Chapter 16: Evolution of Populations and Speciation

16-1 Variation of Traits in a Population16-2 Disruption of Genetic Equilibrium16-3 Formation of Species

I. Variation of Traits in a Population• Variation ALLOWS for natural selection to act upon a population.

16-1 Genetic Equilibrium

(1) Population Genetics (population is SMALLEST unit capable of evolving)• Study of evolution from a GENETIC perspective (i.e., gene pool changes).

(2) Bell Curve (within a LARGE sample population)• Measurable TRAITS display a BELL-CURVE pattern (i.e., MEDIAN form is FAVORED).

(A) Causes of Genotypic Variation in a Population• Genetics CAN increase VARIATION in a population in THREE ways:

(1) MUTATION from mutant copies of individual GENES.

(2) RECOMBINATION AND CROSSING-OVER during meiosis add variation to sex cells.

NOTE: On average, between 2-3 crossovers occur on each pair of chromosomes during meiosis.

(3) RANDOM fusion of GAMETES ensure offspring are NOT clones of parents.

II. Allele Frequencies and the Gene Pool• Some alleles are MORE NUMEROUS than other alleles in gene pools.

(1) Gene Pool• TOTAL genes AVAILABLE in a population of a species (a dynamic NOT static pool of genes).NOTE: SUM of ALL changes in a GENE POOL over a LONG period of TIME is known as “EVOLUTION.”

(2) Allele Frequency (a calculation)• Frequency of an ALLELE among ALL alleles in a POPULATION. (e.g., PREDICT the likelihood of a certain PHENOTYPE being expressed).

(A) Predicting Phenotype• Certain PHENOTYPES are EXPRESSED more often than OTHER TYPES.

(1) Phenotypic Frequency• Frequency of a SPECIFIED phenotype within an entire POPULATION.

(1) Do you think it is easier to analyze genotype in organisms with complete dominance or in organisms with incomplete dominance?

Critical Thinking

III. Hardy-Weinberg Genetic Equilibrium (i.e., a theoretical state)• A model showing HOW allele frequencies in a population CAN remain CONSTANT. (i.e., evolution does NOT occur).

(1) NO mutations occur; (NO allelic frequency change).(2) Individuals neither ENTER nor LEAVE the population. (3) Population is LARGE (ideally, infinitely large)(4) Individuals mate RANDOMLY.(5) Natural selection does NOT occur.

NOTE: This model ALLOWS us to consider what forces DISRUPT genetic equilibrium in a population (i.e., allow a POPULATION to EVOLVE).

Sample Problem: Using the Hardy-Weinberg equation of…p2 + 2pq + q2 = 1 (p = dominant allele)

(q = recessive allele)

Calculate the expected number of heterozygotes (Aa) and homozygous dominant (AA) individuals in a population of 100 tigers, 10 of which are albino white (homozygous recessive, aa).

q2 = (10/100) = 0. 1 Therefore q = 0.32

p = 1-q or 0.68

*Therefore, the frequency of homozygous dominant individuals is p2 or 0.46, or approximately 46%

*And the frequency of heterozygous individuals is 2pq or 0.44, or approximately 44%

16-2 Disruption of Genetic Equilibrium• A VIOLATION of Hardy-Weinberg equilibrium can result in

EVOLUTION (i.e., FIVE evolutionary forces).

I. Mutation (introduces VARIATION, disrupts equilibrium)• Mutations produce NEW alleles INTO a population.

II. Migration (disrupts equilibrium)• REMOVES or ADDS new members to a gene pool (i.e., new alleles/genes).

(1) Immigration (WIDENS gene pool)• MOVEMENT of NEW genes INTO a population.

(2) Emigration (SHRINKS gene pool)• Movement of genes OUT of a population.

(3) Gene Flow (FLUIDITY of the waves in the gene pool)• NET movement of genes BETWEEN populations of SAME species.

III. Genetic Drift (MORE likely experienced in SMALL populations)• CHANGE in allelic frequencies as a result of RANDOM events (chance).

IV. Nonrandom Mating (includes Assortative Mating)• Results from GEOGRAPHIC PROXIMITY and a SUITABLE MATE

(negative drawback, inbreeding).

(1) Assortative Mating (e.g., NON-random mating)• Selection of a MATE with SIMILAR physical traits (to ITSELF).

V. Natural Selection (selective forces of NATURE; for OR against)• An ONGOING process that DISRUPTS genetic equilibrium. (NOTE: 4 types of natural selection include…)

(A) Stabilizing Selection(B) Directional Selection(C) Disruptive Selection(D) Sexual Selection

(A) STABILIZING Selection (selects AGAINST the extremes)• Individuals with AVERAGE form of a trait have the HIGHEST fitness

(i.e., Selected FOR by nature)

(B) DIRECTIONAL Selection (selection is SHIFTED one side)• Individuals with a MORE EXTREME form of a trait (one extreme OR the other, NOT BOTH) have GREATEST fitness.

(C) DISRUPTIVE Selection (mean is selected AGAINST)• Individuals with EITHER extreme variation have GREATEST fitness.

http://research.umbc.edu/~farabaug/biol100/overheads/stabil.gif

(2) Human newborns with either a very high or very low birth weight are more likely to die in infancy. What type of selection does this seem to be?

Critical Thinking

(D) Sexual Selection (survival ALONE does NOT further evolution)• GENES of REPRODUCERS, rather than those of merely SURVIVORS, are FAVORED through natural selection.NOTE: The development of trait that may SEEM harmful (coloration patterns) can enhance the reproductive fitness if it encourages MATING.

I. The Concept of Species• Existing species are MODIFIED versions of ANCESTRAL species.

16-3 Formation of Species

NOTE: As species EVOLVE in the world, it has been a challenge to “exactly” DEFINE a species.

(1) Speciation (an evolutionary process—forming a NEW species)• Members become ISOLATED (over time) AND evolve into a NEW species (Allopatric and Sympatric Speciation).

(3) What effect might a very short generation time, such as that of bacteria, have on speciation?

Critical Thinking

(A) Morphological Concept of Species• DEFINED on evidence of STRUCTURE and APPEARANCE (i.e., its morphology).

(1) Morphology• Structure AND appearance of an organism—using as SPECIES identifier has 2 LIMITATIONS:(1)Phenotypic variation AMONG members of a single population, and (2) Interbreeding of “different” species producing FERTILE offspring.

(B) The Biological Species Concept (Ernst Mayr)• A population of INTERBREEDING organisms producing fertile offspring BUT cannot breed with OTHER groups.NOTE: Limitations include our LACK of ability to classify FOSSILS as species (unable to determine EXTINCT reproductive compatibilities) as well as with ASEXUAL organisms.

II. Isolating Mechanisms• Speciation begins with ISOLATION—TWO TYPES of isolation DRIVE speciation.

(A) Geographic Isolation (Allopatric Speciation)(B) Reproductive Isolation (Sympatric Speciation)

(A) Geographic Isolation• PHYSICAL separation due to geographical barriers (can lead to DIVERGENT evolution)

(B) Reproductive Isolation (TWO types: FOLLOWS geographical isolation) • Genetic BARRIERS between members DUE to mutations OR incompatible recombinations (TAKES TIME).

(1) Prezygotic Isolation (FIRST type of reproductive isolation)• Reproductive isolation that occurs BEFORE fertilization of members of TWO populations. (Ex: Incompatible MATING CALLS in wood frogs)

(2) Postzygotic Isolation (SECOND type of reproductive isolation)• Reproductive isolation that occurs AFTER fertilization. (Ex: Offspring are born STERILE, never develop, OR die early).

(4) Where populations of two related species of frogs overlap geographically, their mating calls are different. Where the species don’t overlap, their calls are identical. What type of isolating mechanism is in operation?

Critical Thinking

III. Rates of Speciation (2 Theories)• MILLIONS of years NEEDED for speciation, HOWEVER, NEW species have formed within a few THOUSAND years.

(5) Highways may provide an effective geographic isolation mechanism for some slow-moving animals. Why are such artificial barriers not likely to result in complete speciation?

Critical Thinking

(1) Punctuated Equilibrium (theory of speciation)• SHORT QUICK bursts of RAPID genetic change followed by LONGER periods of LITTLE to NO change.

(2) Gradualism (Darwin’s belief of speciation)• Occurs over a LONGER period of a million years or so, resulting in SMALLER, GRADUAL genetic changes at a STEADY pace.

Extra Slides AND Answers for Critical Thinking Questions(1) Incomplete dominance is easier because heterozygotes have a

different phenotype from homozygous dominant individuals.(2)This is an example of stabilizing selection because the intermediate phenotype is favored.(3) A very short generation time speeds up the rate of evolution and therefore can increase the rate of speciation.(4) This is an example of a prezygotic isolating mechanism—it prevents mating between the two species.

(5) Such artificial barriers are unlikely to be in place long enough for speciation to occur.

Revisiting Reproduction and Inheritance

• The distribution of the genes in successive generations is determined by the characteristics of the genes and by the interaction between genes and environment.

Assessing Prior Knowledge

• In tiger populations, a recessive allele causes an absence of fur pigmentation, producing an albino or white tiger with pink eyes. What genotype would an orange, heterozygous tiger have?• Would you suggest the white phenotype in tigers to be selected FOR or selected AGAINST by natural selection?