Mechanisms of Evolution: Population Selection and Change

Mechanisms of Evolution:Mechanisms of Evolution:

Population Selection and Change

Overview of this SectionOverview of this Section

• Defining Population and Species• Mutations• Natural Selection• Gene Flow• Genetic Drift• Sickle Cell Anemia and Other Deleterious

Genes• Speciation

Populations, Gene Pools, and SpeciesPopulations, Gene Pools, and Species

• Population: In a species, A community of individuals where mates are usually found

• Gene pool: all the alleles in a population• Species: a population of organisms whose

members can interbreed under natural circumstances reproduce fertile (viable) offspring

• The species is the basic unit of all breeding populations

Counterexample: Related Animals Counterexample: Related Animals whose Offspring are Sterilewhose Offspring are Sterile

• .

• Female horses and male donkeys can interbreed,

• But their offspring (mules)• cannot reproduce fertile

offspring• This cartoon shows a male

donkey attracted to a female horse (mare)

• The mule (right) will not reproduce

• Mules are bred for their strength and their even temperament

Related Animals That Can But Do Related Animals That Can But Do Not Interbreed in the WildNot Interbreed in the Wild

• Lions and tigers have interbred in captivity and reproduced viable offspring (top left); this couple calls the hybrid “liger”

• But they don’t interbreed in the wild• Shades of Gray: Dogs and wolves can

interbreed and reproduce—in captivity and in the wild

• Recently, Canis familiaris has taxonomically been lumped with Canis lupus (“wolf dog”, lower left)

Sources of Variation in Species:Sources of Variation in Species:• Mutation: change in DNA structure• Segregation: Separation of the DNA strands in

sex cells at meiosis• Independent Assortment: What occurs when

genes on different chromosomes segregate to gametes independently of one another.

• Genes on the same chromosomes do not segregate independently of one another.

• Recombination: the exchange of genetic material between pairs of chromosomes during the first stage of meiosis.

MutationsMutations• Mutation: Any change in the genetic code• Point Mutation: Change in a single triplet of a

DNA chain, often with no consequence.• Sickle cell anemia is one significant

exception:• The amino acid Glutamic acid that changes to

valine by one molecule in one triplet is the cause of this disease: CTC to CAC.

• The effect: the normal red blood cells (circle) take on the shape of a sickle (left diagram)

• The sickle-shaped cells cannot carry sufficient oxygen and block capillaries, the small blood vessels that carry oxygen to the cells.

• Consequences: fatigue, stunted growth, fever, and death by the 20s

Chromosomal MutationChromosomal Mutation• Chromosomal Mutation:

Change in a chromosome or a large portion of one

• The change is usually deleterious

• For example the replication of three copies of chromosome 21 (of the 23 pairs) is associated with Down Syndrome (upper left)

• Occurs when a pair of parental chromosomes fail to segregate at the second stage of meiosis, resulting in the 3 copies. (lower left)

Mutations as “Errors”Mutations as “Errors”• Mutations are random, frequent, and occur at a

constant rate. • Mutations are important to evolution if• they involve changes in gametes (sex cells)• These are passed on to the next generation• Mutations are usually deleterious; most Down

syndrome victims miscarry. • New forms are usually poorly adaptive to an

environment, so that they may not have a chance at all to reproduce

Mutations as “Necessary Errors”Mutations as “Necessary Errors”

• Mutations may also prove neutral, or they may yield even better (more adaptive) phenotypes

• If they occur in sex cells, they will be selected and passed on to the next generation.

• They add variation to a gene pool—all the alleles in a population have a better chance to survive in several niches (microenvironments)

• Bottom Line: No mutation, no change, no evolution

Sources of Variation in Gene PoolsSources of Variation in Gene Pools

• Gene Pool: All the alleles in a population• Natural Selection: The environmental selection

of species that influence differential reproductive success

• Gene Flow: The process in which alleles from one population are introduced into another population

• Genetic Drift: Random changes in a small population that are the products of chance.

Natural SelectionNatural Selection

• Natural Selection: textbook definition• Evolutionary change based on differential

reproductive success of individuals within a species

• Generally, the differential reproductive success is the product of environmental pressures.

• Summary of Principle• Adage: man proposes, God disposes.• Mutation proposes, natural selection disposes

Example: Drought on GalExample: Drought on Galáápagos pagos Islands: 1977Islands: 1977

• Natural selection can be best illustrated in a 1977 drought on Galapagos Island

• Prior to 1977, there were Finches of various sizes in body and in beak

• The selection factor, drought, brought about a drop in the insect population and favored seeds with a tougher cover to minimize water loss.

• These changes favored the finches with larger body size and longer beaks, better at cracking nuts (such as Geospiza fortis, left)

• Selection results were dramatic: only 14% of the total population survived

• The finches that reproduced had those two attributes-large body size and long beaks. .

• This does not mean that the others were wiped out; when the drought ended, the population of other species of finches returned.

Example: Galapagos 2003-2004 Example: Galapagos 2003-2004 DroughtDrought

• But natural selection continued to work to this day.

• In 1982, another population of large finches, G. magnirostris (No. 1, left photo) arrived on the Galapagos Islands and ate three times as many seeds as G. fortis (No. 2) and the smaller finches

• Smaller finches again went into steep decline (no. 3 and 4).

• Then in 2003 and again in 2004, another drought struck, increasing the mortality rate of the larger finches

• This left the seeds for the smaller finches, who increased their population at the expense of the larger species.

Types of Natural Selection: Types of Natural Selection: Direction and StabilityDirection and Stability

• Directional Selection: Selection characterized b a generation-after-generation shift in a population in a specific direction

• Examples: Beak size among finches in 1977; bipedalism among hominins (from Lucy to us)

• Stabilizing Selection: Selection characterized by a generation-after-generation in a population toward a mean (average).

• Example: long-term stabilization of finch size and beak length, especially induced in the droughts of 2003 and 2004.

Natural Selection and Sickle Cell Natural Selection and Sickle Cell AnemiaAnemia

• As noted above for sickle cell anemia, mutation often has deleterious effects• However, natural selection has a part in tropical areas where malaria

predominates• Homozygotes for sickle cell anemia are selected against—they die off• Homozygotes for normal red blood cells are subject to elimination by

malaria• But heterozygotes with sickle cells are immune to malaria and so are more

likely to survive than homozygotes wth normal red blood cells

Sexual Selection: Sexual Selection: • Intersexual Selection: Traits that

make males more attractive to females (Peacock wooing peahen, top).

• Intrasexual Competition: Sexual selection that make males better able to compete for sexual access to females (as in this gorilla fight).

• Sexual Dimorphism or physical difference of males and females of the same species, play a role in both species.

• Peacocks (male) are showier than peahens; male gorillas are twice the size of females.

Kin SelectionKin Selection• Kin Selection: Behavior which increases

an individual’s chances of his/her genes being propagated into the next generation.

• Altruism: Behavior characterized by self-denial or self-sacrifice to benefit others; seen especially among close kin.

• Inclusive Fitness: An individual’s own fitness and his or her effect on the fitness of any biological kin.

• Grooming behavior among these Japanese macaques is one example of altruistic behavior, though it extends to non-kin as well.

Gene FlowGene Flow

• Evolution: Change in allele frequency is one part of definition of the term. They involve:

• Gene flow: Exchange of genes among populations through interbreeding

• Breeding populations: populations within a species that to some extent are genetically isolated from other species

• Demes: same definition as breeding populations with emphasis on smallest of such populations

Sources of Gene FlowSources of Gene Flow

• Migration of new populations into existing ones

• Interbreeding without migration• Removal of natural barriers between

populations• Removal of reproductive barriers.

Mating as a Factor of Gene FlowMating as a Factor of Gene Flow• Non-Random Mating: A preferential form of

mating• Consanguineal Mating: Mating between

biological relatives. • Incest Tabu: The prohibition against mating with

close relatives, especially primary relatives (father-daughter, mother-son, and brother-sister

• Cross-Cousin Marriage: A practice often observed, even required, between offspring of brother and sister. Gene flow is limited here

• In this diagram, one sees the men marrying their female cross cousins who are both their mother’s brother’s and father’s sister’s daughters. This is common among Indians of Brazilian Amazonia

Marriage Promotes Gene FlowMarriage Promotes Gene Flow

• E.B. Tylor: Groups have always had the choice of marrying out or dying out.• Matrilateral cross-cousin marriage (man marries mother’s brother’s

daughter) increases the network of kinship betweenthree or more groups (left)• Marriage is one way to cement an alliance between two groups—be they

lineages, tribes, or nations

Genetic DriftGenetic Drift

• Definition: A process in a small population whereby the frequency of alleles in the junior generation will differ from that of the senior generation due to sampling error

• Sampling Error: When a sampling error does not accurately represent the population from which the sample was taken

• This requires some background in statistics

Genetic Drift: Statistical Genetic Drift: Statistical ConceptsConcepts

• Statistical Concepts important here• Population: total number of individuals being

researched• Sample: Portion of population selected for study• Random sample: selection whereby everyone has a

chance to be included• Representative sample: selection whereby every group

has a chance to be included• Sampling Error (in context): Condition whereby

sample chosen for study does not accurately reflect the population from which the sample was taken either because the sample is nonrandom or non-representative.

Genetic Drift: Founder EffectGenetic Drift: Founder Effect

• When a population splits, each population will show a non-representative sample of the genes

• Fission: Splitting up of population to form new populations; usually populations are small

• Founder Effect: Genetic differences between population produced by the fact that genetically different individuals established (founded) the population

• Of 300 original North American Hutterites (a rural religious group of Anabaptists), only 90 individuals contributed genes to future generations

• Most of today’s 35,000 Hutterites trace their ancestry to those 90

Genetic Drift: Bottleneck EffectGenetic Drift: Bottleneck Effect• Bottleneck Effect: Following a severe reduction in

population size, only certain genes survive, which come to characterize the descendant population

• Pennsylvania Dutch• Of a total number of 333 Pennsylvania Dutch in one

study, 98 were found to have an allele for Tay-Sachs disease, a genetic condition usually found among Jews of Eastern European descent

• Tay-Sachs disease involves an enzyme deficiency of lipid metabolism in which fatty acids attack nerve cells; untreated, it usually causes death at an early age.

• All 333 were descended from a single couple, one of whom apparently carried the allele that now affects 98 individual.

Other Evolutionary Processes: Other Evolutionary Processes: Genetic Drift 4Genetic Drift 4

• Gamete sampling: Genetic changes are caused when genes are passed to new generations In frequencies unlike those of the parental generation

• This is a type of sampling error• Example: PTC taster versus nontaster of phenylthiocarbamide (the

bitter substance in brussels sprouts) • A heterozygous couple will possess each allele• But that couple won’t necessarily pass on an equal number of

alleles to the next generation; chance determines the outcome of the couple’s mating

• With large populations, chance plays less of a role.• Genetic drift affects small populations

Summary and ConclusionsSummary and Conclusions

• Source of new alleles is mutation• Other sources of variation include• Natural selection, based on survival value

of traits• Gene flow, based on interbreeding • Genetic drift, based on fission and

sampling error