SPECIATION 6

SPECIATION 6

several conditions - self-fertilisation, vegetative propagation, higher fitness than the diploid, or niche separation from the diploid - may enable new polyploidy to increase & form a viable population
1. because polyploidy apparently confers new physiological & ecological capabilities it may play important role in plant evolution
  1. their increased number of genes may enhance their adaptability
2. polyploidy doesn't confer major new morphological characteristics eg differences in structure of flowers or fruits, so seems unlikely to cause evolution of new genera or other higher taxa
recombination speciation - arises when hybrid genotypes, that are fertile but reproductively isolated from the parent species, increase in frequency forming a distinct population (aka hybrid speciation) - more common in plants than animals
1. hybridisation, by generating diverse gene combinations on which selection can act, can be source of new species with novel morphological & ecological features
speciation rates vary greatly:
1. expected to be very slow if it proceeds by mutation & drift of neutral alleles
2. expected to be faster if driven by ecological or sexual selection
3. expected to be accelerated if reinforcement plays a role
allopatric speciation could be slow or very rapid, depending on strength of divergent selection & on genetic variation in relevant traits
1. polyploidy, recombination speciation & sympatric speciation should be very rapid when they occur - but may occur rarely, resulting in long intervals between speciation events
  1. ecological speciation can be very rapid
characteristics that seem most likely to increase speciation rate include:
1. pollination by animal (rather than wind) in plants
2. features that indicate intense sexual selection in animals
3. these observations suggest diversification in some groups of animals owes more to simple evolution of reproductive isolation (due to sexual selection) than to ecological diversification
MOST IMPORTANT CONSEQUENCE OF SPECIATION IS THAT ITS THE SINE QUA NON (ESSENTIAL CONDITION OR PREREQUISITE) OF DIVERSITY
for sexually reproducing organisms, every branch on phylogenetic tree represents a speciation event
1. speciation stands at border between microevolution - genetic changes within & among populations - & macroevolution - evolution of higher taxa in all their diversity
evidence that speciation contributes to both rates of molecular & morphological evolution, based on expected differences between patterns of divergence expected under models of phyletic gradualism vs punctuated equilibrium
1. in gradual model, rate of evolution of lineage is unaffected by number of speciation events
  1. in contrast, evolutionary change requires speciation & increases with number of speciation events, in pure punctuated equilibrium model
agreed no reason to think speciation triggers morphological evolution
1. nevertheless, morphological change might be associated with speciation because reproductive isolation enables morphological differences between populations to persist in the long term

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