nbb unit2

Evolution (simple definition)

Change in allele frequencies in a population over time. It requires a population of breeding individuals and genetic change

Do individuals evolve?

No. Populations evolve. Individuals do not evolve genetically during their lifetimes

Allele

An alternative version of a gene (e.g. A vs a)

Allele frequency

The proportion of all copies of a gene in a population that are a given allele (e.g. % A vs % a)

Genotype frequency

The proportion of individuals in the population with each genotype (AA, Aa, aa)

Genotype vs phenotype

Genotype = the genetic makeup (alleles carried). Phenotype = the observable trait or behavior produced by genotype and environment

Gene pool

All the genetic material (all alleles) within a population

How to calculate genotype frequencies

Count individuals of each genotype and divide by total individuals. Example: 2 AA, 6 Aa, 2 aa out of 10 gives AA 0.2, Aa 0.6, aa 0.2

How to calculate allele frequencies

Count allele copies and divide by 2N. A = (2×AA + Aa) / 2N. Example: (2×2 + 6)/20 = 0.5, so a = 0.5

Practice: 5 AA, 8 Aa, 2 aa. Find p (A) and q (a)

N = 15, so 30 alleles. A = (10 + 8)/30 = 0.6, a = (4 + 8)/30 = 0.4

Lightning example: after a strike, 4 AA, 8 Aa, 0 aa. New p and q?

24 alleles. A = (8 + 8)/24 = 0.67, a = 8/24 = 0.33. This is a change by chance (genetic drift), not selection

Four forces of evolution

Mutation, gene flow, genetic drift, natural selection. Natural selection is ONE force of evolution, not evolution itself

Mutation

A mistake during DNA replication that permanently changes the DNA sequence. It can be deleterious, beneficial or neutral, and it is the only force that creates NEW genetic variation

Is mutation directed toward what organisms need?

No. Mutation is random with respect to need. Selection sorts among existing heritable variation

Chromosomal alterations

Crossover/nondisjunction errors that produce trisomies (extra chromosome, including sex-chromosome trisomies) or monosomies (e.g. Turner syndrome = missing X)

Gene flow

Movement of alleles between populations through migration. Called "admixture" in humans and "hybridization" in other organisms. It makes populations more similar

Genetic drift

Change in allele frequency due to chance, i.e. random sampling of which alleles get passed to the next generation

Why is drift strongest in small populations?

Random sampling error is larger in small samples, so chance can wipe out or fix alleles (e.g. only 2 aa individuals means one random event can eliminate them)

Founder effect

A special kind of drift. A small group becomes reproductively isolated from the larger population and the new population diverges genetically because the founders carried a non-representative sample of alleles

Huntington's disease, Lake Maracaibo, Venezuela

About half the villagers have Huntington's, all traced to one woman ~200 years ago (Nancy Wexler). A founder effect/drift example. Late onset (after ~40) means carriers reproduce before symptoms

O blood type in South America

High frequency of type O likely reflects the founder effect. As humans migrated out of Africa, founding populations carried less of the original allele variety

Three requirements for natural selection

Variation, heritability, and differential reproductive success (some heritable variants leave more descendants)

Natural selection: random or nonrandom?

Nonrandom with respect to phenotype. The variation arises randomly (mutation), but which variants survive and reproduce is not random

How does differential survival/reproduction change allele frequencies?

Individuals with favorable heritable traits leave more offspring, so their alleles become more common in the next generation

Peppered moth (Biston betularia)

Before 1848 all moths were light. Industrial soot darkened trees, dark moths were better camouflaged from predators, and by 1950 about 90% were dark. A classic case of directional natural selection

Directional selection

Favors one extreme, shifting the trait in one direction (e.g. peppered moth)

Balancing / stabilizing selection

Favors the middle (average) value

Disruptive / diversifying selection

Favors both extreme values over the average

Sexual selection

Selection from differences in mating success (e.g. peacock tail). It can favor traits that reduce survival

Is selection uni-directional?

No. If the environment changes, the direction of selection can change (e.g. Darwin's finch beak size went up during drought and back down when rains returned)

Microevolution vs macroevolution

Micro = changes in allele frequencies within a population. Macro = changes producing new species and differences between species. Same processes, longer timescale

Sickle-cell and malaria (balanced polymorphism)

AA = susceptible to malaria. SS = sickle-cell anemia. AS heterozygotes resist malaria and are healthy, so heterozygotes have the highest fitness and both alleles persist

Adaptation

A trait favored by natural selection BECAUSE of the function being explained. Key question: what evidence shows direct selection for this function?

Exaptation / co-option

A trait that existed first (for another function or no function) and was later co-opted for a new use. Key question: what was its earlier role or origin?

Evolutionary byproduct

A feature that arose as a consequence of something else rather than through direct selection on it. Key question: what other trait or process could generate it?

Spandrels of San Marco (Gould & Lewontin)

Triangular spaces between arches exist because of the dome's architecture, not to hold mosaics. An analogy critiquing overly adaptationist "just-so" explanations: not every trait is an adaptation

Why avoid assuming every trait is an adaptation?

Traits may be byproducts, exaptations, results of drift, constrained by history/development/physics, or mismatched to current environments

Why aren't organisms perfect?

Other forces (drift, etc.) operate, and adaptation is limited by historical/phylogenetic, developmental, physical and genetic constraints plus trade-offs. Selection works with what is available

Historical / phylogenetic constraint

Evolution must modify what was inherited from ancestors. Selection doesn't start from scratch

Developmental constraint

Developmental pathways limit which phenotypes are possible (e.g. a mutation changes skull-formation timing, but only certain skull shapes can be produced because tissues form through interconnected processes)

Physical constraint

The laws of physics and chemistry limit possible adaptations (e.g. volume/mass grows faster than bone cross-section, so large animals need disproportionately thick bones)

Genetic constraint / correlated characters

Selection can only act on available genetic variation, and pleiotropic genes link traits (Darwin's finches: beak depth and width are correlated because the same genes affect both)

Correlated response

Selection on one trait changes another trait because they share genes (pleiotropy), e.g. selecting deeper beaks also widened them

Trade-off

Organisms have limited time, energy and resources, so investing in one function reduces investment in another. You can't maximize everything

Four classic trade-offs

Growth vs reproduction. Current vs future reproduction. Offspring number vs investment per offspring. Mating effort vs parental effort

Life-course energy allocation

Energy is divided among growth, maintenance/repair/immune function, and reproduction/parental investment. Heavy reproduction now can reduce future survival

Mate guarding

Staying with or guarding a mate (especially around estrus/ovulation) to stop rivals mating. Benefit = higher paternity. Cost = can't search for more mates

Soapberry bugs and the sex ratio

Males guard more when males outnumber females (few other mates available) and less when females are plentiful

Soapberry bugs: Oklahoma vs Florida

Oklahoma (variable sex ratios) bugs evolved behavioral flexibility. Florida Keys (stable sex ratio) bugs did not. What evolved in Oklahoma was plasticity itself

Canalized vs plastic behavior

Canalized = genetically fixed, the same response regardless of conditions. Plastic = individuals adjust behavior to conditions

Why does the optimal mate-guarding strategy change?

Its costs and benefits depend on circumstances (e.g. sex ratio, availability of other mates)

Specialist

Exploits a narrow range of resources or conditions. High peak performance there, but vulnerable when conditions change

Generalist

Functions across a broad range of conditions. Tolerates variation but sacrifices peak performance for versatility

Performance curve

Plots performance vs an environmental variable. Specialist = narrow breadth, higher peak. Generalist = broad breadth, lower peak

Environment-dependent fitness

Fitness is not an intrinsic property of a trait. The same phenotype can have different fitness in different environments, so "adaptive" means adaptive under particular conditions

Adaptive landscape

A map of fitness across phenotypes. Height = fitness, peaks = high-fitness phenotypes, valleys = low fitness. There can be multiple peaks, and the landscape depends on the environment

Why does a trait's fitness depend on other traits?

Organisms are interconnected systems. Selection acts on the whole organism, so a trait's value depends on other traits, the environment and its costs

Environmental cues

Information organisms use (light, temperature, food, predators, competitors, social signals). Selection favors responding to cues that reliably predict conditions

Ecological trap

A formerly reliable environmental CUE now directs behavior toward a poor outcome (e.g. mayflies laying eggs on shiny surfaces that look like water)

Evolutionary mismatch

Traits shaped under past conditions are poorly suited to a changed environment because environments change faster than populations evolve

Ecological trap vs mismatch

Trap = a specific cue becomes unreliable and lures behavior to a bad choice. Mismatch = the broader environment changed, so formerly adaptive traits are now neutral or harmful

Mismatch examples

Artificial light disrupting light-cued behavior. Failing to recognize novel predators. Human preference for energy-rich food when calorie-dense food is abundant

Habitat vs niche

Habitat = where an organism lives. Niche = how it makes its living: resources, conditions, predators, competitors, other organisms and behavior

Fundamental niche

The full range of conditions and resources where a species COULD survive and reproduce

Realized niche

The part actually occupied after interactions with competitors, predators and other ecological factors (smaller than fundamental)

Niche construction

Organisms change their environment, which changes selection on themselves and future generations, creating a feedback loop. The change must persist to affect selection

Niche construction examples

Earthworms altering soil chemistry. Beaver dams changing water, plants and fish. Humans (master niche constructors through culture: agriculture, domestication, pollution)

Yam cultivation niche-construction sequence

West African yam farmers clear forest, which creates standing water, which brings more malaria-carrying mosquitoes, which selects for the sickle-cell allele (AS protects against malaria)

Bantu migration (Frank Livingstone)

Bantu agriculturalists spread both farming (forest clearing and standing water, so more malaria) and the sickle-cell allele. Culture shaped biology

Cultural response to niche construction

Humans can respond culturally (water treatment, filtration, regulation) to environments they altered, which reduces or removes the selection pressure. Genetic adaptation is not the only response

Niche construction controversy

Is niche construction its own evolutionary process, or does it only change the conditions under which selection operates? Natural selection still produces the allele-frequency change

Dawkins' "weasel" (monkeys typing Shakespeare)

Random typing almost never works, but if correct letters are kept each round (cumulative selection), the phrase appears in about 43 trials. Selection is cumulative, not random

Evolution of the eye

Intermediate stages are useful (5% of an eye beats none). A fish eye could evolve in about 1,800 steps, roughly 364,000 generations