Evolution (Chapters 17-19)

Darwin & his Ideas Darwin’s Gang 1-J. Hutton—GRADUALISM (profound change results from slow, continuous processes)

2-J. Lamarck— visionary; so close! [except for (1) inheritance of acquired characteristics, (2) theory of use & disuse, i.e. “use it or lose it”]

3-A. Wallace—Malaysia, June 1958: identical theory, no qualms about publishing 4-T. Malthus—wrote a 1798 essay in which he suggested that the number of people on the planet is

increasing than food supply; thus, much human suffering (disease, famine, homelessness, war) was the inescapable consequence of the potential for human populations to increase faster than food supplies and other resources

5-C. Lyell—uniformitarianism--geological processes had not changed throughout Earth’s history

Darwin!

Darwin’s Life & Journeys -Dad wanted him to be a doctor; he liked nature, so he went into the clergy instead. •He became the conversation companion to Captain Robert FitzRoy, who was preparing the survey ship Beagle for a voyage

around the world. -The main mission of the five-year voyage of the Beagle was to chart poorly known stretches of the South American

coastline -Darwin explored the Brazilian jungles, the grasslands of the Argentine pampas, the desolation of Tiera del Fuego, and the

heights of the Andes.

•Darwin made several observations -Organisms from temperate regions of South America were more similar to those from the tropics of South America than

to those from temperate regions of Europe. -The origin of the fauna of the GALAPAGOS, 900 km west of the Ecuadorean coast, especially puzzled him; he noted

that while most of the animal species on the Galapagos lived nowhere else, they resembled species living on the South American mainland

***It seemed that the islands had been colonized by plants and animals from the mainland that had subsequently diversified on the different islands

-Lyell’s ideas and his observations on the voyage led Darwin to doubt the church’s position that the Earth was static and only a few thousand years old.

• After his return to Great Britain in 1836, Darwin began to perceive that the origin of new species and adaptation of species to the environment were closely related processes.

-e.g. Clear differences in the beaks among the 13 types of finches that Darwin collected in the Galapagos were adaptations to the foods available on their home islands.

•On November 24, 1959, Charles Darwin quickly finished On the Origin of Species by Means of Natural Selection & published it that same year.

Darwin’s Ideas •Central to Darwin’s view of the evolution of life is descent with modification.

-This idea states that all present day organisms are related through descent from unknown ancestors in the past.

-Further, descendents of these ancestors accumulated diverse modifications, or adaptations, that fit them to specific ways of life and habitats.

•Ernst Mayr, an evolutionary biologist, has dissected the logic of Darwin’s theory into three inferences based on five observations.

Darwin’s Five Observations & His resulting Three Inferences •Observation #1—Tremendous fecundity: All species have such great potential fertility that their population size would

increase exponentially if all individuals that are born reproduced successfully. •Observation #2—Stable populations sizes: Populations tend to remain stable in size, except for seasonal fluctuations. •Observation #3— Limited environmental resources: Environmental resources are limited.

-Inference #1: Production of more individuals than the environment can support leads to a struggle for existence among the individuals of a population, with only a fraction of the offspring surviving each generation.

•Observation #4— Variation among individuals: Individuals of a population vary extensively in their characteristics; no two individuals are exactly alike.

•Observation #5— Heritability of some of this variation: Much of this variation is heritable. -Inference #2: Survival in the struggle for existence is not random, but depends in part on the hereditary constitution of the

individuals. Those individuals whose inherited characteristics best fit them to their environment are likely to leave more offspring than less fit individuals.

-Inference #3: This unequal ability of individuals to survive and reproduce will lead to a gradual change in a population, with favorable characteristics accumulating over the generations.

Darwin’s main ideas can be summarized in three points. 1-Natural selection is differential success in reproduction (unequal ability of individuals to survive and reproduce). 2-Natural selection occurs through an interaction between the environment and the variability inherent among the individual

organisms making up a population. 3-The product of natural selection is the adaptation of populations of organisms to their environment. Summary of Natural selection: 1. There is ___________________________ within a population & has several sources

(a) Main source--_____________________________ reproduction, which involves

-crossing over

-mixing genes from an organism’s ________________ & ______________

(b) also results from ______________________________; if this involves special genes called Homeobox Genes, which

control an organism’s __________________ development, then even minor mutations can produce

__________________ changes!

2. Some variations are ___________________________, meaning it improves an organism’s ability to function, and more

importantly to _________________________________ in its environment.

3. More young are produced than can ____________________________, dying from disease or starvation or being killed by

predators before they can ______________________

4. Those that survive to reproduce are those with favorable variations--b/c the offspring will inherit these characteristics, a greater proportion of each new generation will have them

5. Over LARGE time, __________________ changes accumulate, and new species appear

Also: ARTIFICIAL SELECTION (crops, farm animals, pets)—powerful, and quick

Evidence for Evolution 1. Biogeography

-Australian marsupials -similar species in widely separated yet similar habitats -different species in nearby but very different habitats

•Species tend to be more closely related to other species from the same area than to other species with the same way of life,

but living in different areas. e.g. Even though some marsupial mammals of Australia have look-alikes among the placental mammals that live on

other continents, all the marsupial mammals are still more closely related to each other than they are to any eutherian mammal.

e.g. While the sugar glider and flying squirrel have adapted to the same mode of life, they are not closely related. →Instead, the sugar glider from Australia is more closely related to other marsupial mammals from Australia than to the flying squirrel, a placental mammal from North America. •The resemblance between them is an example of CONVERGENT EVOLUTION. *Find an example on the net:

•Often islands have many species of plants and animals that are endemic, or found nowhere else in the world

2. Fossils--LAW OF SUPERPOSITION—Sedimentary rocks;

-fishamphibiansreptilesbirdsmammals

3. Comparative Anatomy -HOMOLOGOUS STRUCTURES -VESTIGIAL ORGANS (e.g.?)

4. Comparative Embryology -gill slits, tail -“ontogeny recapitulates phylogeny” (but not really)

5. Molecular Biology

-proteins, DNA

- - - - •”Just a Theory” applies only to Darwin’s 2nd idea: natural

Selection being the main mechanism for the facts of evolution, (change over time)

-What was missing in Darwin’s explanation was an understanding of inheritance that could explain how chance variations arise in a population while also accounting for the precise transmission of these variations from parents to offspring

-Although Gregor Mendel and Charles Darwin were contemporaries, Mendel’s discoveries were unappreciated at the time, even though his principles of heredity would have given credibility to natural selection. (ch.23)

How Evolution by natural selection actually occurs

(or, So much to Do, So little Time) Terms to Know a-Population genetics--emphasizes the extensive genetic variation within populations and recognizes the importance of

quantitative traits b-Population—a localized group of individuals that belong to the same species c-Gene pool—The total aggregate of genes in a population at any one time; this includes all alleles at all gene loci in all

individuals of a population -Each locus is represented twice in the genome of a diploid individual -If all members of a population are homozygous for the same allele, that allele is said to be fixed -Often, there are two or more alleles for a gene, each contributing a relative frequency in the gene pool

e.g. If the percentage of alleles in California were: 30% IA, 24% IB & 46% I, then the relative frequencies would be 0.3, 0.24 & 0.46

-Population vs. species

The Hardy-Weinberg Theorem •States that allele frequencies will remain the same iff the population is not evolving; i.e. if it meets the following 5 criteria:

1-Has a large population In small populations, chance fluctuations in the gene pool, genetic drift, can cause genotype frequencies to change over time.

2-Has no migration Gene flow, the transfer of alleles due to the movement of individuals or gametes into or out of our target population can change the proportions of alleles

3-Has no net change in the gene pool due to mutation If one allele can mutate into another, the gene pool will be altered

4-Engages in completely random mating If individuals pick mates with certain genotypes, then the mixing of gametes will not be random and the Hardy-Weinberg equilibrium does not occur.

5-Has no natural selection If there is differential survival or mating success among genotypes, then the frequencies of alleles in the next variation will deviate from the frequencies predicted by the Hardy-Weinberg equation.

(The five above restrictions might be summarized by simply requiring that the population not be experienceing microevolution, which will be discussed below.)

•As long as the above requirements are met, the processes of meiosis and random fertilization will maintain the

same allele and genotype frequencies with each generation. -When a population’s genetic structure is in this state of equilibrium,it is called a Hardy-Weinberg equilibrium. -Theoretically, the allele frequencies should remain constant forever.

The Hardy-Weinberg Equation p2 + 2pq + q2 = 1

p = % of dominant alleles in gene pool

q = % of recessive alleles in gene pool

∴p + q = 1

∴If the equation doesn’t work, then…

*H-W enables us to calculate: Allele frequency in gene pool ⇔ Frequencies of

genotypes -e.g. #1--If 1/10,000 U.S. babies born with PKU,

then what are the allele frequencies?

e.g. #2--If 60% of the alleles for attached earlobes, what are the genotypic frequencies?

Causes of Microevolution Defined as: generation-to-generation change in a

population’s frequencies of alleles -Note that the first two are the main causes

-All of them represent deviation from the conditions required for the Hardy-Weinberg equilibrium

1. Genetic Drift

…occurs when changes in gene frequencies from one generation to another occur because of chance events (*sampling errors) that occur when populations are small

(Use the coin analogy)

•There are 2 special cases of genetic drift: a-Bottleneck Effect--disaster kills victims nonselectively; e.g. cheetahs—clones--

no rejection (above); this reduces individual variation & adaptability b-Founder Effect-- occurs when a new population is started by only a few

individuals that do not represent the gene pool of the larger source population (→) 2. Natural Selection (the only 1 of these 5 that is likely to adapt the individual to its

environment) 3. Gene flow

= Genetic exchange due to migration of fertile individuals or gametes between populations -This tends to reduce differences between populations that arose from others in this list

4. Mutation -the original source of ALL genetic variation

•Nonrandom mating -Inbreeding (or even selfing)—how does it affect Hardy-Weinberg?

-With each generation, the heterozygote % ________, while the % of homozygotes _______________ -more and more express the _________________________ phenotype

-Assortative mating—select like partners -allele proportion remains unchanged; genotypic makeup changes (more homozygotes) -humans? Types of Genetic Variation -not all is heritable (e.g.?) 1. Variations within a population

-quantitative characters usually indicates polygenic inheritance -qualitative (single gene locus); Polymorphism—2+ distinct morphs

(forms) 2. Variations between populations

-Geographical variation -cline— Geographic variation in the form of graded change in a trait

along a geographic axis (e.g. mammal body size ~ latitude) Sources of Genetic Variation 1. Mutation—rare & random

-often harmful (a random change is not likely to be helpful to a product refined by evolution for millennia)

*when helpful? When a changing environment makes beneficial a trait that was previously selected against

-FYI--bacteria reproduce ~ every 20 minutes 2. Sexual recombination—for sexual reproducers, this is the source of most

variation

3 Other Important Aspects of Variation: 1. Polyploidy prevents lots of variation from being affected by selection pressure, by covering recessive alleles

-Why is this advantageous to a population?

2. Balanced polymorphism— occurs when two or more discrete traits are present and noticeable in a population; it is maintained in several ways:

a-Heterozygote Advantage e.g. sickle-cell & malarial resistance (20% of alleles in some areas! So what are the genotype frequencies?)

e.g. corn’s hybrid vigor

b-Frequency-dependent Selection: ↑ proportion of a certain morph → ↓ numbers of that morph (a perfect examples of negative feedback loops!) -e.g. African swallowtail butterfly’s various forms—how are these maintained?

-e.g. right- & left-mouthed Japanese fish—how do they maintain an even division?

3. Neutral variation -e.g. human fingerprints -RELATIVE! (A variation may be neutral in one environment, yet beneficial or harmful in another)

Finally…Natural Selection!!Finally…Natural Selection!!

The effect of selection on a continuously varying trait: 1. Stabilizing Selection—reduces variation; the most common, b/c it resists change that is not needed

e.g. human birth weight (why?)

2. Directional Selection—favors one phenotypic extreme

-This is most common during periods of environmental change

e.g. darker snails as volcanic

rock increases

e.g. European black bear size during glacial & interglacial periods

3. Diversifying Selection—

favors both extremes of a phenotype

-can cause balanced polymorphism -e.g. snail color as white sand is studded with black volcanic rock

-e.g. seedcracker finches—short beaks eat the soft seeds, long beaks eat the hard—medium are inefficient at both

Sexual Selection & Sexual Dimorphism—often size (male) or adornment (male)

*Every time a female chooses a mate based on particular phenotypic traits, she increases those alleles -e.g. jungle chickens—wattles & combs

-e.g. peacock plumage

Things to Remember about Natural Selection

1. Populations are the smallest unit that can evolve, individuals do not evolve-all an individual can do is either

____________________________ or ___________

2. Evolution occurs when there is a change in the ________________________________ of a population.

3. Organisms do not purposely change with the conscious intention of survival; they do not evolve new traits in order to

survive. There is no drive toward speciation for its own sake.

4. The contribution that an individual organism can make toward evolution is: die or live to __________________

5. Evolution will not happen unless something changes in the _______________________________.

6. It can only work with existing and heritable variations. NEW ALLELES DON’T ARISE ON DEMAND!

7. Fitness is relative—adaptation to a situation may be neutral or detrimental in another (e.g.?)

8. Survival is irrelevant; only reproduction matters

9. Only phenotype affects an individual’s fitness, not its genotype

10. Adaptations are often compromises, for different purposes

-e.g. human prehensile hands & flexible limbsboth agility & versatility, also injury-prone

-e.g. seals—leg-like flippers for land, or fin-like for water?

11. Natural selection cannot predict the future—it can only improve a structure for its current utility (see novelty)

12. An evolutionary trend does not mean that evolution is goal-oriented; rather, the branches that didn’t work just aren’t around anymore (e.g. horses)

The Origin of Species Generally, there are 2 types of evolution

a-anagenesis—1 species1 different species b-cladogenesis—1 speciesoriginal species plus

another (responsible for diversity) Species—potential to interbreed…to produce viable

AND fertile offspring…in nature -Species are based on interfertility, not physical

similarity. -The key to a new species is reproductuve

isolation!

Barriers to reproduction→different species These barriers are classified according to which stage of the reproductive process is prevented:

1-Prezygotic Barriers—impedes mating

a-habitat isolation--Two organisms that use different habitats even in the same geographic area are unlikely to encounter each other to even attempt mating

e.g. garter snakes that occur in the same areas but because one lives mainly in water and the other is primarily terrestrial, they rarely encounter each other

b-behavioral isolation—results from elaborate species-specific behaviors used to attract mates e.g. female fireflies only flash back and attract males who first signaled to them with their species’ rhythm of light

signals c-temporal isolation—breed at different times of day, days (e.g. tropical orchid),seasons (e.g. skunks) or years cannot mix

gametes d-mechanical isolation—they just don’t fit (bummer) → sperm transfer is not possible

e.g. many insects the male and female copulatory organs of closely related species do not fit together e.g. chihuahua & St. Bernard?

e-gametic isolation—gametes cannot create a zygote due to inability to fuse e.g. In species with internal fertilization, the environment of the female reproductive tract may not be conducive to the

survival of sperm from other species -e.g. with external fertilization, gamete recognition may rely on the presence of specific receptors on the egg’s coat,

which adhere only to specific molecules on sperm cells of the same species

2-Postzygotic Barriers –mating OK, development into viable fertile adult not a-reduced hybrid viability—spontaneous abortion b-reduced hybrid fertility—mules; (hinny?)

Additional bits about the concept of the species

a-Why are asexual organisms’ species difficult to define? b-e.g. coyotes, wolves, dogs c-The text goes on about all sorts of definitions—philosophically interesting, but not relevant

B. Modes of Speciation

There are 2 general types:

I. Allopatric speciation—geographic isolation restricts gene flow

Geographical Isolation—could result from any of the following: mountain range emerges, creeping glacier, land bridge disappears, lake dries & fragments

-e.g.—rainy S. Cal becomes deserta big lake fragmentseach pool contains own species of pupfish

e.g. Galapagos

e.g. Grand canyon squirrels

•Worth noting: Very few small, isolated populations will

develop into new species; most will simply perish in their new environment

S.J. Gould—“Status as a peripheral isolate merely gives a lottery ticket to a small population. A population can’t win [i.e. speciate] without a ticket, but there are very few winners.”)

Q: So…why might a small isolated population change into a new species? A: 3 reasons--founder effect, genetic drift & (usually) different environmental pressures than original habitat

A fascinating little example from our own back yard of the complexities of speciation:

•Ring species provide examples of what seem to be various stages in the gradual divergence of new species from common ancestors.

-In ring species, populations are distributed around some geographic barrier, with populations that have diverged the most in their evolution meeting where the ring closes.

-Some populations are capable of interbreeding, others cannot.

•One example of a ring species is the salamander, Ensatina escholtzii, which probably expanded south from Oregon to California, USA.

-The California pioneers split into one chain of interbreeding populations along the coastal

mountains and another along the inland mountains (Sierra Nevada range).

-They form a ring around California’s Central Valley.

-Salamanders of the different populations contrast in coloration and exhibit more and more genetic differences the farther south the comparison is made.

-At the northern end of the ring, the coastal and inland populations interbreed and produce viable offspring → In this area they appear to be a single biological species.

-At the southern end of the ring, the coastal and inland populations do not interbreed even when they overlap → In this area they appear to be two separate species.

Adaptive radiation—the evolution of many diversely adapted species from

a common ancestor -ISLANDS! Compare Hawaii to the Florida Keys.

An example of induced isolation: a-Diane Dodd has demonstrated the ability of ________zygotic

reproductive barriers to develop due to adaptive divergence by separate populations in fruitflies, Drosophila pseudoobscura. (what a name!)

b-She divided a sample of fruit flies into groups that were cultured for several generations on media containing either starch or maltose.

c-Through natural selection acting over several generations, the population raised on starch improved their efficiency at starch digestion, while the maltose populations improved their efficiency at malt sugar digestion.

d-Females from populations raised on a starch medium preferred males from a similar nurturing environment over males raised in a maltose medium after several generations of isolation, demonstrating a newly-established prezygotic barrier to interbreeding.

II. Sympatric speciation—intrinsic isolation

(e.g. genetic, mating) Polyploidy—extra sets of chromosomes

-postzygotic barrier may arise in a single generation!

-allopolyploid—zygote due to 2 different species’ gametes fusing successfully

-responsible for up to 50% of new plant species

-e.g. spontaneous hybrid of cultivated wheat & wild grassmodern bread wheat (8000 years ago)

-e.g. birds with different beaks—why might they choose mates with similar beaks? -Many current food plants are a result of chemically-induced polyploidy (oats, wheat, cotton, potatoes, tobacco)

Punctuated equilibrium (vs. gradualism) •Proposed by Stephen Jay Gould (brilliant & knows it) (knew it, that is)

-species diverge in spurts of relatively rapid change, then remain unchanged for the rest of its existence (note—“rapid” might mean only a few thousand years)

-In the fossil record, many species appear as new forms rather suddenly (in geologic terms), persist essentially unchanged, and then disappear from the fossil record.

•Evidence: paleontology shows few intermediate forms

-Climbing Mt. Improbable, one of many works by Richard Dawkins, (aka “The Man”)

The Origin of Evolutionary Novelty •Adaptations are only modified versions of older structures

-e.g. birds’ light bones’ initial use? -e.g. wings’ initial use? -Interesting idea: some structures that evolve in one context, but become co-opted for another function are called

exaptations.

•Embryonic genes—very important -homeotic genes -allometric growth—difference in relative growth rates of body parts

•Evolution of morphology by modification of allometric growth is an example of heterochrony, an evolutionary change in the rate or timing of developmental events.

-Heterochrony appears to be responsible for differences in the feet of tree-dwelling versus ground-dwelling salamanders. -The feet of the tree-dwellers with shorter digits and more webbing may have evolved from a mutation in the alleles that

control the timing of foot development. -These stunted feet may result if regulatory genes switched off foot growth early. -Thus, a relatively small genetic change can be amplified into substantial morphological change.

Paedomorphosis—a sexually mature adult retains juvenile characteristics

-e.g. axolotl