Population Change and Natural Selection

Mechanisms of Population Change

• Populations are affected by natural selection.
• Three main mechanisms causing changes in allele frequencies are natural selection, genetic drift, and gene flow.

Microevolution

• Microevolution is the change in allele frequencies within a population over generations.

Natural Selection

• Natural selection consistently causes adaptive evolution.
• It's a prerequisite for evolution by natural selection that traits are heritable.

Mendel's Pea Experiments

• Mendel's pea experiments demonstrated discrete hereditary units (genes).

Genetic Variation

• Variation among individuals is caused by differences in genes or other DNA segments.
• Genetic variation is necessary for a population to evolve, but it doesn't guarantee evolution will occur.

Population Definition

• A population is a localized group of individuals capable of interbreeding and producing fertile offspring.
• The gene pool consists of all alleles for all loci in a population.
• A locus is fixed when all individuals in a population are homozygous for the same allele.

Hardy-Weinberg Equilibrium

• The Hardy-Weinberg equilibrium describes the genetic composition expected in a non-evolving population at a particular locus.
• If a population's genetic composition differs from Hardy-Weinberg expectations, the population is evolving.
• In a Hardy-Weinberg equilibrium, gametes contribute to the next generation randomly, Mendelian inheritance occurs, and allele and genotype frequencies remain constant over generations.
• Natural populations can be evolving at some loci while remaining in Hardy-Weinberg equilibrium at other loci.

Factors Altering Allele Frequencies

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

Adaptive Evolution

• Adaptive evolution is a process wherein characteristics increasing survival or reproduction become more frequent over time; caused by natural selection.

Genetic Drift

• Genetic drift describes how allele frequencies fluctuate unpredictably from one generation to the next.
• Genetic drift is random.
• It tends to reduce genetic variation by randomly losing alleles.
• Significant in small populations.
• Can cause random changes in allele frequencies.
• Can lead to loss of genetic variation within populations.

Founder Effect

• The founder effect occurs when a few individuals isolate themselves from a larger population.

Bottleneck Effect

• A bottleneck effect occurs when a population drastically shrinks due to a sudden environmental change.

Gene Flow

• Gene flow is the movement of alleles between populations.
• Gene flow tends to reduce variation between populations over time.
• Gene flow can also increase the fitness of a population.

Molecular Evolution

• Molecular evolution is the process of change in the sequence composition of macromolecules (DNA, RNA, proteins) over generations.

Evolutionary History

• Darwin (1859) provided evidence for evolution, with natural selection as the mechanism.
• Rediscovery of Mendel's laws and development of chromosome theory (1900-1916).
• Origin of population genetics (alleles, genotype frequencies) (Hardy-Weinberg, 1908).
• Mathematical bases of population and evolutionary genetics (Fisher, Wright, Haldane, 1920s, 1930s).
• Emergence of molecular biology (1960s, 1970s).

Phylogenetics

• Phylogeny is the evolutionary history of a species or group, especially its lines of descent.
• A phylogenetic tree is a diagrammatic hypothesis of relationships that reflects the evolutionary history of an organism group.

Homology vs. Homoplasy

• Homology is the similarity between species due to a common ancestor (genes, proteins, anatomical structures). Shared ancestry.
• Homoplasy is similarity that arises by chance or convergent evolution; misaligned alignments. Mathematical tools help distinguish these.

Cladistics and Phylogenetic Trees

• A shared derived character is a novel evolutionary trait (characteristic) unique to a particular clade, inherited from a common ancestor; define monophyletic groups that include all descendants of that ancestor.
• A shared ancestral character is a characteristic that originated in an ancestor of the taxon.
• A polyphyletic group includes distant species but not their most recent common ancestor.
• A paraphyletic group is the ancestral species and some, but not all, of its descendants.
• A monophyletic group is the ancestral species and all its descendants; a valid clade.

Molecular Clock

• The molecular clock uses constant rates of evolution in some genes to estimate the absolute time of evolutionary change.
• It's calibrated based on branches whose dates are known from the fossil record.

Orthologous vs. Paralogous Genes

• Orthologous genes are found in a single copy in the genome and are homologous between species; can diverge only after speciation.
• Paralogous genes result from gene duplication, so they are found in more than one copy in the genome; can diverge within the species carrying them, and often evolve new functions.

Heredity and Environment

• Heritability estimates the genetic influence, measuring how much genetic differences explain phenotype variations; applicable to populations and their variance (a statistic describing a population at a given time), and cannot predict.
• Genes determine physical and behavioral characteristics.
• "Nature" (genes) has the largest influence on who we are.
• "Nurture" (environmental variables) plays a more significant role, including upbringing, individual experiences, and social relationships.
• An inherited trait is determined genetically; passed from parents to offspring according to Mendelian genetics.

Human Genome

• The human genome is the total DNA of an organism; has about 3 billion base pairs.
• Only 3% of the human genome codes for proteins.
• 40-50% of the human genome contains repetitive sequences.

Human Chromosomes

• Humans have 23 pairs of chromosomes.

GWAS Studies

• GWAS studies collect data to find common variants in individuals with and without a specific trait (e.g. a disease) across the genome, using genome-wide SNP arrays.
• Variants associated with the disease or within the same haplotype as a disease-associated variant will be found at a higher frequency in cases (affected individuals) than in controls (unaffected individuals).
• Statistical analyses are carried out to indicate how likely a variant is to be associated with a trait.

Sexual vs. Asexual Reproduction

• Prokaryotes reproduce sexually and asexually.
• Eukaryotes reproduce sexually and asexually.

Gametes

• A gamete is a haploid cell that fuses with another haploid cell during fertilization in sexually reproducing organisms.
• An ovum is substantially larger than a sperm cell.

Sexual Reproduction Types

• Isogamy is the state where gametes of both sexes have the same size and form, arbitrarily designated mating types.
• Anisogamy is the state where two types of gametes differ in size, structure, and/or function.

Genetic Transfer in Prokaryotes

• Conjugation is the unidirectional transfer of genetic material (plasmids) from a donor to a recipient via a conjugation tube called a sex pilus.
• Transformation is the uptake of extracellular DNA from the environment that is integrated into the bacterial genome.
• Transduction is the transmission of genetic material from one bacterial cell to another by a viral vector.

Asexual Reproduction

• Asexual reproduction is when an organism reproduces without the involvement of another organism; creates a genetically similar or identical copy of itself.
• Multicellular organisms can perform asexual reproduction (yes).

LECA (Last Eukaryotic Common Ancestor)

• The LECA became sexually reproducing during evolution.

Outcrossing

• Outcrossing is the process of mating between unrelated individuals, typically of different genetic lines or populations; increases genetic diversity within a species.

Human Sex Determination

• Genes, chromosomes: heredity.
• Hormones: chemical effectors of development.
• Reproductive organs: anatomy.

Importance of Sex in Evolution

• When selection changes over time.
• When selection changes in different locations.
• When organisms are less adapted to the environment.
• When populations are smaller.
• Sex and recombination allow genes from different individuals to form new genotype combinations that selection can act upon.

Sexual Reproduction Efficiency

• Sexual reproduction is less efficient than asexual reproduction.

Theory Characteristics

• Broader in scope than a hypothesis.
• General, can lead to testable new hypotheses.
• Supported by a larger body of evidence compared to a hypothesis.

Theoretical Knowledge vs. Empirical Knowledge

• Theoretical knowledge is abstract, not practically applicable, lacks supporting evidence, and typically lacks real-world application.

Australopithecus Characteristics

• Lived in a variety of habitats (forests to savannas).
• Fully bipedal.
• Lost key characteristics that help primates climb trees.
• Jaws and teeth adapted for eating tough, plant-based foods.
• Diet probably included fruits, leaves, possibly roots, tubers, insects, and small animals.

Homo Characteristics

• Large brains, small teeth and jaws.
• Tools for processing a wide variety of plant and animal foods.

Neanderthal and Human Evolution

• Neanderthals and modern humans evolved from a common ancestor (Yes, evidence).

Neanderthal and Modern Human Coexistence

• Neanderthals and modern humans coexisted in Western Asia for 30,000-50,000 years.

Homo Sapiens Origin

• Homo sapiens first evolved in Africa 315,000 years ago.

Homo Sapiens Migration

• Homo sapiens migrated out of Africa 65,000 years ago.

Human Key Characteristics

• Major anatomical and ecological changes: increased brain size, hunting and gathering, increasingly complex stone tools, bigger body size.
• Major physiological changes: adaptation to exercise, dependence on physical activity for survival.

Theories on Human Evolution

• Savanna hypothesis: Hominins were forced out of trees to expanding savannas → started walking upright →savanna expansion due to drier conditions → hominin adaptation.
• Cooking hypothesis: Cooking improves food quality, making it softer (smaller teeth) and easier to digest (smaller intestines); provided more energy than raw foods.
• Social brain hypothesis: Improvements in cognitive abilities allowed hominins to improve social behavior and group cooperation.
• Expensive tissue hypothesis: Cooking breaks down plant fibers (cellulose and lignin) and connective tissues in meat (collagen), allowing early humans to extract more nutrients with less digestive effort. → reduced energetically expensive intestinal tissue. → smaller intestines are cheaper energetically, allowing calories to be directed to the brain, enlarging its size.

Bipedalism Advantages

• Freed hands for manual work and less strenuous movement.
• Allowed long-distance travel and hunting.
• Wider field of vision.
• Reduced area of exposed skin to sun.

Skeletal Changes from Bipedalism

• Changes in legs, knee and ankle joints.
• Changes in spinal vertebrae, toes, and arms.
• Pelvis became shorter and more rounded.

Bipedalism Impact on Childbirth

• Smaller birth canal, making childbirth more difficult.
• Shorter gestation period.
• Babies are more helpless and less developed at birth.

Encephalization Trends

• Encephalization began in Homo habilis and continued through the Neanderthal line (1500 cm³ capacity).

Patterns in Human Evolution

• Sharing food.
• Bipedalism.
• Cooperation and division of labor.
• Adaptation to exercise.
• Propensity to store fat.
• Faster metabolism.
• Dietary shift → cooking → larger brain.

Brain Structure

• Gray matter is primarily responsible for processing and interpreting information; composed of neuron clusters.
• White matter transmits information to other parts of the nervous system; composed of myelinated axons.

Cerebrum (Brain)

• The cerebrum comprises gray matter (cerebral cortex) and white matter in its center; the largest part of the brain.
• Coordinates movements.
• Regulates temperature.
• Speech.
• Judgment.
• Thought and reasoning.
• Problem-solving.
• Emotions and learning.
• Senses

Neocortex

• The neocortex is a set of mammalian cerebral cortex layers involved in higher brain functions like sensory perception, cognition, motor command generation, spatial reasoning, and language.
• Humans have a 60:1 ratio; chimpanzees have a 30:1 ratio.

Reptilian Complex

• Instinctive behavior, aggression, and dominance.

Explore the mechanisms of population change, including natural selection, genetic drift, and gene flow. This quiz covers fundamental concepts of microevolution, genetic variation, and Mendel's experiments, emphasizing their significance in understanding evolution. Test your knowledge on how populations evolve over generations.