CHAPTER 13CHAPTER 13

How Populations EvolveHow Populations Evolve

Taking a closer look at organisms.

•Notice differences from other organisms

•Features that show the relationship of an organism to where it lives, what it does

Ex: marine iguanas to land iguanas

Darwin & His Boat TripDarwin & His Boat Trip

• Spent 4 ½ years traveling to different places

• Had awareness of how each organism’s adaptations allow it to fit into its envrionment

• Came to see how species change over time & that living species arose from earlier species

• Based on ideas of interrelatedness dating back to over 2500 years ago

Who Thought What Prior to Who Thought What Prior to DarwinDarwin

• Early Greek: (2500 years ago) life began in water w/simpler forms then more complex forms

• Aristotle Greek: (2000 + years ago) life was fixed and didn’t evolve

• Judeo-Christian: all species were created in a single act of creation about 6000 years ago

• Lamarck: (French, early 1800s) supported evolution but believed in acquired characteristics as mechanism for change

The Man of ControversyThe Man of Controversy• Published The Origin

of Species by Means of Natural Selection or the Preservation of Favoured Races in the Struggle For Life in 1859 based on ideas of those before him, evidence he had accumulated, & publishing pressure

Charles Darwin

Natural SelectionNatural Selection“As many more individuals of each species are born than can possibly survive; and as, consequently, there is a frequently recurring struggle for existence, it follows that any being, if it vary however slightly in any manner profitable to itself, under the complex and sometimes varying conditions of life, will have a better chance of surviving, and thus be naturally selected.” – Charles Darwin, The Origin of Species

Sect 13.2

Natural SelectionNatural Selection

•In a varied population, individuals whose inherited characteristics adapt them best to their environments are more likely to survive and reproduce leaving more offspring than less fit individuals do

Natural Selection = DifferentialNatural Selection = DifferentialReproductionReproduction

• Indivduals in populations vary• Some are more suitable to a given

environment & reproduce more easily and abundantly

• favored characteristics are passed on to next generation (less-favored are not)

• Gradual accumulation of favored characteristics among individuals in a population occurs over vast amounts of time

Fossil Evidence for EvolutionFossil Evidence for Evolution Kinds of Fossils

1. imprint in a rock (leaf, feather, organism)

2. cast made that filled in hollows of an animal track or shell

3. petrified wood or bone (actually replaced w/minerals)

4. frozen in ice,…these are the remains of “Lucy” found in the mountains of Asia

5. Insect or other organism trapped in plant resin (amber) …tar pits

•Fossil record is an array of fossils w/in strata of sedimentary rock

- each stratum has deposits of material that has accumulated

- younger strata sit on top of older strata*sedimentary rock contains the most fossils because it’s formed by layers of fine particles settling out of a liquid (shale, sandstone,limestone – most of the 3)

Mass Evidence for EvolutionMass Evidence for Evolution

1. Biogeography-observations about distribution of different but related forms around the world & in neighboring geographical regions

• Island forms are more similar to forms found on the closest mainland than those found on similar but more distant islands

Sect 13.4

2. Molecular Biology- demonstrates universality of genetic code•Conservation of amino acid sequences in proteins (hemoglobin) are compared•Some processes virtually the same in all organisms (glycolysis)

Table 13.4 p. 263

3. Comparative Anatomy- comparison of homologous structures

• All mammals have same basic limb structures

• Structure not function

• Leads to descent w/modification

• Vestigial organs

4. Comparative Embryology- comparison of embryonic stages of different organisms•All vertebrates have pharyngeal pouches in their throat regions

Natural Selection: The Natural Selection: The Prominent Force in NatureProminent Force in Nature

•Populations tend to adapt to local environments during one time period

•These same adaptations may be pointless in other locales or times

2 Examples: Cepaea nemoralisCepaea nemoralis (land snail) & Biston betulariaBiston betularia

(peppered moth)• European

land snail’s shell patterns camouflage in different areas (striped for well-lit, dark for shady areas)

• Light & dark variety• Light was

predominant before Industrial Revolution

• Darkened trees w/pollution allowed for dark variety to become predominant

The Evolution of The Evolution of PopulationsPopulations

•Population – group of individuals of the same species living in the same place at the same time

•Evolution is measured by the frequency of a given characteristic w/in a population over successive generations not an individual

Sect 13.6

•Darwin realized this but didn’t know the genetic mechanism – which is??

•population genetics (1920s)- rediscovery of Mendel’s genetic principles & Darwin’s natural selection (how populations change genetically over time)

Modern synthesis (1940s) – incorporates population genetics & ideas from paleontology, taxonomy, biogeography

•Focuses on populations as the units of evolution & that central role of natural selection

biological species – a group of populations whose individuals have the same potential to interbreed & produce fertile (viable) offspring

•Opportunities for breeding among populations will vary, depending on species and extent of isolation of populations

MicroevolutionMicroevolution

•A change in a population’s gene pool – all of its alleles in all individuals making up a population that can be inherited by the next generation

•Most gene loci have 2 or more alleles in a population

•During microevolution, the relative frequencies of alleles can change (light to dark peppered moths)

Hardy-Weinberg PrincipleHardy-Weinberg Principle

• In a large population, the gene pool will remain constant unless acted upon by some outside agent

•Separation of alleles during meiosis, recombining during fertilization has no effect on frequency of alleles in a population

Is this the case in nature?

Sect 13.7

5 Conditions Required by Hardy-Weinberg

1. Very large population2. Isolated population (no migrations

into or out)3. Mutations do not alter gene pool4. Mating is random5. All individuals are equal in

reproductive success (no natural selection)

Then why use it?Public health agencies:

•estimate the frequency of harmful alleles in a population

•establish programs to deal w/genetic diseases

•real populations are not the ideal (static) Hardy-Weinberg, stats must be constantly re-evaluated.

Sect 13.8

4 Potential Causes of 4 Potential Causes of MicroevolutionMicroevolution

1. Genetic Drift – change in a gene pool of a small population due to chance (effect of losing a few individuals is much greater than in a large population)

Bottleneck effect: an event that drastically reduces population size (earthquake, fire, flood, humans)

Founder effect: small group moves locations, genes not representative

Sect 13.9

2. Gene Flow – gain or loss of alleles in a population due to immigration or emigration of individuals or gametes

3. Mutations – rare events that it changes the whole population

4. Nonrandom Mating – more often the cause, particularly among animals (choosing mates w/particular traits, ex: healthiest)

- differential success in reproduction is probably always the case in natural populations (leads to adaptive elvolution)

Adaptive ChangeAdaptive Change

•results when natural selection upsets genetic equilibrium

•degree of adaptation that can occur is limited only by amount and kind of genetic variation in a population

•endangered species often have less variation because of a small population

•not all genetic variation is due to natural selection: - some neutral variation shows no apparent selective advantage (fingerprints)

- frequency of characteristics may be due to genetic drift

- difficult to show an allele benefits in one environment but not in another

Extensive Variation in Most Populations

•a single characteristic can be caused by one or more genes or by environmental action causing a phenotypic change

•population is polymorphic for a characteristic if 2 or more morphs (contrasting forms) are present

•populations can exhibit geographic variation (preference of flower color by the local pollinator)

Cline- graded change in an inherited characteristic due to geographic location

2 Random Processes Generate Variation

•mutations normally are harmful but could be adaptive in a changing environment

•sexual recombination shuffles mixture of alleles (such as?)

Independent assortment, crossing over, random fertilization

Sect 13.12

How does Natural Selection Affect Variation?

•starts w/varied ancestral population•over successive generations, those

w/characteristics best suited for environment leave more offspring behind than ill-suited

•recessive alleles (and their effects) can remain in population over long periods of time

Sect 13.14

Overall Effects of Natural Overall Effects of Natural SelectionSelection

•Perpetuation of Genes Defines Evolutionary Fitness

•Sexual Selection & Sexual Dimorphism•Natural Selection Acts on Whole

Organisms•Natural Selection’s 3 Modes of Action•Resistant Populations of Pests &

Parasites

Perpetuation of Genes Defines Perpetuation of Genes Defines Evolutionary FitnessEvolutionary Fitness

•emphasis is survival of genes over time, not individual organisms

•fitness - the relative combination that an individual makes to the gene pool of the next generation (passes on the greatest number of genes)

Sexual Selection & Sexual Dimorphism

•Secondary sexual characteristics (size, male adornment)

•Differences between male & female

Natural Selection Acts on Natural Selection Acts on Whole OrganismsWhole Organisms

•acts on phenotypes•each phenotype is the sum of the

organism’s genotype•there is no way for natural

selection to select individual gene loci (it favors the whole organism)

Natural Selection’s Natural Selection’s 3 Modes of Action3 Modes of Action

(1)• Stabilizing Selection - tends to narrow ranges in population variability toward some intermediate form (in stable environments, most common)

Sect 13.16

(2)

• Directional Selection - tends to move the modal form toward one of the extremes (most common during times of environmental change)

(3)

• Disruptive Selection - occurs when environmental factors are varied in a way that favors extremes over the intermediate form

disruptive

Resistant Populations of Pests Resistant Populations of Pests & Parasites& Parasites

•directional selection is the mode for pesticide, antibiotic, and drug resistant pests and disease causing organisms

•these life forms are adaptable because they are numerous, multiply rapidly, and have short generation times

Sect 13.13

So Why Aren’t We Perfect?

1.Historical Constraints – we can only use what our ancestors gave us

2.Adaptations & Compromises – no rain, no rainbows

3.Chance & Natural Selection – nature can change things

4.Selection w/Variations – populations can only use what is available (see # 1)

“The key is man’s power of accumulative selection: nature gives successive variations; man adds them up in certain directions useful to him. In this sense he may be said to have made for himself useful breeds....for hardly any one is so careless as to breed from his worst animals.”

- Charles Darwin The Origin of Species