evolution and speciation in life sciences
TRANSCRIPT
Speciation• Formation of a new species• Species:
– a population that can breed freely and produce fertile offspring
• Speciation often occurs when part of the population is isolated from another part– Selective pressures of the environment in one
area may be different from pressures in another area
What is a Species?• Definition :• Morphospecies - based on appearance• Biologic species - a population that can breed
freely and produce fertile offspring• The largest unit of population in which gene flow is
possible• Limitations:
– doesn’t work for asexual organisms– extinct life forms– populations that are geographically isolated - sometimes
call subspecies• No clear answer; idea is arbitrary
Patterns of Speciation
• Fossil record shows 2 patterns:• Anagenesis (phyletic evolution)
– the transformation of an unbranched lineage of organisms, sometimes creating an organism different enough to be a new species
• Cladogenesis – branching evolution; budding of one or more
new species from a parent species that continues to exist.
Causes of Speciation• Speciation often occurs when part of the
population is isolated from another part• Geographic Isolation
– most common– a physical barrier develops (changing course of
a river; separation of an island)– Selective pressures in one area are different
from pressures in another area• Reproductive Isolation
– another form of isolation
Geographic Isolation• Biogeography of Speciation • Classified according to geographic relationship
between new and old species• Sympatric
– population becomes reproductively isolated in the midst of the parent population
– ranges of new and old species overlap.• Allopatric
– species are physically separated– more likely in small populations
• Adaptive radiation is allopatric :– emergence of numerous species from a common ancestor
that spreads to several new environments.
Reproductive Isolation
• Example: organisms breed at different times
• Reproductive barriers are of 2 types:• Prezygotic
– before the formation of fertilized eggs– impedes mating or fertilization
• Postzygotic – after
Prezygotic Isolation• Impedes mating or fertilization• Habitat isolation
– not geographically separated, but occupy different niches within an area, e.g. trees versus ground
• Temporal isolation – breed at different times
• Behavioral isolation– don’t produce appropriate courtship signals
• Mechanical isolation – anatomically incompatible
• Gametic isolation – mating occurs but gametes rarely fuse to form zygotes
Postzygotic Barriers
• Hybrid inviability – offspring don’t make it
• Hybrid sterility – e.g. mules
• Hybrid breakdown – F2 are sterile or weak
Introgression
• Alleles pass a reproductive barrier when a fertile hybrid mates with a parent species
• Increases variation• Rare
– 2 species remain distinct
Post Speciation Evolution• Divergent Evolution
– Process by which related organisms become less alike
– occurs after speciation– at first 2 new species are very similar, but over
time become more & more different.• Adaptive radiation is a special type of
divergent evolution– Many new species from a single parent
species
Timing of Evolution• Most scientists accept natural selection as the process
of evolution• The timing is controversial• Gradualism
– the traditional view– a slow, steady accumulation of changes, leads to new
species• Punctuated Equilibrium
– long periods of inactivity followed by big jumps• Fossil record provides evidence that the pace of
evolution varies– The same evidence is used to support different ideas– Could be some of both