Crush AP Bio Unit 7: Evolution

Introduction to AP Bio Unit 7: Evolution

Overview of Topics

• The unit covers the extensive timeline of biological evolution, focusing on how change occurs over time.

• Key topics include natural selection, artificial selection, sexual selection, population genetics, and Hardy-Weinberg equilibrium.

• The course will explore evidence for evolution and the process of speciation—how species diverge into multiple descendant species.

• Discussion includes both small-scale extinctions (single species) and mass extinctions caused by geological or astronomical events.

Instructor Introduction

• Glenn Wolkenfeld (Mr. W), a retired AP biology teacher, introduces himself and expresses his passion for teaching biology.

• A downloadable checklist is available at apbio succombs for students to aid their study.

Understanding Selection in Evolution

Types of Selection

Artificial Selection

• Defined as selective breeding where breeders select organisms with desired traits over generations.

• Example: Brassica oleracea has been bred into various forms like cauliflower and broccoli based on specific traits.

Natural Selection

• Involves inherited variation within populations that arises from recombination and mutation.

• The concept "many are born but few survive" summarizes the survival advantage of individuals with beneficial traits leading to adaptation.

Sexual Selection

• Focuses on traits that enhance reproductive success, resulting in sexual dimorphism between males and females.

Mechanisms of Sexual Selection

Intersexual vs. Intrasexual Selection

Intersexual Selection

• Females choose mates based on attractive traits displayed by males; example includes male turkeys displaying tail feathers to attract females.

Intrasexual Selection

• Competition among males for access to females or territory leads to aggressive behaviors; example includes large elephant seals competing for dominance.

Distribution of Phenotypes in Populations

Types of Phenotypic Distribution

Directional Selection

• Selective pressure against one extreme phenotype shifts the population towards another trait variant.

Stabilizing Selection

• Favors average phenotypes while selecting against extremes; an example is birth weight where average-sized babies have higher survival rates.

Disruptive Selection

Understanding Directional Selection and Adaptive Melanism

Directional Selection and Adaptive Melanism

• Directional selection can occur when the average phenotype is maladaptive, leading to a split in the population towards two extremes.

• Adaptive melanism refers to the darkening of body coloration in response to environmental changes, evolving over time within a gene pool due to natural selection.

• The rock pocket mouse serves as an example where populations on dark substrates evolved mutations for increased melanin production, resulting in darker coloration.

Evolutionary Fitness

• Evolutionary fitness is measured by the number of offspring that survive to reproduce, rather than physical strength or speed.

• Fitness is observed at every life cycle stage of organisms, such as penguins during feeding and breeding.

Case Study: The Peppered Moth

• The peppered moth illustrates directional selection and adaptive melanism through observable changes in response to environmental shifts.

• Before the Industrial Revolution, light-colored moths were predominant due to camouflage against light tree trunks; this changed with pollution darkening tree surfaces.

• As soot from factories covered trees, dark-colored moths gained a selective advantage, shifting the population's mean phenotype from light to dark during industrialization.

Historical Reversal of Selection

• In the 1960s, pollution control measures led to lighter tree trunks re-emerging; thus, the mean phenotype of peppered moth shifted back from dark to light colors.

• Research by Michael Majerus confirmed these observations as clear examples of adaptive melanism linked with environmental change.

Introduction to Population Genetics

Understanding Population Genetics

• Population genetics studies how genes are distributed within populations and how they evolve over time; key measurements include allele frequency.

• A gene pool encompasses all alleles present in a population; evolution reflects changes in allele frequencies over time.

Common Misconceptions

• A prevalent misconception is that dominant alleles must be more common than recessive ones; however, frequency depends on advantages conferred by alleles or random historical factors.

Understanding Allele Frequencies and Evolutionary Change

The Hardy-Weinberg Principle

• The equation p + q = 1 represents the frequencies of dominant (p) and recessive (q) alleles in a population, where p^2 + 2pq + q^2 = 1 describes genotype frequencies.

• In a sample problem, if 49% of mice have the recessive trait, then q^2 = 0.49 . Taking the square root gives q = 0.7 , leading to p = 0.3 .

• From these values, we can calculate heterozygote frequency as 2pq = 42% , and homozygous dominant frequency as p^2 = 9% .

Conditions for Hardy-Weinberg Equilibrium

• The Hardy-Weinberg principle states that allele frequencies remain constant unless one of five conditions is violated: infinite population size, no selection (harmful or beneficial alleles), random mating, no immigration/emigration, and no net mutation.

• If any condition is not met, allele frequencies can change due to factors like genetic drift, natural selection, sexual selection, gene flow, or directional mutation.

Genetic Drift and Population Bottlenecks

• Genetic drift refers to random changes in allele frequencies typically seen in small populations; it can lead to significant shifts over time.

• A population bottleneck occurs when a large portion of individuals are wiped out by an event (biotic or abiotic), leaving few survivors whose alleles may not represent the original population's diversity.

• Cheetahs exemplify this phenomenon; they show low genetic diversity likely due to a historical bottleneck event that drastically reduced their numbers.

Founder Effect

• The founder effect happens when a small group from a larger population establishes a new one. This limited sampling can result in different allele frequencies compared to the parent population.

• An example illustrates how certain alleles may be lost entirely in new populations founded by only a few individuals.

Gene Flow and Its Impact on Populations

• Gene flow involves the movement of alleles between populations through individual migration or gamete transfer (e.g., pollen).

• It alters allele frequencies in recipient populations and reduces differences between adjacent groups.

Mutation's Role in Evolution

• Mutations are crucial as they introduce genetic variation within populations; directional mutations can shift allele frequencies significantly.

Heterozygote Advantage and Evolutionary Evidence

Heterozygote Advantage in Malaria Resistance

• The homozygous dominant genotype (big S big S) is selected against due to its susceptibility to malaria.

• The heterozygous genotype (big S little s) is favored as it provides protection against malaria without the symptoms of sickle cell anemia.

• Homozygous recessive (small s small s) individuals are selected against because they suffer from sickle cell disease.

• A correlation exists between the frequency of the little s allele and malaria intensity across various African regions.

Evidence of Evolution: Homologous Traits

• Homologous traits share a common structure and embryological origin, indicating descent with modification from a common ancestor.

• Examples include forelimb bones in humans, dogs, birds, and whales that have similar structures but evolved differently through natural selection.

Adaptive Radiation

• Adaptive radiation occurs when one ancestral species diversifies into multiple descendants with unique adaptations for different ecological niches.

• Galapagos finches illustrate this concept; they evolved distinct beak shapes from a common ancestor to exploit various food sources.

Vestigial Structures as Evolutionary Evidence

• Vestigial structures are inherited features that no longer serve their original function, providing evidence for evolution.

• Whales possess pelvic bones from ancestors with hind limbs; humans have a tailbone homologous to tails in other mammals.

Homology vs. Analogy

• Homologous features arise from shared ancestry while analogous features evolve independently through convergent evolution.

• Bird wings and bat wings are analogous due to similar functions but differ structurally; however, their forelimbs are homologous as they derive from a common ancestor.

Molecular Homologies

• Molecular homologies indicate common ancestry at the molecular level through similarities in structure and sequence among molecules like hemoglobin across vertebrates.

• Differences in amino acid sequences among species reflect evolutionary relationships; closer relatives show fewer differences.

Pseudogenes: Nonfunctional Genes as Evolutionary Remnants

• Pseudogenes are nonfunctional variants of functional genes found in related species, exemplifying descent with modification.

Evidence for Evolution: Homologies and Biogeography

Common Ancestry and Genetic Evidence

• The sequence of pseudogenes shows different mutations, indicating convergent features that are analogous rather than homologous. This suggests a shared ancestry among living organisms.

• Key features supporting common ancestry include DNA as genetic material, ATP for energy coupling, a universal genetic code, ribosomes for protein synthesis, and shared metabolic pathways like glycolysis.

• Eukaryotes share ancient traits such as nuclei, mitochondria derived from endosymbiosis, linear chromosomes, and sexual reproduction involving gamete fusion.

Embryological Development as Evidence

• Early vertebrate embryos exhibit similarities (e.g., fish, reptiles, birds, humans), suggesting a common ancestor through embryonic development leading to diverse adult forms.

• Vestigial features in embryos (like human tails and gill slits) indicate descent with modification from a common ancestor.

Shared Genes in Animal Development

• Certain genes control developmental processes across species separated by millions of years. For example, the eyeless gene can induce eye development in both fruit flies and mice.

• Homeotic genes dictate body plan organization across various animal groups (arthropods to vertebrates), reinforcing the idea of a shared evolutionary history.

Understanding Biogeography

• Biogeography studies the geographic distribution of species. It reveals patterns where populations evolve in one area before spreading to adjacent regions.

• A notable example is marsupials predominantly found in Australia due to geographical isolation preventing placental mammals from populating the continent.

Parallel Evolution in Isolated Ecosystems

• In Australia, marsupials fill ecological niches similar to placental mammals elsewhere. Examples include convergent evolution seen between marsupial moles and Eastern moles or sugar gliders and flying squirrels.

Evidence for Evolution

Understanding Fossils as Evidence

• Fossils are petrified remains of living organisms, providing evidence for evolution by demonstrating changes over time.

• Transitional forms in fossils illustrate the descent with modification, showing how modern species evolved from ancestral forms, such as whales evolving from land mammals.

Dating Fossils: Relative and Absolute Methods

• Relative dating is based on the principle of superposition; younger sedimentary layers are found above older ones, allowing scientists to determine the relative ages of fossils.

• Absolute dating utilizes radioactive isotopes' decay rates (e.g., Carbon-14 to Nitrogen-14), measuring half-lives to establish precise ages of fossils.

Observing Evolution Today

Resistance in Mosquitoes

• The evolution of DDT resistance in mosquitoes exemplifies ongoing evolutionary processes; initially effective, DDT becomes less so as resistant individuals survive and reproduce.

• This rapid genetic change occurs due to short generation times in mosquitoes, leading to a significant increase in resistance within months.

Broader Implications of Resistance

• Similar patterns of resistance have been observed across various species, including bacteria (antibiotic resistance), weeds (herbicide resistance), and cancer cells (chemotherapy resistance).

Speciation and Extinction Concepts

Biological Species Concept

• The biological species concept defines a species as groups that can interbreed naturally to produce viable and fertile offspring while being reproductively isolated from other groups.

Limitations of the Biological Species Concept

• Limitations include cases where closely related species hybridize or when assessing extinct or asexual species where reproductive viability cannot be determined.

Reproductive Isolating Mechanisms

Speciation Mechanisms and Their Implications

Prezygotic Isolating Mechanisms

• Prezygotic barriers prevent breeding altogether, stopping the formation of a zygote. These mechanisms ensure that gene pools remain separate.

• Temporal isolation occurs when species breed at different times (e.g., one in winter, another in summer), preventing mating opportunities.

• Mechanical isolation involves structural differences that hinder sperm or pollen from reaching an egg, such as incompatible flower structures.

• Habitat isolation arises when species occupy different environments (e.g., one in forests and another in meadows), reducing chances of encounter for mating.

• Gametic isolation prevents fertilization due to molecular mismatches between sperm and egg, inhibiting successful fertilization.

Postzygotic Isolating Mechanisms

• Postzygotic barriers exist between species that can mate but produce unviable or sterile offspring, maintaining separate gene pools.

• Hybrid inviability occurs when hybrid organisms fail to develop properly or survive to maturity.

• Hybrid sterility results in healthy hybrids (e.g., mules from horses and donkeys), but they cannot reproduce successfully.

• Hybrid breakdown leads to viable hybrids that can reproduce; however, their subsequent generations are often inviable or infertile.

Modes of Speciation

Allopatric Speciation

• Allopatric speciation is driven by geographical barriers leading to reproductive isolation. Initially, a species has gene flow across its range until a barrier splits it into isolated populations.

• Environmental differences create distinct selective pressures on each population, resulting in genetic differentiation over time.

Sympatric Speciation

• Sympatric speciation occurs without geographical barriers; it can arise through mechanisms like polyploidy in plants which causes instant reproductive isolation.

• An example includes cichlid fish in Lake Victoria where sexual selection has led to reproductive isolation among subspecies.

Microhabitat Adaptation and Speciation

• Adaptation to specific microhabitats can lead to speciation; for instance, lice evolving on different parts of birds demonstrate specialization without geographic separation.

Adaptive Radiation

Concept Overview

• Adaptive radiation describes how one ancestral species diversifies into multiple descendant species with unique adaptations filling various ecological niches.

• The Galapagos finches exemplify this concept as they evolved from a single South American ancestor into numerous species adapted to different environments.

Evolutionary Evidence

• Homologous and vestigial traits provide evidence for evolution through adaptive radiation by showing descent with modification from common ancestors.

Importance of Phenotypic Variation

Natural Selection and Phenotypic Variation

The Role of Natural Selection

• Natural selection acts on phenotypes, not genotypes; advantageous phenotypes lead to higher survival and reproduction rates.

• Without phenotypic variation, natural selection cannot occur, making loss of variation dangerous for species.

Importance of Phenotypic Variation in Adaptation

Phospholipid Structure in Browsing Mammals

• Variations in phospholipid structure help mammals adapt to snowy environments by maintaining membrane fluidity at low temperatures. More saturated tails are found in the body core, while unsaturated tails are present in extremities.

Hemoglobin Variations for Oxygen Absorption

• Fetal hemoglobin has a higher affinity for oxygen than adult hemoglobin, facilitating oxygen transfer from maternal blood to fetal blood during development. This transition occurs post-birth as beta chains replace gamma chains.

Chlorophyll Types and Photosynthesis Efficiency

• Green plants possess chlorophyll A and B; variations allow them to maximize light absorption under different conditions (shade vs direct light). This adaptability enhances photosynthesis efficiency across seasons.

Extinction: Processes and Implications

Understanding Extinction Vortex

• Extinction is a normal part of life; over 99% of species that have ever lived are extinct. The extinction vortex begins with population decline due to environmental changes or competition, leading to genetic drift and reduced fitness. This creates a feedback loop accelerating extinction risk.

Mass Extinctions vs Normal Extinction Processes

Understanding Mass Extinctions and Phylogenetics

The Impact of Mass Extinctions

• A mass extinction event leads to the loss of biological diversity, followed by adaptive radiation in surviving species.

• An example is the diversification of placental mammals after the Cretaceous Extinction, which eliminated dinosaurs.

• Human activity is now causing a sixth extinction, comparable to previous mass extinctions due to habitat destruction and overexploitation.

Causes of Current Extinction Rates

• Key human activities contributing to extinction include:

• Habitat destruction and fragmentation.

• Overhunting and overharvesting of species.

• Introduction of invasive species into new habitats.

Understanding Phylogeny

• Phylogeny refers to evolutionary history; phylogenetic trees illustrate these relationships based on morphological, molecular, or genetic evidence.

• Evidence shows that hippos and whales share a more recent common ancestor than either does with deer.

Clades and Shared Derived Characters

• A clade consists of a common ancestor and all its descendants; for instance, all humans form a clade due to their shared ancestry.

• Shared derived characters are traits that distinguish a clade from others, such as lungs in certain vertebrates.

Nodes, Sister Groups, and Outgroups

• Nodes represent points where branches diverge in phylogenetic trees; they indicate common ancestors for those lineages.

• Sister groups are taxa that split from the same node; for example, the common cactus finch and large ground finch are sister species.

Understanding Phylogenetic Trees and the Origin of Life

The Structure of Phylogenetic Trees

• A horizontal tree indicates evolutionary closeness, but proximity on the tree (e.g., frogs and lizards) does not necessarily reflect close relatedness; only recency of common ancestry matters.

• Nodes in phylogenetic trees can rotate without changing the relationships depicted; this means that different arrangements can represent the same evolutionary connections.

• Visualizing phylogenetic trees as kinetic sculptures helps understand their flexibility; always focus on recency of common ancestry to determine relationships accurately.

Ancestral Features and Evidence for Relationships

• An ancestral feature is a trait shared by members of a clade that also appears in larger inclusive clades, such as claws or nails in mammals.

• Morphological similarities were historically used to construct phylogenetic trees, but since the 1960s, molecular evidence like DNA sequences has become crucial for determining relationships.

Molecular Clocks and Evolutionary Timing

• Molecular clocks measure mutation rates in proteins or nucleic acids over time, allowing scientists to estimate when species diverged based on genetic changes.

• For example, if 40 amino acid substitutions are observed between two species' hemoglobin, it suggests they split approximately 230 million years ago.

Exploring the Origin of Life

Key Questions About Life's Emergence

• Understanding how life emerged naturally involves addressing how the first cell formed from non-living materials and what chemical processes led to biological complexity.

Steps Leading to Life's Emergence

• Earth needed stabilization after its formation about 4.5 billion years ago before life could begin; early conditions were hostile due to asteroid impacts.

• Chemistry leading to biology required abiotic synthesis of monomers (like amino acids), which then combined into polymers necessary for cellular structures.

Formation of Early Cellular Structures

• The emergence of vesicles was essential for creating proto-cells—early forms that exhibited some characteristics of living cells but weren't fully functional yet.

Final Steps Toward Living Cells

The Miller-Urey Experiment and the Origin of Life

Overview of the Miller-Urey Experiment

• The experiment conducted in the 1950s demonstrated that amino acids could be synthesized in a controlled, abiotic environment simulating early Earth conditions. Despite some inaccuracies in its setup, it served as a crucial proof of concept for life's building blocks.

• The apparatus included a chamber representing early oceans heated to produce steam, which circulated through tubes into an atmosphere chamber filled with gases like methane, ammonia, hydrogen, and water vapor—reflecting the then-current understanding of Earth's primordial atmosphere.

• Notably, oxygen was absent from this setup since it was believed to have been produced by photosynthesis later on. Electrodes were used to create sparks mimicking lightning, facilitating chemical reactions among the gases.

• After sampling the liquid from the apparatus over time, Stanley Miller discovered amino acids present in the solution. This finding indicated that essential monomers for life could form without pre-existing life.

• Although some details of the experiment were incorrect based on modern geology, it laid groundwork for future studies exploring various inorganic catalysts and gas mixtures leading to other biological molecules like nucleotide bases and fatty acids.

Understanding Heredity: The Role of RNA

• To comprehend life's origin fully, one must also explore heredity's beginnings. It is widely accepted that RNA—not DNA—was likely the first hereditary molecule due to its ability to store genetic information and act as an enzyme.

• RNA's dual functionality allows it not only to carry genetic information (as seen in viruses) but also to catalyze reactions within ribosomes where proteins are synthesized from amino acids.

• The hypothesis suggests there existed self-replicating systems composed solely of RNA before cellular life emerged. This phase is referred to as the "RNA world," leading up to what we recognize as common ancestors of all life forms today.

Evolutionary Pathway Towards Cellular Life

• Initially formed inorganic precursor molecules would combine through abiotic processes into RNA monomers. These monomers would further evolve into polymers capable of folding into complex shapes with enzymatic properties.

• Eventually, these self-replicating RNA systems may have become encapsulated within lipid bilayers forming proto-cells—a significant step towards cellular organization and complexity.

• Natural selection would drive further evolution resulting in diverse organisms leading ultimately to archaea and bacteria; these domains eventually fused giving rise to eukaryotes.

Key Components Identified in Early Life Forms

• A diagram illustrates critical components necessary for cellular function:

• Lipid Bilayer: Essential for cell membrane structure.

• DNA: Serves as genetic material across living organisms.

• RNA: Functions in information transfer and catalysis.

• Ribosomes: Translate mRNA into proteins.

• Membrane Channels: Regulate matter/energy flow across membranes.