Evolution and Speciation

Define evolution

• Evolution = change in heritable characteristics of organisms over generations

• Heritable = passed from parent to offspring via genes

• Caused by:

  • Mutation (random DNA change)

  • Natural selection

• Non-heritable changes (e.g. leaves eaten) ≠ evolution

Define natural selection

• Individuals show variation due to random mutations

• Selection pressure creates competition

• Individuals with advantageous traits survive & reproduce

• These traits become more frequent in population

• Traits must be heritable

Outline Darwin’s theory of evolution

• Species show variation due to mutations

• Organisms compete for survival

• Best-adapted survive and pass on advantageous alleles

• Over time, allele frequency changes → evolution

• Evolution is gradual and requires heritability

What is Lamarckian evolution and why is it incorrect?

• Traits used frequently become stronger

• Traits not used disappear

• Acquired traits passed to offspring

• Example: giraffes stretched necks → passed on

• ❌ Rejected: acquired traits are not heritable

What is sequence data and how is it used as evidence for evolution?

• DNA: base sequences from nucleus, mitochondria, chloroplasts

• RNA: shows gene expression

• Proteins: amino acid sequence reflects gene info

• More similarity = more closely related

• Must compare conserved regions (e.g. haemoglobin)

How do scientists compare DNA sequences?

• Extract DNA (blood, tissue, fossils)

• Sequence the base pairs

• Compare with other species

• Build evolutionary trees

• E.g. Humans and chimps share ~99% DNA

Define selective breeding and explain its connection to evolution

• Humans breed organisms with desirable traits

• Traits passed on over generations

• Example of artificial selection

• Faster than natural selection

• Provides evidence for evolution via DNA changes

Outline the steps of selective breeding

• Start with variation in population

• Select individuals with desired traits

• Breed selected individuals

• Test offspring → select best

• Repeat over many generations

What are homologous structures?

• Same structure, different function

• Suggests common ancestry

• Example: pentadactyl limb (5-digit limb)

  • Human foot, whale fin, bird wing, frog leg

What is convergent evolution?

• Different species evolve similar traits

• Caused by similar selection pressures

• Traits evolved independently

• Traits = analogous structures

List examples of convergent evolution

• Dolphin & shark: streamlined shape

  • Dolphin = mammal, shark = fish

• Cactus & euphorbia: spiny stems

  • Different plant orders, different continents

Define speciation

• Formation of new species from existing ones

• Occurs when gene flow stops

• Populations become reproductively isolated

• Accumulation of genetic differences → speciation

What is allopatric speciation?

• Caused by geographical isolation (natural or man-made barrier)

• No gene flow → different environments = differential selection

• Over time → new species

• Example: mountain range splitting tree population

What is sympatric speciation?

• Occurs without geographical barrier

• Isolation due to:

  • Temporal isolation (different mating seasons)

  • Behavioral isolation (different courtship)

• Prevents mating → no gene flow → new species

What is reproductive isolation?

• Individuals can no longer breed to produce fertile offspring

• Causes:

  • Different mating times (temporal)

  • Different courtship behavior

  • Geographic separation

Describe a real example of allopatric speciation

• Bonobos (south of Congo River)

• Chimpanzees (north of Congo River)

• River = barrier

• Different environments = different traits

• Eventually → reproductive isolation

What prevents hybridisation between different species?

• Different chromosome numbers → infertile offspring

• Example: mule (horse × donkey) = sterile

• Different courtship behaviors → no mating

• Example: birds with mismatched mating dances

What is adaptive radiation?

• One species evolves into many to fill different niches

• All share common ancestor

• Traits allow them to coexist in same habitat

• Example: Darwin’s finches, Hawaiian honeycreepers

What is polyploidy and how can it cause speciation in plants?

• Polyploidy = >2 sets of chromosomes

• Polyploid individuals can’t breed with diploid ones

• → Reproductive isolation = new species

• Often more vigorous or mutation-resistant

What is the difference between autopolyploidy and allopolyploidy?

• Autopolyploidy: Extra chromosomes from same species

  • Caused by nondisjunction during meiosis

• Allopolyploidy: Chromosomes from different species

  • Caused by hybridisation