Anthropology 101: Human Biological Evolution

Lecture 4: ModernEvolutionaryTheory

Dr. Leanna WolfeLAWolfe@aol.com

Office AHSDrop In Hours:Thursday 5:00-6:30 PM AHS 303

All this variation! Where does it come from?

• Mutation• Meiosis

Recombination of alleles into unique gametes increases genetic variation at a faster rate than mutation

• Sexual Reproduction Recombination of alleles from unique gametes into

unique offspring New phenotypes for NS to act upon

• Complex genetic inheritance Polygenic traits Environment interacts with genotype

Hidden Variation allows species change

• Normal sized wolves carry some alleles for small body size (some – alleles, and many + alleles)

• As big wolves die (or people prefer small ones), frequency of – alleles increases

• Variation is shuffled, some new combinations arise

• As – alleles become more common, more – alleles likely to be combined in a single individual

• New combinations with more – alleles will be outside initial range of variation

What forces produce and redistribute variation in a population?

• Mutation introduces variation at low rates• Recombination allows different combinations of traits and alleles to

appear (meiosis, independent assortment, sexual reproduction)

• Natural selection works with existing variation to change its representation in a population

• Gene flow introduces variation from other populations• Genetic drift can eliminate variation in small populations

Founder Effect• Sexual selection favors traits that enhance success in reproductive

competition• Nonrandom Mating is bias in how we choose our mates

Gene Flow

• Exchange of genes between populations

• Can be one-way or two-way

• Not exactly the same as migration Amish: Migration without gene flow Vietnam War: Gene flow without migration

• Maintains variation within populations

• Reduces variation between populations

Genetic Drift

• In small populations, random processes affect variation

• Frequencies in small samples show small random differences from large source population

5 red:5 black 6 red:4 black 3 red:7 black 7 red:3 black

Population Bottlenecking

• Population crash• Sampling error• Reduces genetic

variability

Population Genetics allows us to track the changes in allele frequencies in a population

• Understand how traits in a population can change over time = Evolution

• Demonstrate how alleles stay hidden in the genome & available for natural selection to act upon

Evolution is the change in the genetic composition of a population over time

• Natural selection acts on phenotypes (genotypes affected)

• Next generation has different traits (and genes) that are common

• When allele frequencies change from one generation to the next, evolution is occurring

• Equilibrium = no change

25% Aa

70% AA5% aa

Let’s follow a Mendelian trait…

• Sickle-cell anemia

• Two alleles A = normal hemoglobin S = sickling hemoglobin

• Three genotypes AA normal blood cells SS sickled blood cells AS slightly impaired blood

cells, but greater defense

against malaria

Selection for AS genotype (and S allele)

• AS people have an advantage• S allele favored as part of the heterozygotes despite SS

disease & death (before reproduction)

F0 generation F1 generation F2 generation

AAAAAA

ASAS AS

SS SS SS

Natural selection is not guaranteed to produce “perfect” adaptations

• NS can only act on existing traits• Traits arise via mutation

• Natural selection does not have “foresight”

• natural selection is limited by variation that is currently available

Pleiotropy

• In Darwin’s finches, beak traits are correlated

Deeper & wider Shallow & narrow

• one gene affects multiple traits = correlated traits

• If environment changes, population is not perfectly suited Playing catch up as NS favors different traits Finches on Galapagos at beginning and end of drought

In Humans!!• Certain kinds of nutrients (fat, salt, sugar) were rare in the past• Selection created strong preferences for them that persist even

though they are no longer rare

Disequilibrium

Laws of Physics & Chemistry

• Some designs may be physically impossible

• Why can’t elephants fly? Heavy animals need strong

bones Strong bones are heavy

• Why can’t male mammals lactate? Hormones that facilitate milk

production inhibit testosterone production

Species are easy to recognize but hard to define

Biological species concept• Fertile interbreeding (gene flow)

(gets stopped by) Reproductive isolation

Ecological species concept• Exploit a single ecological niche• Natural selection keeps

individuals in a species similar (same useful traits)

Biological Species Concept:Reproductive isolation prevents gene flow

• Isolated populations can’t exchange genes Geographical barriers (pre-mating) Behavioral differences (pre-mating) Viability of offspring (post-mating)

Premating isolating mechanisms prevent accidental matings between species

• Mechanical incompatibility

• Habitat isolation• Temporal isolation• Behavioral isolation

Postmating isolating mechanisms reduce the fitness of accidental matings between species

• Sperm-egg incompatibility• Zygote failure• Embryonic failure• Offspring death• Offspring sterility

Problems:• Gene flow can occur

between “good” species

• Medium finches mate with large ground finches 10% of time

• Why aren’t they one species?

Medium ground finch

Large ground finch

Biological Species Concept is not perfect

Ecological species concept is based on selection

• Selection favors certain phenotypes in a particular niche• Hybrids do poorly (post-mating reproductive isolating mechanism!)

food available to birds

Ecological species concept is based on selection

• Selection maintains differences between species even if gene flow occurs

Darwin’s finches

• Selection maintains similarity between species even if no gene flow occurs

Checkerspot butterfly

Checkerspot butterfly

How does a new species form?

• Speciation - process by which a new species evolves from an earlier species = macroevolution

Anagenesis Cladogenesis

• Adaptive Radiation Competition pushes some individuals to exploit new habitats Diversify and separate from original species

• Adaptive potential (species & traits) Generalized Specialized

Generalized vs. Specialized Traits:Hands vs. Feet

Generalized vs. Specialized Species:Norwegian Rat vs. Naked Mole Rat

Three kinds of speciation

• Allopatric speciation

• Parapatric speciation

• Sympatric speciation

Allopatric Speciation

• Closely linked to biological species concept

• Gene flow interrupted

• Isolated population adapts (allopatric species)

• If the two populations reunited (sympatric species now)

• No longer able to interbreed

food available to birds

Sympatric species kept distinct via Reproductive Isolating Mechanisms

• Hybrids do poorly

• Selection for pre-mating barriers

The tempo of evolutionary change

• Gradualism - Microevolution leads to the accumulation of small changes over time

Fossil record one long series of small changes from one generation to the next

Small differences eventually add up to a new species

The tempo of evolutionary change

• Punctuated Equilibrium - evolutionary change proceeds through long periods of stasis punctuated by rapid periods of change

Transition to new species is rapid

Evolution leads to new species (rather than gradual changes in same species)