Evolution of Sexual Reproduction, Species Concepts, and Macroevolutionary Patterns

What is sexual reproduction?

Eukaryotic reproduction via meiosis and fertilization producing genetically mixed offspring.

When did sex evolve?

In early eukaryotes around 1.2-2.0 billion years ago.

Which lineages have sex?

Most eukaryotes; not prokaryotes.

Genetic consequences of sex

Recombination segregation increased variation and reduced linkage.

What is asexual reproduction?

Reproduction without meiosis or gamete fusion.

What is parthenogenesis?

Development from unfertilized eggs; not all asexual reproduction is _______.

Two-fold cost of sex

Females transmit half as many genes and produce non-reproductive males.

Advantages of sex

Increases genetic variation faster adaptation breaks up bad alleles.

Disadvantages of sex

Energy costs finding mates disease risk and the two-fold cost.

Muller's ratchet

Accumulation of harmful mutations in asexual lineages avoided by recombination.

Red Queen hypothesis

Sex helps hosts keep up with parasites as in the snail-trematode system.

Sexual vs natural selection

Sexual selection is about mating success; natural is about survival/reproduction.

Who proposed sexual selection?

Charles Darwin in 1871.

Sexual dimorphism

Differences between males and females due to anisogamy.

Parental investment differences

Females invest more; males limited by access to mates.

Intersexual vs intrasexual selection

Intersexual = mate choice; intrasexual = competition within a sex.

Forms of intrasexual selection

Combat mate guarding sperm competition alternative strategies.

Forms of intersexual selection

Mate choice based on ornaments displays gifts or territories.

Direct vs indirect benefits

Direct = resources or care; indirect = genetic quality.

Good genes hypothesis

Females choose males with high genetic quality increasing offspring fitness.

Sex role reversal examples

Seahorses jacanas phalaropes where males provide most care.

Linnaeus' binomial system

Two-part scientific names that standardized taxonomy.

Biological species concept

Species are groups that can interbreed; not useful for fossils or asexuals.

Morphological species concept

Defines species by physical traits; subjective but works for fossils.

Phylogenetic species concept

Smallest monophyletic group; precise but may oversplit.

Cryptic species

Morphologically identical but genetically distinct species.

Why species are hard to define?

Speciation is gradual hybridization occurs and concepts differ.

Speciation definition

Formation of new species through isolation divergence and reproductive isolation.

Five evolutionary processes

Mutation selection drift migration recombination.

Allopatric speciation

Speciation caused by geographic isolation.

Dispersal vs vicariance

Dispersal = movement to new area; vicariance = barrier splits population.

Sympatric speciation

Speciation without geographic separation; harder due to gene flow.

Rhagoletis flies example

Host shift from hawthorn to apple causing reproductive isolation.

Howea palms example

Divergence on different soil types on the same island.

Polyploidy

Whole genome duplication.

Allopolyploidy

Hybridization followed by chromosome doubling.

Macroevolution

Large-scale evolutionary patterns above the species level.

Standing diversity

Number of taxa existing at a given time.

Turnover

Origination plus extinction rates.

Raup's dataset

Marine invertebrate fossils used to study extinction patterns.

Sepkoski's curve

Fossil diversity curve showing increases and mass extinction events.

Background vs mass extinction

Background = normal rate; mass = rapid global high-loss events.

Permian extinction

Occurred 252 mya; largest mass extinction affecting marine and terrestrial species.

End-Cretaceous extinction

66 mya; wiped out dinosaurs and many marine taxa.

Sixth extinction

Current human-caused extinction crisis.

Life spans of taxa

Duration between first and last fossil appearances.

Living fossils

Lineages with little morphological change (e.g., coelacanth horseshoe crab).

Lineage survivorship curves

Show probability of persistence through time often exponential.

What is behavior?

Responses to stimuli; behavior evolves when heritable.

Nervous system and behavior

Nervous system mediates behavior but some organisms behave without one.

Tinbergen

Founder of ethology known for four questions about behavior.

Belding's ground squirrels example

Alarm calls explained by kin selection.

Forms of social behavior

Selfishness altruism spite mutualism.

Group vs kin selection

Group selection acts on groups; kin selection favors helping relatives.

Eusociality

Social system with castes cooperative care and overlapping generations.

Why eusociality is a problem?

Sterile workers seem counter to natural selection; solved by inclusive fitness.

Direct vs indirect fitness

Direct = personal reproduction; indirect = helping relatives reproduce.

Human phylogenetic placement

Primates → apes → great apes → African apes → hominins → Homo.

Five Hs

Hylobatidae Hominidae Hominoidea Hominins Homo.

Great ape species

Chimpanzee bonobo gorilla species orangutan species humans.

Where great apes live?

African apes in Africa; orangutans in SE Asia.

Closest relatives

Chimps and bonobos; closest extinct relatives include Neanderthals and Denisovans.

Steps to Homo sapiens

Bipedalism tools brain expansion culture; some traits changed nonlinearly.

Key fossil species

Ardipithecus Australopithecus Laetoli footprints Oldowan tools H. erectus naledi floresiensis Neanderthals.

Out of Africa events

H. erectus ~1.8 mya and H. sapiens ~60-70 kya.

Evidence for human evolution

Fossils DNA archaeology each with strengths and limitations.

Neanderthal/Denisovan admixture

Non-Africans have Neanderthal DNA; Oceanians have Denisovan ancestry.

What is cancer?

Uncontrolled cell growth caused by mutations.

Cancer as clonal evolution

Tumors evolve via mutation selection and drift within the body.

Mutagens and carcinogens

UV tobacco chemicals radiation viruses.

Where cancer occurs most?

Rapidly dividing tissues; more common in older individuals.

Three mutation classes in cancer

Oncogenes tumor suppressors DNA repair genes.

Why cancer predisposition persists?

Late-onset weak selection and evolutionary trade-offs.

BRCA1 and BRCA2

DNA repair genes whose mutations increase cancer risk.

Infectious cancers examples

Devil Facial Tumor Disease and CTVT.

Cancer-resistant organisms

Naked mole rats and elephants.