Wednesday, March 06, 2013
So far, we've been talking about populations (level at which evolution occurs)
•Study of evolution in populations: MICROEVOLUTIONS
How do you get new species?
•One of the fundamental questions in MACROEVOLUTION
What is a species?
•You know: A species is the smallest fundamental unit in the Linnaean hierarchy - all
individuals of a given specific type
•BUT: Species means different things to different biologists.
•Biological Species Concept
o Biological species is a group of organisms that are (1) actually or potentially
interbreeding and (2) are reproductively isolated from other groups
o If you CAN or COULS mate and produce fertile offspring, you're in the same
•What is reproductive isolation?
o Prevention of gene flow between populations by genetically determined
differences between them.
o Something that prevents act of mating/production of viable offspring other than
o Parts font fit
o Different body sizes
o Different courtship rituals
o Genetic differences
• Cause offspring to be sterile (ex: mule)
• Male: mule donkey x female horse
Problems: Some very different groups CAN produce viable offspring.
• Liger: M Lion x F Tiger - sterile
• Tigon: M Tiger x F Lion - sterile
o Some guenon species can produce fertile offspring even though they look and
behave differently AND even have different number of chromosomes…
•Ecological species concept
o Emphasizes the role of natural selection in maintaining species boundaries
o Gene flow CAN occur, but separate species are recognizes because natural
selection acts against hybrid individuals.
o Ex: Baboon species are ecological species but not biological species
How different species arise
•Three modes of speciation
•Mode #1: ALLOPATRIC speciation: occurs when a population is divided by some type of
physical barrier (river, mountain range, ocean) and then the separated populations diverge
•Mode #2: PARAPATRIC speciation: occurs when two populations are physically
adjacent, but adapt to different environmental conditions over time. Hybrid zones occur
between different species ranges.
•Mode #3: SYMPATRIC speciation: individuals of a single population in one habitat
gradually diverge over time (no large-scale spatial separation or physical barrier- rare) o Tree hopper: six sympatric species. Each lays eggs on different genus of host
plant in same forest. Eggs hatch at different times in response to spring sap flow.
Diff. species reach sexual maturity and mate at different times.
• In each of these three modes of speciation, one ancestral species gave rise to two or
more descendant species.
o Cladogenesis: speciation by splitting of lineages
• Adaptive Radiation: Special type of cladogenesis, Rapid series of multiple
cladogenic events. Often occurs following colonization of new landmass.
Primate Example: Lemurs.
Anagenesis: gradual transformation of one species into another. How you decide
where to recognize a species boundary is a problem… (How much change do you
need in order to tell which is a different species?)
• What is an Adaptation?
o Any trait that increases an organism's fitness (lifetime reproductive potential)
• Do adaptations have to arise by natural selection?
o No, but they must be maintained by natural selection (because adaptations by
definition beneficial/increase fitness)