BackGeneral Biology: Evolution & Phylogenetics
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Terms in this set (29)
A morphogen is a molecule in a concentration gradient that tells cells their position. Bicoid protein is highest at the anterior of Drosophila embryos and directs anterior structure formation by acting as a transcription factor.
Pattern formation determines the spatial organization of an embryo, specifying which cells become head, tail, dorsal, or ventral. Morphogen gradients establish the three body axes: anterior-posterior, dorsal-ventral, and left-right.
A chain of regulatory genes where one gene activates the next. In Drosophila, it proceeds: maternal effect genes → gap genes → pair-rule genes → segment polarity genes → Hox genes → effector genes → differentiated cells.
Hox genes are master regulators specifying segment identity along the body axis. The homeobox is a conserved DNA sequence encoding the DNA-binding domain of Hox transcription factors.
A mutation causing a body part to form in the wrong place. Example: Antp mutation in Drosophila causes legs to grow where antennae should be; Ubx mutation causes a second pair of wings instead of halteres.
The same set of conserved regulatory/signaling genes (toolkit genes) exist across animals. Different body forms arise from changes in how these genes are regulated, not from different genes.
Hox genes originated early in animal evolution. For example, mouse Hoxb6 can cause legs to grow in place of antennae in flies, showing deep homology across species.
Snakes lost limbs due to mutations in an enhancer controlling Sonic hedgehog (SHH) expression in limb buds. The SHH coding sequence is unchanged; only regulation differs, altering the body plan.
Three ways gene expression changes drive evolution: heterometry = change in amount of gene product; heterochrony = change in timing of expression; heterotopy = change in location of expression.
Typological thinking views species as fixed types; variation is noise. Population thinking (Darwin) sees variation among individuals as the raw material for evolution.
Lamarck proposed inheritance of acquired characters (traits gained during life passed to offspring), which is incorrect. Only heritable genetic variation is passed on.
1) Species change over time. 2) Divergent species share common ancestry. 3) Natural selection is the mechanism of change.
1) Vast geologic time (radiometric dating). 2) Extinction shows species are dynamic. 3) Transitional fossils (e.g., Tiktaalik). 4) Vestigial traits (e.g., whale hip bones).
A reduced or nonfunctional structure homologous to a functional one in related species (e.g., human tailbone). Supports descent with modification, contradicting special creation.
Uses decay rates of radioactive isotopes (e.g., C-14) to calculate absolute ages. Shows Earth is ~4.6 billion years old; earliest life ~3.4-3.8 billion years ago.
1) Variation exists among individuals. 2) Some variation is heritable. 3) Survival and reproduction vary. 4) Survival/reproduction is non-random, favoring advantageous heritable traits.
Fitness: ability to produce surviving fertile offspring relative to others. Adaptation: heritable trait increasing fitness. Selection: differential reproduction due to heritable variation.
1) Variation: resistant and non-resistant strains exist. 2) Heritable: resistance encoded in rpoB gene. 3) Differential survival: only resistant reproduce. 4) Non-random: resistant allele frequency increases.
During drought, finches with deeper beaks survived better. Average beak depth increased in one generation, showing directional selection on a heritable trait.
Selection acts on individuals via differential survival/reproduction, but evolution (allele frequency change) occurs in populations.
Predicts allele/genotype frequencies remain constant if 5 conditions hold. It is the null hypothesis because it describes no evolution; deviations indicate evolution.
1) Random mating. 2) No natural selection. 3) Large population (no genetic drift). 4) No gene flow. 5) No mutation. Violation of any causes evolution.
\(p^2 + 2pq + q^2 = 1\), where p = frequency of dominant allele, q = frequency of recessive allele, p + q = 1; p^2 = homozygous dominant frequency, 2pq = heterozygous frequency, q^2 = homozygous recessive frequency.
The sum of all copies of all alleles at all loci in a population; the source of genetic variation on which natural selection acts.
Stabilizing: favors intermediate traits, reduces variation. Directional: favors one extreme, shifts mean. Disruptive: favors both extremes, increases variation. Balancing: maintains multiple alleles.
Also called stabilizing selection at the genetic level; removes deleterious mutations to conserve gene sequences. It is the most common form of selection.
Selection maintaining multiple alleles in a population. Example: heterozygote advantage in sickle-cell allele where Hb-S/Hb-A heterozygotes have higher fitness in malaria regions.
Random changes in allele frequencies due to chance. Strongest in small populations where sampling error is large. Drift is random and not adaptive.
Alleles become either fixed (frequency 1.0) or lost (frequency 0.0), reducing genetic variation over time.