Chapter 11

Effects of genetic drift

- Population loses genetic variation; little genetic variation = less likely to have some individuals able to adapt to changing environment - Alleles that are lethal in homozygous individuals may be carried by heterozygous individuals, become more common in gene pool due to chance alone

Evolutionary arms races

A form of competitive coevolution; each species responds from the other through better adaptations over many generations; the giraffe and bull-thorn plant is an example, but there's also the crab and snail shells; crabs are predators of snails, crack shells; snails shells get harder and spikier, crab claws become stronger, keeps happening over and over

What does natural selection act on?

Acts on diff. phenotypes in a population; in order to have diff. phenotypes, must have genetic variation; a lot of genetic variation = wide range of phenotypes; greater variation of phenotypes = more likely some individuals can survive in changing environment

Why is natural selection not classified as a random event?

Acts on diversity produced by random events; individuals w/ traits that are better adapted for their environment have a better chance of surviving/reproducing than do individuals w/o these traits

Genetic drift, Hardy-Weinberg

Allele frequencies can change due to chance alone

Hybridization

Another possible source of genetic variation; crossing of two diff. species that share common genes; occurs within many groups of animals when similar species live in the same area and individuals cannot easily find mates of their own species

Geographic isolation and the Isthmus of Panama

Barrier for many marine species; no longer easily cross between the Atlantic and the Pacific; these two isolated populations eventually became genetically different; snapping shrimp have evolved, almost physically identical, but when males and females from opposite sides are placed together, they snap at each other instead of courting, no longer mate = diff. species

Drug-resistant bacteria and directional selection

Before antibiotics, trait for varying levels of drug resistance existed among bacteria, but there was no advantage to having drug resistance; once antibiotics were used, resistant bacteria had an advantage; early success of antibiotics led to overuse, overuse favored even more resistant phenotypes, new drugs were developed for these, resulting in the evolution of superbugs that are highly resistant to drugs

What are three barriers that can prevent mating between populations (leading to reproductive isolation)?

Behavioral, geographic, and temporal isolation

What pattern has been found in history of life that is reflected in the fossil record?

Bursts of evolutionary activity are followed by long periods of stability

Natural selection, Hardy-Weinberg

Certain traits may be an advantage for survival; alleles for these traits increase in frequency

Sexual selection, Hardy-Weinberg

Certain traits may improve mating success; alleles for these traits increase in frequency

Genetic drift

Changes in allele frequencies due to chance; causes a loss of genetic diversity in a population; commonly caused by Bottleneck Effect and the Founder Effect

Red fox and kit fox, Divergent Evolution

Closely related, have different appearances that have resulted from adapting to different environments; red fox lives in temperate regions like forests, dark reddish coat helps hide from predators; sandy-colored coat of kit fox allows blending in with desert surroundings, and their large ears helps keep cool in desert heat

Adaptive radiation

Diversification of one ancestral species into many descendant species; descendant species are usually adapted to wide range of environments

What happens during directional, stabilizing, and disruptive selection?

Effects of natural selection add up over many generations, pushes a population's traits in an advantageous direction; alleles associated with these traits add up in population's gene pool

Extinction

Elimination of a species from Earth; often occurs when a species as a whole is unable to adapt to a change in its environment; two categories: background and mass extinctions, differ in degree, both result in permanent loss of species from Earth

Why is having direction not the same thing as having purpose?

Environment controls direction taken by natural selection; environment changes = diff. traits become advantageous; response of species to environmental challenges and opportunities is not random

Punctuated equilibrium

Episodes of speciation occur suddenly in geologic time and are followed by long periods of little evolutionary change; proposed by Niles Eldredge and Stephen Jay Gould, written as revision of Darwin's idea that new species arise through gradual transformations of ancestral species

Convergent evolution

Evolution toward similar characteristics in unrelated species; analogous structures (wings in birds vs. wings in insects)

Temporal isolation

Exists when timing prevents reproduction between populations; some members of population show signs of courtship at diff. times if there is a lot of competition for mates; reproductive periods may change to a different time of year or day, differences in timing can lead to speciation

Gall flies and stabilizing selection

Flies lay eggs in developing shoots of tall goldenrod; larvae produce chemical that causes plant tissue to swell around them; resulting mass of the plant tissue is called a call, serves as a home where larvae can develop; range of phenotypes for body size in gall fly larvae, each body size causes a certain fall to form, and each of the two main predators of gall flies specializes on a specific gall size Downy woodpeckers attack larger galls and eat them Parasitic wasps lay its own eggs inside small galls, wasp larvae emerge and eat gall fly larvae Selective pressure from predators work against fly phenotypes that produce galls at both extremes (large, small); flies that produce mid-sized galls become more common; stabilizing selection results in higher frequency of flies that produce mid-sized galls; small and large galls become less common

What has to occur for a population to become isolated?

Gene flow between two populations stops; as these populations adapt to their own environments, their gene pools may change; these changes add up over many generations and they become more nad more genetically different; the two populations may begin to look and behave differently from one another

Founder Effect

Genetic drift that occurs after a small number of individuals colonize a new area; gene pools of these populations are often very different from those of the larger populations

Bottleneck Effect

Genetic drift that occurs after an event greatly reduces the size of a population; results in very little genetic variation; certain alleles will become fixed while others have been completely lost from the gene pool

Stabilizing selection

Intermediate phenotype is favored and becomes more common in the population; distribution becomes stable at the intermediate phenotype rather than shifting toward one of the extremes; increases number of individuals with intermediate phenotypes; selection against both extremes decreases as well, extreme phenotypes may be lost all together

Geographic isolation

Involves physical barriers that divide a population into two or more groups; most commonly studied type of isolation

Behavioral isolation

Isolation caused by differences in courtship or mating behaviors; changes in signals like chemical scents, courtship dances, and courtship songs can prevent mating between populations

What happens when an organism joins a new population and reproduces?

Its alleles become part of that population's gene pool; at the same time, these alleles are removed from the gene pool of its former population

Bull-thorn acacia and coevolution

Main source of food for giraffes, but have thorns; giraffes developed tongue that can just slide the leaves off without getting hurt by the thorns; the plant then hollowed the thorns so that stinging ants could live in them, so that the giraffe would get stung;

Behavioral isolation and fireflies

Male and female fireflies produce patterns of flashes that attract mates of their own species; other species won't be attracted to certain flash patterns

Why is the cost of reproduction different in each sex?

Males - produce many sperm continuously, value of each sperm is relatively small; make many investments at little cost Females - more limited in number of offspring they can produce in each reproductive cycle; each investment made is more valuable, want a good return This difference in reproductive cost makes females choosy about mates, which is known as sexual selection

Traits that increase mating success

May not always be adaptive for survival of individual, but some showy traits may be linked with genes for good health and fertility; other traits in males can offer care for offspring or defense from predators; females may use showy traits as signs of quality and health in males; showy traits can become very exaggerated over time through sexual selection

Allele frequency

Measure of how common a certain allele is; count number of allele occurrences in gene pool, divide by total number of alleles for that gene in gene pool

Reproductive Isolation

Members of diff. populations can no longer mate successfully w/ one another; they cannot produce offspring that survive and reproduce; final step in becoming separate species

Gene flow, Hardy-Weinberg

Movement of alleles from one population to another changes the allele frequencies in each population

Gene flow

Movement of alleles from one population to another; occurs when individuals move between populations (animals) and when spores or seeds are spread to new areas (plants, fungi); increases genetic variation of receiving population, between neighboring populations = keeps gene pools similar; less gene flow = more genetically different two populations become, increases chance that the two populations will evolve into diff. species

Mass extinctions

Much more rare than background; much more intense; operate at global level; destroy many species, entire orders/families; thought to occur suddenly in geologic time because of catastrophic events (ice age, asteroid); 5+ in last 600 mill. years

Where does genetic variation come from?

Mutation and recombination

Random events

Mutations and genetic drift = cannot be predicted, random events, sources of genetic diversity

Mutation, Hardy-Weinberg

New allele can form through mutation; mutations create genetic variation needed for evolution

Conditions needed for Hardy-Weinberg Equilibrium

Not commonly found in nature; some parts of a population may stay the same, others will likely change; populations that are not at equilibrium are evolving as a response to their environment; five factors that can lead to evolution: genetic drift, gene flow, mutation, sexual selection, natural selection

Microevolution

Observable change in the allele frequencies of a population over time; occurs on small scale, within a single population; natural selection can lead to it, can change the distribution of a trait with directional, stabilizing, or disruptive selection; can have major effects on how a population looks and behaves

Background extinctions

Occur continuously but at a very low rate; part of Earth's life cycle; occur at roughly the same rate as speciation; usually affect only one or a few species in a relatively small area; can be caused by local environmental changes, introduction of new predator, or decrease in food supply; seem to occur randomly but at a fairly constant rate

Adaptive radiation at end of Cretaceous

Occurred after mass extinction at end of Cretaceous period 65 million years ago; Mammals evolved for ~150 mill years before end of Cretaceous, barely resembled anything we know today; tiny, insect eaters, mostly nocturnal; these characteristics allowed them to coexist with dinosuars; extinction of dinosaurs left environments full of opportunities for other types of animals; first 10 mill after dinosaur extinction = 4000 mammal species had evolved (whales, bats, rodents, primates)

Disruptive selection

Occurs when both extreme phenotypes are favored, while individuals with intermediate phenotypes are selected against by something in nature; middle of the distribution is disrupted; can lead to the formation of new species

Sexual selection

Occurs when certain traits increase mating success. Two types are inter/intra sexual selection Intrasexual - Involves competition among males, whoever wins competition wins female Intersexual - Males display certain traits that attract female

What happens as environments change?

Populations either adapt or go extinct; extinct = a diff. species can take its place, allowing for the continuation of the cycle

Coevolution

Process in which two or more species evolve in response to changes in each other; specialized relationships can form

Mutation

Random change in the DNA of a gene; can form a new allele; occurs in reproductive cells = can be passed on to offspring; increases genetic variation within gene pool; many genes in each individual and many individuals in population = new mutations form frequently in gene pools

Speciation

Rise of two or more species from one existing species

Godfrey Hardey and Wilhelm Weinberg

Showed that genotype frequencies in population stay the same over time as long as certain conditions are met, and that these frequencies can be predicted; identified five conditions needed for a population to stay in equilibrium; if met, populations are not evolving

Which populations are more likely to be affected by genetic drift?

Small; due to chance, some alleles will likely decrease in frequency and become eliminated, while others will increase in frequency and become fixed

Recombination

The formation of new allele combinations in offspring; most occurs during meiosis; when gametes are made, each parent's alleles are arranged in new ways; shuffling of alleles results in many diff. genetic combination

Directional selection

The type of selection that favors phenotypes at one extreme of a trait's range; causes a shift in a population's phenotypic distribution; an extreme phenotype that was once rare in a population becomes more common; mean value of a trait shifts in the direction of the more advantageous phenotype

Phenotype

Trait produced by one or more genes; may be a wide range of phenotypes in a population

Hardy-Weinberg Equation

Traits in simple dominant-recessive systems; can be used to predict genotype frequencies; values predicted by equation are those that would be present if population were at equilibrium p = frequency of dominant allele q = frequency of recessive allele p^2 + 2pq + q^2 = 1 When comparing predicted genotype frequencies with actual frequencies, same = population is at Hardy-Weinberg equilibrium for that trait, do not match = population not at equilibrium, is evolving

Normal distribution

Type of distribution where the frequency is highest near the mean value and decreases toward each extreme end of the range; bell-shaped curve; all phenotypes provide an equal chance of survival = generally a normal distribution; phenotypes near middle of range tend to be most common, extremes are less common; environmental conditions can change, resulting in another phenotype possibly becoming an advantageous one, nature will favor individuals with that advantageous phenotype (able to survive and reproduce at higher rates than those w/ less favorable phenotype); alleles associated with favorable phenotypes increase in frequency

Male lazuli buntings and disruptive selection

Vary widely in brightness of their feathers (dull brown to light blue); adult males have brightest blue feathers, so they get pick of best territories and are most successful at attracting females; young males that are bright blue and dull brown have the best chance at attracting a mate compared to bluish brown due to the fact that the adult males attack bluish brown and bright blue (but bright blue attracts females anyway so it doesn't matter that they are attacked) since they perceive them as a threat, but dull brown birds are left alone and therefore attract mates as well; bluish brown doesn't attract a mate because they are not bright enough and they are attacked; so, the extreme phenotypes (bright blue, dull brown) are favored in this situation, while intermediate bluish brown phenotypes are selected against; bluish brown males are not as well adapted to compete for mates because they are too blue to be left alone by adult males, but not blue enough to win a mate based on color alone

Hardey-Weinberg Equilibrium Conditions

Very large population - no genetic drift can occur No emigration/immigration - no gene flow can occur No mutations - No new alleles can be added to the gene pool Random mating - No sexual selection can occur No natural selection - All traits must aid equally in survival Real populations rarely meet the five conditons; still very important concept as biologists can compare research to data predicted by the model; can learn about how the population is evolving; gives framework for testing the factors that can lead to evolution

Divergent evolution

When closely related species evolve in different directions, become increasingly different

Gene pool

Where genetic variation is stores; combination of alleles of all of the individuals in a population; diff. combinations of alleles in gene pool can be formed when organisms mate and have offspring

One mutation and its effect on speciation, flies

ds2 gene affects how well fruit flies deal with cold temperatures and pheromones; fruit flies living in tropical areas (high comp for food) have a tropical allele, while flies living in cooler regions (low food comp) have a temperature allele; pheromones are used to attract mates of own species; speciation occurred within these two fruit fly populations; flies migrating north from Africa encountered cooler temperatures and less food comp, resulting in a mutation in the ds2 gene that produced the temperate allele, allowing the migrating flies to survive in cooler climates; the mutation also affected the pheromones that the gene controlled, so the mating behaviors changed, so the tropical-allele flies and the temperate-allele flies rarely mated together, and the two populations eventually became reproductively isolated