BIO EXAM 1 review

Gene

A segment of DNA that carries the instructions for a trait or protein

Allele

One of the alternative versions of a gene (e.g. a flower-color gene may have a purple allele and a white allele)

Locus

The physical location of a gene on a chromosome

Chromosome

A long coiled molecule of DNA that carries many genes

Genotype

The combination of alleles an individual carries for a gene (such as AA Aa or aa)

Phenotype

The observable trait produced by an organism's genotype interacting with its environment

Dominant allele

An allele that shows up in the phenotype even when only one copy is present (written as a capital letter)

Recessive allele

An allele expressed only when two copies are present; it is masked by a dominant allele (written lowercase)

Homozygous

Having two identical alleles for a gene (AA or aa)

Heterozygous

Having two different alleles for a gene (Aa)

Trait

A measurable characteristic of an organism (like height or color) that can differ among individuals

Heritable

Able to be passed from parent to offspring through genes; only heritable traits can evolve by natural selection

Population

A group of individuals of the same species living in the same area and interbreeding

Species

A group of organisms that can interbreed and produce fertile viable offspring and are reproductively isolated from other groups

Evolution

A change in the heritable characteristics (allele frequencies) of a population across generations

Descent with modification

Darwin's phrase for evolution — all species share common ancestors and accumulate changes over time

Common ancestor

An ancestral species from which two or more later species descended

Natural selection

The process by which individuals with heritable traits better suited to the environment survive and reproduce more; the main mechanism of adaptive evolution

Requirement 1 for natural selection

There must be variation among individuals in the population

Requirement 2 for natural selection

Some of that variation must be heritable (genetically passed on)

Requirement 3 for natural selection

More offspring are produced than can survive (a struggle for existence)

Requirement 4 for natural selection

Survival and reproduction are non-random — certain traits give a reproductive advantage

Fitness

An individual's reproductive success — how many surviving fertile offspring it leaves (not strength or speed for its own sake)

Adaptation

A heritable trait shaped by natural selection that improves survival or reproduction in a particular environment

Artificial selection

Humans choosing which organisms reproduce to promote desired traits (like breeding low-fat corn or dog breeds)

Variation

Differences in traits/alleles among individuals; the raw material that evolution acts on

Misconception - individuals evolve

False — individuals do not evolve during their lifetime; only populations evolve across generations

Misconception - evolution has a goal

False — evolution is not goal-directed or progressive and does not strive toward perfection

Misconception - traits arise because needed

False — mutations occur randomly and the environment then selects among the variation that already exists

Evidence for evolution

Fossils and transitional forms; homologous structures; shared genes and development; vestigial structures; biogeography

Transitional fossil

A fossil showing intermediate features between an ancestral group and its descendants

Tiktaalik

A famous transitional fossil (a fish-to-tetrapod intermediate) with fins containing wrist-like bones and a neck; documents the move onto land

Homologous structures

Body parts inherited from a common ancestor with the same developmental origin even if they now have different functions (like the tetrapod limb bones in a human arm bat wing and whale flipper)

Analogous structures

Body parts that look or function similarly but did NOT come from a common ancestor; they arose independently (like a bird wing vs an insect wing)

Vestigial structure

A reduced nonfunctional remnant of a structure that was useful in an ancestor (like the pelvic and leg bones inside a boa constrictor or the human tailbone)

Homology

Similarity between species due to shared ancestry; the key evidence of evolutionary relatedness

Homoplasy

Similarity between species that is NOT from common ancestry but from convergent evolution (an analogous trait)

Convergent evolution

When distantly related species independently evolve similar traits because they face similar environments (produces analogous structures)

Divergent evolution

When related lineages become increasingly different over time and a shared structure takes on new functions

Biological species concept

Defines a species as populations whose members can interbreed and produce fertile viable offspring but cannot breed with other groups

Reproductive isolation

The inability of members of different species to successfully interbreed; maintained by reproductive barriers

Prezygotic barrier

A reproductive barrier that blocks mating or fertilization before a zygote can form

Habitat isolation

A prezygotic barrier in which two species live in different habitats and rarely encounter each other

Temporal isolation

A prezygotic barrier in which two species breed at different times of day season or year

Behavioral isolation

A prezygotic barrier in which different courtship rituals or mating signals keep species from attracting each other

Mechanical isolation

A prezygotic barrier in which reproductive structures are physically incompatible

Gametic isolation

A prezygotic barrier in which sperm and egg are chemically incompatible and cannot fuse

Postzygotic barrier

A reproductive barrier that acts after fertilization by making hybrids unfit

Reduced hybrid viability

A postzygotic barrier in which hybrid offspring die early or are weak

Reduced hybrid fertility

A postzygotic barrier in which hybrids survive but are sterile (such as a mule)

Hybrid breakdown

A postzygotic barrier in which first-generation hybrids are healthy but their offspring are weak or sterile

Speciation

The evolutionary process by which one species splits into two or more new species

Allopatric speciation

New species forming when a population is split by a geographic barrier (such as squirrels separated by the Grand Canyon)

Sympatric speciation

New species forming without geographic separation while living in the same area (often by polyploidy or a host shift)

Polyploidy

Having one or more extra full sets of chromosomes; can instantly create a new plant species in the same area (as in the potato which doubled its chromosomes)

Host shift speciation

Sympatric speciation when a population specializes on a new host and stops interbreeding (as in Rhagoletis flies on apple vs hawthorn trees)

Adaptive radiation

The rapid diversification of one ancestral species into many new species that exploit different niches (like Galapagos finches or Anolis lizards)

Hybrid zone

A region where two species meet and interbreed producing some hybrid offspring

Reinforcement

A hybrid-zone outcome where selection strengthens prezygotic barriers because hybrids are unfit so the species stay separate

Fusion

A hybrid-zone outcome where weak barriers let two species merge back into one

Stability (hybrid zone)

A hybrid-zone outcome where hybrids keep being produced over time without fusing or fully separating

Gradualism

The model that species evolve slowly and steadily through small changes over long periods

Punctuated equilibrium

The model that species stay mostly unchanged for long periods then change rapidly in short bursts often during speciation

Population genetics

The study of allele and genotype frequencies in populations and how they change over time

Gene pool

All of the alleles of all genes in every individual of a population

Allele frequency

The proportion of a given allele among all copies of that gene in a population

Genotype frequency

The proportion of individuals in a population that have a particular genotype

Microevolution

A change in allele frequencies within a population from one generation to the next

Hardy-Weinberg principle

The idea that in a non-evolving population allele and genotype frequencies stay constant across generations; used as the baseline null model

Hardy-Weinberg allele equation

p + q = 1 where p is the dominant allele frequency and q is the recessive allele frequency

Hardy-Weinberg genotype equation

p squared + 2pq + q squared = 1 for the three genotype frequencies

p squared

The frequency of the homozygous dominant genotype

2pq

The frequency of the heterozygous genotype

q squared

The frequency of the homozygous recessive genotype (the only genotype directly visible under full dominance)

Hardy-Weinberg condition 1

No mutation (no new alleles are being created)

Hardy-Weinberg condition 2

Random mating (no mate choice or inbreeding)

Hardy-Weinberg condition 3

No natural selection (all genotypes survive and reproduce equally)

Hardy-Weinberg condition 4

Very large population size (so chance/drift is negligible)

Hardy-Weinberg condition 5

No gene flow (no alleles entering or leaving the population)

Solving Hardy-Weinberg problems

The visible recessive phenotype equals q squared; take its square root to get q; then p = 1 minus q; then compute 2pq and p squared

HW example - heterozygotes

If the recessive allele frequency q = 0.3 then p = 0.7 and heterozygotes = 2pq = 0.42

HW example - homozygous dominant

If the recessive allele frequency is 0.8 then the dominant allele is 0.2 and homozygous dominant = p squared = 0.04

HW example - disease frequency

If 1 in 2500 people are homozygous recessive then q squared = 0.0004 so q = 0.02 and p = 0.98

Why Hardy-Weinberg matters

It predicts a non-evolving population so any deviation from it means the population is evolving at that gene

Deviation from Hardy-Weinberg

A sign that one of the five conditions is violated and the population is evolving at that locus

Sources of genetic variation

Mutation (makes new alleles); recombination and independent assortment (reshuffle alleles); gene flow (brings in alleles)

Why heritable variation is required

Natural selection can only change a population across generations if the favored trait can be inherited

Mutation

A random change in DNA sequence; the ultimate source of all new alleles

Genetic drift

Random change in allele frequencies due to chance events; strongest in small populations and not adaptive

Bottleneck effect

A form of genetic drift where a disaster sharply reduces population size leaving survivors with a random unrepresentative set of alleles

Founder effect

A form of genetic drift where a few individuals start a new isolated population carrying only part of the original variation

Gene flow

The movement of alleles between populations through migration and interbreeding (such as a baboon leaving to join another troop)

Effect of gene flow

It increases genetic variation within a population and makes different populations more similar to each other

Effect of genetic drift on variation

It decreases variation within a population and increases genetic differences between populations

Non-random mating

Choosing mates based on traits or inbreeding; it changes genotype frequencies (for example inbreeding raises the number of homozygotes)

The five evolutionary forces

Natural selection; genetic drift; gene flow; mutation; non-random mating

Only adaptive force

Natural selection is the only evolutionary force that consistently produces adaptation (a better fit to the environment)

Directional selection

Selection that favors one extreme of a trait and shifts the whole population toward it (like lizards evolving shorter legs on treeless islands)

Stabilizing selection

Selection that favors the intermediate phenotype and removes both extremes (like a population of butterflies all becoming medium gray or average human birth weight)

Disruptive selection

Selection that favors both extremes of a trait against the intermediate and can split a population into two