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Same species or different species? 2A ? 2B Speciation
Population of a species Microevolutionary Forces Population evolves; adaptation How do Species Form? • Speciation (process of a species splitting into ≥ 2 more species) bridges microevolution with macroevolution.
• Biological species concept: naturally interbreeding individuals of a population that produce fertile offspring
Emphasizes reproductive isolation: a biological barrier that prevents members of two species from producing viable offspring Gene flow occurs (or can potentially occur) among populations of the same species Applicable only to sexually reproducing types; can’t apply to fossils or prokaryotes • Morphological species concept: characterize species due to similarities in structures; applicable to all life forms • General Speciation Process
– A reproductive barrier is established in a population such that 2 populations become separated; gene flow between them is stopped. – Over time, the separated gene pools diverge genetically. – Reproductive isolation established if members of each species cannot form viable hybrids or form hybrids that are sterile or do not survive well. Reproductive barrier established Two species if: members of each population no longer capable of producing viable, fertile offspring • Reproductive isolating mechanisms
– Prevent gene flow between species & limit formation of hybrids – Typically a combination of barriers work to isolate a species’ gene pool. • Prezygotic barriers: prevent mating or fertilization
– Temporal isolation – Spatial isolation – Mechanical isolation – Gametic isolation – Behavioral isolation • Postzygotic barriers: reproductive failure following fertilization & hybrid zygote formation
– reduced hybrid fertility – reduced hybrid viability – hybrid breakdown. Pre‐zygotic barriers
• Temporal isolation
– Mating at different times of the day or year • Habitat (Spatial) isolation
– Species occupy different habitats in the same area (e.g., tree v. ground dweller) • Gametic isolation: prevents zygote formation.
– environment of female reproductive tract may not be conducive to the survival of sperm from other species. – fertilization requires gametes recognize each other via specific molecular interactions Mechanical isolation: incompatible genital organs
Example: White sage v. Black sage 2 different plant species
Only small bees can land on the petal of the black sage Only large bees brush against the stamens of the white sage Cross‐pollination can’t occur • Behavioral isolation
– Courtship behaviors are species specific
The male satin bowerbird builds a bower (shelter) of twigs to attract females • Postzygotic barriers
– Reduced hybrid viability
• Hybrid dies when genetic regulation fails during development of the – Reduced hybrid fertility
• Problems during meiosis cause abnormal gametes (ex: horse x donkey sterile mules) – Hybrid breakdown
• 1st generation hybrids are viable & fertile, but matings with other hybrids yield offspring that are sterile or don’t survive well. Allopatric speciation:
Geographic barrier (water, land) splits population; alternately, some members of a population migrate and colonize a new, isolated area. Allopatric speciation Sympatric speciation Sympatric speciation: Divergence of two populations in the same geographic region. Occurs via polyploidy, habitat /resource differentiation, or sexual selection. Parent Population Allopatric Speciation
• The extent of the geographic barrier (large or small) to allow for speciation depends on the mobility of the species. • Once geographic separation occurs, gene pools diverge via natural selection & genetic drift changes allele frequencies.
– Reproductive isolation mechanisms may evolve • Reproductive isolation between populations increases as a function of distance. • Regions highly divided geographic barriers generally have more species. • Geographic isolation is not itself a biological barrier to reproduction. Sympatric speciation by polyploidy in plants
Allopolyploidy • Mating of species with different chromosome number; creates a hybrid with multiple sets of chromosomes. • Hybrid reproductively isolated from parent species • Autopolyploidy: diploid individual produces diploid gametes, yielding tetraploid offspring. Sympatric Speciation via Habitat Differentiation & Sexual Selection
• Habitat differentiation: some members of a population exploit a habitat or resource not used by the parent population.
– Example: change in insect : plant host relationship • Sexual Selection: females preference for male type changes based differing male characteristics related to appearance.
– Example: Lake Victoria cichlids Male coloration change becomes selected for as some females preferentially choose them as mates; gene pools diverge new species Contact of Allopatric Populations Hybrid Zone • Hybrid zones form where members of previously isolated populations intermittently mate & produce hybrids. The results of these matings has three outcomes:
– Reinforcement: hybrids are less fit that parent species; strengthening reproductive barriers favored – Fusion: hybrids as successful as parent populations gene pools merge – Stable hybrid zone: hybrids continue to be produced Hybrid Zone of Two European Toad Species Macroevolution
• As species diverged and speciation continued, differences accum‐ ulated and large‐scale phenotypic changes appeared. • The changes to life on Earth were influenced by:
– Continental drift: altered habitats, changed climate on land masses as their position shifted, & promoted allopatric speciation – Mass extinctions: disappearance of evolutionary lineages; altered balance of organism types; paved the way for new speciation. – Adaptive radiations: evolution of many diversely adapted species upon introduction to various new environmental opportunities.
• • Worldwide in scope – followed mass extinctions Regional in scope – colonization of a new, distant areas (e.g., islands) possessing little competition. Evolutionary Developmental Biology
• Major changes in body form resulted from changes in developmental genes. – Developmental genes are conserved among animals (fruit flies to humans); regulate timing , pace, & pattern of development from zygote to adult. • Change in rate or timing of develop‐ mental events (heterochrony): – Allometric growth: differential growth of body parts; change in body shape if altered. – Paedomorphosis: retention of juvenile traits in the adult form; certain salamander species retain gills from early development. • Changes in spatial patterns of development
– Homeotic genes: master regulators that determine the location & arrangement of body parts during development. – One type Hox genes: control development of structure at proper location; altering genes dramatically changes morphology. Evolution of vertebrates due to Hox gene duplications Evolution of insect body plan from crustacean ancestor Change in Ubx gene (a Hox gene); led to development of 6‐legged form common to insects. Evolution Is Not Goal Oriented
• Evolution is the “tweaking” of existing forms giving rise to descen‐ dant forms that work to match the organism to its environment. • In many cases, complex structures gradually arise from simpler, previous forms (e.g., the eye). • Q: The eye is so complex, requiring many parts to function – how could this have been of use in a simpler, ancestral from? • A: simple eyes with less complex function do function in ways that benefit its owner. • Eyes evolved independently many times; linked to various lineages by developmental genes common to all animals that possess eyes. Evolution is Branching
• Evolution of the horse shows there are many “dead ends” in the progression to the modern horse (Equus) [follow the yellow line] Trends in horse evolution: larger body size; fusion of toes into one toe. Other phenomena: evolutionary reversals (e.g., loss of limbs by snakes) Pre‐adaptations: structure adapted for use in one way, is later used a different way (e.g., evolution of bird wings) Summary
• Defining a species: biological species (reproductive isolation); also morphological, ecological, phylogenetic species. • Speciation process: formation of a biological barrier; genetic divergence; elimination of gene flow. • Reproductive isolation
– Prezygotic: mechanical, gametic, temporal, spatial (habitat), behavioral – Post‐zygotic: reduced hybrid viability, reduced hybrid fertility , hybrid breakdown • Allopatric v. Sympatric speciation • Hybrid zones‐ consequences • Macroevolution (forces of change; evolutionary developmental biology) • Evolution does not work to seek perfection. ...
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This note was uploaded on 04/15/2010 for the course BIOL 1362 taught by Professor Loeblich during the Spring '08 term at University of Houston.
- Spring '08