Natural Selection and Phenotypes

• Natural selection determines the survival of phenotypes and genotypes.
• Natural selection acts on the phenotypes of individuals.
• Organisms with phenotypes suited to the environment have greater fitness.
• Natural selection can lead to permanent genetic changes in a population over time.
• Natural selection is a dynamic process that changes with environmental selection.

Types of Natural Selection

• Natural selection includes stabilizing, directional, disruptive, and mate selection.

Stabilizing Selection

• Original populations have varied phenotypes, with the middle range most common.
• Extreme variations are selected against, retaining middle-range phenotypes.
• Stabilizing selection decreases variation and prevents divergence, frequently occurring to maintain adaptive phenotypes.

Directional Selection

• Adaptive phenotypes shift in one direction.
• Selection becomes directional when environmental conditions trend, such as from warm to cold, especially during abrupt changes.

Disruptive Selection

• Favors extreme phenotypes over intermediate forms.
• It occurs when environments or resources fluctuate distinctively.

Mate Selection

• Sexual selection influences an organism's ability to find a mate or successfully copulate.
• Breeding passes on alleles/phenotypes, increasing fitness.
• Sexual selection can produce features harmful to individual survival.

Speciation

• Speciation creates new species through evolution.
• It occurs when a group becomes reproductively isolated and develops unique traits.

Species

• A biological species interbreeds successfully to produce fertile offspring.
• This concept applies to animals more than plants due to plant hybridization.

Populations as Units of Evolution

• Natural operates on individuals, but evolutionary impacts affect populations over time.
• A population is a group of a single species in the same area.
• A deme is a group within a population isolated from others.
• Geographical barriers often limit gene flow between demes, but not enough for speciation.
• Gene flow maintains members as the same species.

Cline

• A cline is a gradual change in phenotypes and genotypes across a geographical gradient.
• Allele frequencies and traits vary slightly due to different selection pressures.
• Members of a cline interbreed with neighbors, forming a continuous genetic gradient.

Ring Species

• Ring species are closely related populations distributed around a barrier.
• Adjacent populations can interbreed, but extremes are reproductively isolated.

Reproductive Isolating Mechanisms (RIMs)

• RIMs prevent interbreeding between different species including prezygotic and postzygotic mechanisms.

Prezygotic Mechanisms

• Prezygotic mechanisms operate before fertilization to prevent gamete wastage.
• They are the most common and can be behavioral, morphological, or temporal.

Postzygotic Mechanisms

• Postzygotic mechanisms operate after fertilization.
• They are less common but maintain species integrity.
• Results involve premature death or infertility.

Allopatric Speciation

• Allopatric speciation involves species divergence after geographic isolation.
• This can be broken down into Isolation, different selection pressures and finally reproductive isolation.

Step 1: Movement to a New Environment

• Species expand ranges due to competition.

Step 2: Geographical Isolation

• Physical barriers like mountains form, and sea levels change with glacial periods.

Step 3: Differential Selection Pressures

• Selection pressures in isolated populations cause different phenotypes to be selected.
• Reproductive isolation mechanisms are not yet established, but subspecies are significantly different.

Step 4: Reproductive Isolation

• Genetic and behavioral differences and RIMs are established, making interbreeding impossible.
• Temporal isolation: breeding at different times.

Sympatric Speciation

• Sympatric speciation forms new species without geological isolation.
• It's rare in animals but occurs via niche specialization.
• Insects may lay eggs on new plants to reduce competition.
• Groups become reproductively isolated due to microhabitat preferences, limiting gene flow.
• Sympatric speciation is more common in plants, usually via polyploidy.

Polyploidy

• Polyploidy shows variations relative to parental species and may lead to instant speciation.

Polyploidy – Instant Speciation

• Polyploidy occurs when an organism has more than two sets of chromosomes.
• It is common in plants and some animals.
• It arises when chromosomes fail to separate during mitosis or meiosis.
• Polyploidy can result in plant species being reproductively isolated from the ‘parent’ species.
• In plants, polyploidy doesn't result in infertility as plants carry both male and female sex organs and can reproduce via self-fertilization.

Non-Disjunction

• Non-disjunction occurs when chromosomes fail to separate during cell division.
• It results in daughter cells with an abnormal chromosome number.
• It can occur during meiosis or mitosis.

Autopolyploidy

• Autopolyploidy occurs when an organism has more than two sets of chromosomes from the same species.

Allopolyploidy

• Allopolyploidy occurs when two different species produce an infertile hybrid
• Chromosomes from the two parents are not identical so cannot pair during meiosis but chromosome doubling allows meiosis to occur normally.
• There are one and two step processes.

Advantages of Polyploidy

• Hybrid vigor shows improvements over the parent and increases heterozygosity, which reduces recessive mutations as well as gene redundancy.

Co-evolution

• Co-evolution, divergent and convergent evolution are covered.

Divergent Evolution

• Divergent evolution is the divergence of species from a common ancestor - this is facilitated by genetic accumulation and new species arising.

Convergent Evolution

• Convergent evolution involves unrelated species evolving similar traits to adapt to similar environments due to similar ecological niches and selective pressures.

Analogous Structures

• Analogous structures are similar traits in different species that evolved independently to perform the same function due to convergent evolution.

Co-evolution

• Co-evolution is the reciprocal evolution of species in an ecological relationship.
• Each party exerts selection pressure on the other and species develop mutual dependency.
• Examples – pollinator/plant and predator/prey.

Adaptive Radiation

• Adaptive radiation involves rapid species divergence from a single ancestor: ecological oppurtunities, mass extinction and novel adaptive traits.

Ecological Opportunities

• Ecological opportunities permit rapid diversification in species number and morphology.

Mass Extinction

• Mass extinction occurs when many species go extinct in a short time due to various causes.

Acquisition of Novel Adaptive Traits

• Evolutionary novelties can cause unique variations that adapt to different roles.
• An example of this is feathers on bird wings evolved from reptilian scales representing preadaptation of flight.

Rates of Evolutionary Change

• New species may arise gradually or suddenly.
• Species that reproduce frequently and quickly can result in higher rates of evolution.

Punctuated Equilibrium

• Punctuated equilibrium involves species staying the same for long periods (stasis).
• These periods are punctuated by bursts of evolution that rapidly provide new species.
• Stimuli for change occur when the environment changes, creating new selection pressures.

Phyletic Gradualism

• Phyletic gradualism assumes populations diverge slowly, accumulating adaptive characteristics.

Evidence of DNA & Fossils

• Fossils – formations
• Organisms die and sediment quickly covers it. Best in an aquatic environment or burial by volcanic ash.
• Usually only hard materials, bone and teeth remain. Layers of sediment start to create pressures
• More layers and increased pressure. Minerals in surrounding sediment moves into bone and replace the bones.
• Erosion of the sediments exposes fossils on the surface.

Index Fossils

• Index fossils are remains of plants and animals that lived during a specific geological time. - They identify geologic periods and correlate rock layers.
• They involve short vertical range, wide geographic distribution and are common and easily identifiable.

Geology – the Study of Rocks

• Strata are sedimentary rock layers that form when sediments deposit on each other.
• How strata forms: weathering, transportation, deposition, compaction and cementation.

Types of Strata

• Bed: Layers of rock with a uniform texture and lithology.
• Bands: Thin strata distinguished by color or lithology.
• Key marker beds: Well-defined strata easily identifiable by lithology or fossil content.

Biostratigraphy

• The study of fossils to determine geological strata's relative ages.
• A biozone is an interval of geological strata characterised by certain fossil taxa.

Relative Dating

• Relative dating is a geological method determining the order of past events.
• Geologists use radiometric dating methods to date ancient events.

Biogeographical Evidence

• Biogeography studies the distribution of species and ecosystems across geographical space and geological time.
• Organisms and communities vary along geographic gradients and barriers.

Continental Drift and Evolution

• Drifting continents create new environments that drive evolutionary pressures on living organisms through:
  • Isolation and speciation: When continents split populations are separate
  • Climate change forcing species to adapt to different environments, that can drive evolutionary changes.

Comparative Anatomy

• Comparative anatomy provides evidence through body structure comparisons.

Homologous Structure

• Homologous structures are similar body parts in different species that share a common ancestor: include divergent evolution.
• Example features can be organs, systems or limbs.

Analogous Structures

• Analogous structures are features similar in function but not necessarily in structure.
• Evolved in response to a similar environmental challenge.

Vestigial Structures

• Vestigial structures are body parts that have lost their main function over time and are often reduced in size.
• Provides hints to the evolution of an organism.

Explore how natural selection shapes the survival of phenotypes and genotypes. Understand the role of environmental adaptation and fitness in driving genetic changes within populations. Learn about stabilizing and directional selection.