference for males with longer mating calls, as call duration correlates with genetic quality, thereby acting as an indicator of fitness.

Balancing Selection

  • Maintenance of genetic variation through diploidy permits recessive alleles to evade selection in heterozygotes.

  • Balancing selection encompasses mechanisms like heterozygote advantage and frequency-dependent selection that stabilize the prevalence of multiple phenotypes within a population.

Frequency-Dependent Selection

  • In this selection paradigm, the fitness of a phenotype diminishes when it becomes overly prevalent.

  • Whichever form is rarer is typically favored, as highlighted in scale-eating fish where both phenotypes are maintained at equilibrium.

Frequency-Dependent Selection in Scale-Eating Fish (Perissodus Microlepis)

  • Figure 23.17 depicts the ecological dynamics of frequency-dependent selection among scale-eating fish.

Heterozygote Advantage (1 of 2)

  • Heterozygote advantage occurs when heterozygotes display a higher relative fitness compared to homozygotes, promoting the retention of multiple alleles at a locus.

Heterozygote Advantage (2 of 2)

  • The sickle-cell allele serves as a notable case where mutation impacts hemoglobin production yet confers malaria resistance; selection favors heterozygous individuals in malaria-endemic regions due to their survival advantage.

Why Natural Selection Cannot Fashion Perfect Organisms

  1. Selection operates only on existing variations.

  2. Historical constraints limit evolutionary pathways.

  3. Adaptations often represent compromises between different evolutionary pressures.

  4. The interaction between chance, natural selection, and environment constrains perfect adaptation.

Selection can act only on existing variations

  • A figure 23.19 illustrates how solely pre-existing variations in a population limit adaptive potential.