Unit 7: Natural Selection

22 terms. Say what each one means, then open the card to check.

• Natural selection

  Individuals whose inherited traits suit their surroundings leave more offspring than others, so over generations those traits grow more common in the population.

  Example. In a fictional dry valley, lizards with heritable thicker scales lose less water, leave more young, and thick scales grow more common each generation.

• Fitness

  An individual's success at passing on its alleles, measured by how many offspring it leaves that survive and reproduce, compared with others in the population.

  Example. A fictional frog that lives only 3 years but raises 8 young to breeding age has higher fitness than one that lives 9 years and raises 2.

• Directional selection

  Selection that favors one extreme of a trait, so the population average shifts toward that extreme over generations.

  Example. After a fictional cold spell, longer-furred hares survive best, and the average fur length of the population rises.

• Stabilizing selection

  Selection that favors the middle of a trait range and acts against both extremes, so the average stays put and variation narrows.

  Example. In a fictional desert cactus, very short plants are shaded out and very tall ones snap in windstorms, so mid-height plants leave the most seeds.

• Disruptive selection

  Selection that favors both extremes of a trait over the middle, which can split a population into two distinct forms.

  Example. On a fictional beach, very light and very dark crabs each blend in on their own patch of sand or rock, while mid-toned crabs are easy for gulls to spot.

• Artificial selection

  Breeding in which humans, rather than the environment, choose which individuals reproduce, so the traits people want become more common.

  Example. A grower saves seed only from the sweetest melons each year, and after several generations the whole crop is sweeter.

• Allele frequency

  The share of all copies of a gene in a population that are one particular allele. For a gene with two alleles, the two frequencies add to 1.

  Example. In 100 fictional beetles (200 gene copies), 50 copies are the allele for red wings, so its frequency is 50 out of 200, or 0.25.

• Hardy-Weinberg equilibrium

  The state of a population that is not evolving at a gene: allele and genotype frequencies stay the same from one generation to the next.

  Example. If a fictional moth population has allele frequencies 0.6 and 0.4 and meets every condition, it keeps 0.36, 0.48 and 0.16 for the three genotypes.

• Genetic drift

  A change in allele frequencies caused by chance events in which individuals happen to reproduce. Its effects are largest in small populations.

  Example. In a fictional pond of 12 newts, a falling branch kills 3 that by chance carried a rare allele, and the allele is lost.

• Bottleneck effect

  Genetic drift that follows a sharp drop in population size, leaving the survivors with allele frequencies that may differ by chance from the original group.

  Example. A fictional flood leaves 20 of 2,000 marsh rats alive, and those 20 happen to carry only a few of the original coat-color alleles.

• Founder effect

  Genetic drift that occurs when a few individuals start a new population, so its allele frequencies reflect that small group by chance.

  Example. Five fictional finches blown to a new island happen to include two carriers of a rare allele, which becomes common there.

• Gene flow

  The movement of alleles from one population to another when individuals or their gametes move between them. It makes populations more alike.

  Example. Wind carries pollen from a fictional hillside meadow to a valley meadow, adding the hillside plants’ alleles to the valley population.

• Homologous structures

  Body parts in different species that share an underlying plan because they were inherited from a common ancestor, even if they now do different jobs.

  Example. The tiny bones that let a fictional mammal hear and the jaw bones of a fictional reptile can be traced to the same set of bones in a shared ancestor.

• Convergent evolution

  The evolution of similar traits in distantly related lineages that face similar demands. It produces analogous structures, which are not evidence of close kinship.

  Example. A fictional desert lizard and a fictional desert beetle both evolve pale, heat-reflecting outer surfaces, though their last shared ancestor was very different.

• Vestigial structure

  A reduced body part with little or no function in a species, kept from an ancestor in which it was useful.

  Example. A fictional cave fish has small eye sockets with no working eyes, a remnant of sighted ancestors that lived in lit streams.

• Phylogenetic tree

  A branching diagram that shows a hypothesis of how groups are related. Each branch point stands for the most recent common ancestor of the groups beyond it.

  Example. On a tree where species R and S branch from the same node and T branches off below it, R and S are each other’s closest relatives.

• Shared derived character

  A trait that first appeared in a common ancestor and is shared by its descendants but not by more distant groups. Such traits are used to group species.

  Example. Among four fictional beetles, only the two with hooked antennae inherited that trait from their shared ancestor, so they are grouped together.

• Outgroup

  A group known to have branched off before the groups being studied. Traits it has are taken as ancestral, which shows which traits are newer.

  Example. To sort four fictional crickets, a researcher uses a related katydid as the outgroup; since it lacks striped legs, stripes are a newer trait.

• Biological species concept

  A way to draw species lines: two groups are one species if, in the wild, they mate with each other and their young can grow up to breed too.

  Example. Two fictional moth populations that look different but mate freely where they meet and have fertile young count as one species.

• Reproductive isolation

  Any barrier that keeps two groups from producing fertile offspring together. Prezygotic barriers act before fertilization; postzygotic barriers act after it.

  Example. Two fictional firefly species flash different light patterns, so females ignore males of the other kind (prezygotic); forced lab crosses give sterile young (postzygotic).

• Allopatric speciation

  The formation of new species after a physical barrier splits a population, so the separated groups evolve apart with no gene flow between them.

  Example. A rising sea cuts a fictional peninsula into two islands, and after many generations the snails on the two islands no longer interbreed when brought together.

• Sympatric speciation

  The formation of a new species within the same area as the parent species, with no physical barrier, often by a change in chromosome number in plants.

  Example. In one fictional pond, some frogs begin calling at a higher pitch and females choose mates by pitch, so high and low callers stop interbreeding while sharing the pond.