Evolution: Unifying Biology - Medium

Evolution as a Unifying Framework

• Evolution is the central tenet that unifies all biological disciplines.
• It explains the patterns of diversity seen across life.
• Evolution explains the unity of biochemical and genetic mechanisms.
• It describes the relationships between organisms over time.
• Evolution is the process by which complex adaptations develop.
• It provides insights into practical problems like antibiotic resistance.
• It explains climate change impacts on species.
• Studying SARS-CoV-2 evolution highlights how genetic mutations and selection lead to new variants.
• Viruses evolve differently than whales due to their different mutation rates.
• Understanding new variants is essential for developing vaccines and pandemic responses.

Multiple Lines of Evidence Support Evolution

• Evolutionary biology integrates evidence from diverse fields.
• This integration strengthens its explanatory power.
• Paleontology gives fossil records such as the Tiktaalik.
• These records show transitional features.
• Comparative anatomy exhibits homologies in vertebrate limbs.
• Genetics displays a shared genetic toolkit across diverse animals.
• Biogeography gives the distribution patterns of species.
• Direct observation such as Grants' finch studies and pesticide resistance can be studied as evidence.
• The evolution to tetrapod limbs from fish fins can be observed through fossil evidence, such as the Tiktaalik.
• Genetic evidence supporting evolution to tetrapod limbs from fish fins is conversed hox gene expression.
• Developmental and molecular evidence is observed.
• Embryonic limb bud formation, Shh, and Hoxd13 gene expression patterns are both examples of evidence.

Evolution Operates at Multiple Scales

• Evolutionary processes vary differently from molecular changes to species diversification.
• Microevolution shows changes in allele frequencies within populations.
• Finch beak size and pesticide resistance are examples of microevolution.
• Macroevolution shows changes that lead to new species, and major evolutionary transitions (whale evolution from land mammals).
• Molecular evolution shows changes in DNA, RNA, and proteins over time.
• The Grants' studies of Galápagos finches show microevolution occurring over observable time periods.
• Beak size changed in response to drought conditions over a long period of time.
• This operation over millions of years produced the diverse beak morphologies among finch species.
• Micro and macroevolution are connected.

Natural Selection as a Primary Mechanism of Adaptation

• Natural selection is a powerful force shaping adaptations.
• It is not the only evolutionary mechanism.
• Natural selection requires heritable variation.
• Differential reproductive success is a result of natural selection.
• Natural selection produces adaptations to environmental challenges.
• It can take different forms such as directional, stabilizing, and disruptive.
• Natural selection leads to trade-offs due to constraints.
• Beach mice evolved light-colored fur that matches their sandy habitat, providing camouflage from predators.
• This adaptation arose independently in Gulf and Atlantic populations through different genetic mutations.
• Natural selection can find multiple genetic "solutions" to the same environmental challenge.

The Tree of Life and Common Descent

• All organisms are connected through a branching tree of life.
• Closely related species share more recent common ancestors.
• Hierarchical classification reflects evolutionary relationships.
• Homology is different than homoplasy.
• Homoplasy is an example of convergent evolution.
• Monophyletic groups form natural classification units.
• Phylogenies are hypotheses based on shared derived characters.
• Cetaceans, such as whales, and dolphins, evolved from terrestrial ancestors.
• Fossil transitions from land to water shows the evolution of cetaceans.
• Vestigial hindlimb structures can be observed.
• The developmental patterns of embryonic dolphin hindlimb buds exist.
• Molecular evidence showing shared genetic markers with artiodactyls indicates evolution from terrestrial ancestors.

Molecular Basis of Evolutionary Change

• Changes in DNA provide the raw material for evolution.
• Mutation causes genetic variation.
• Sources of variation include point mutations, which are substitutions, insertions, and deletions.
• Gene duplications can lead to new functions.
• Changes in gene regulation affect development.
• Recombination occurs through sexual reproduction.
• Horizontal gene transfer occurs especially in microbes.
• Snake venom evolved through a series of gene duplication of the defensin gene.
• Mutations altered protein function.
• Regulatory mutations express in venom glands rather than other tissues.
• Complex adaptations can evolve through stepwise genetic changes.

Constraints and Trade-offs in Evolution

• Evolution is constrained by genetic, developmental, and physical factors.
• Imperfect adaptations are the result of these constraints, and trade-offs.
• Physical and chemical constraints such as laws of physics exist.
• Developmental constraints like conserved pathways are factors.
• Genetic constraints such as pleiotropy and linkage are factors of evolution.
• Historical constraints such as path dependency exists.
• Antagonistic pleiotropy in mammals limits the number of cervical vertebrae to seven (even in giraffes with 2m necks).
• Mutations that allow more vertebrae affect other developmental processes.
• Evolution often involves trade-offs rather than perfection.

Evolution and Development (Evo-Devo)

• Changes in developmental processes occur through alternations in gene regulation.
• These changes can produce major evolutionary innovations.
• Key evo-devo concepts include a conserved genetic toolkit across animals.
• Regulatory changes can alter timing and expression patterns.
• Homologous genes can be co-opted for new functions.
• Small developmental changes can have large phenotypic effects.
• The evolution of tetrapod limbs from fish fins involved changes in the expression patterns of genes like Hoxd13 and Shh.
• Rather than evolving new genes, evolution modified the regulation of existing ones.
• New anatomical structures from ancient genetic materials were created.

Humans as Agents of Selection

• Human activities create selective pressures that drive rapid evolution in many species.
• Examples include antibiotic and pesticide resistance.
• Evolutionary responses to overharvesting can be seen in fished populations being smaller.
• Domestication of plants and animals is an example.
• Climate change driving selection on life history events occurs.
• Commercial fishing targets larger fish, creating selection for earlier maturation at smaller sizes.
• This anthropogenic selection has led to evolutionary changes in many fish populations.
• These changes reduce their reproductive capacity and threaten fishery sustainability.

The Dynamic Nature of Evolution

• Evolution is an ongoing process, not a historical event, that continues to shape all life on Earth.
• Evolutionary rates can vary dramatically.
• Selection pressures change over time and space.
• Environmental change can reverse selection patterns.
• Multiple mechanisms operate simultaneously.
• The Grants' study of Darwin's finches shows that selection can fluctuate in direction and intensity.
• During drought years, selection favoured larger beaks.
• In wet years, the pattern reversed.
• Evolution is dynamic rather than directional toward a fixed "optimal" form.