Speciation and
Macroevolution
Chapter 17
Defining a Species
• Morphological Species Concept- species are distinguished from each
other by one or more distinct physical characteristics.
• Cryptic species- look very similar but vary in other traits, such as reproductive
strategies.
• Evolutionary Species Concept- species are distinguished from each other
by certain morphological traits. Abrupt changes in traits indicate a new
species
• Phylogenetic Species Concept- an evolutionary family tree is used to
identify species bases on a common ancestor- is not based on physical
characteristics- can be based on DNA
• Biological Species Concept- relies on reproductive isolationphysiological, behavioral, and genetic processes inhibit interbreeding
Prezygotic Isolating Mechanisms
• Prevent reproductive attempts or make it unlikely for fertilization
to be successful if mating occurs.
• Habitat isolation- If two species occupy different habitats, they are less likely to
meet and attempt to reproduce.
• Temporal isolation- If species reproduce at different times of the year, they will
not interbreed
• Behavioral isolation- courtship patterns are specific to species
• Mechanical isolation- If animal genitalia or plant floral structures are
incompatible, reproduction cannot occur
• Gamete isolation- in the event of reproduction, some gametes will not be able
to fuse to form a zygote.
Postzygotic Isolating Mechanisms
• Operate after the formation of a zygote
• Hybrid inviability- developing embryo may receive incompatible instructions
from maternal and paternal genes, so it is unable to develop properly
• Hybrid sterility- zygote may develop into a sterile adult.
Speciation- The transformation of one
species into one or more species
• Allopatric- Speciation occurs due to geographic isolation or
another physical barrier
• Sympatric- Speciation occurs due to divergence of diet or
microhabitat
• Polyploidy• Autoploidy- diploid plant forms diploid gametes due to nondisjunction and a
sterile triploid plant is made.
• Alloploidy- two different species of plants hybridize and the number of
chromosomes doubles.
Speciation- The transformation of one
species into one or more species
• Adaptive Radiation- a single ancestral species gives rise to a
variety of new species rapidly.
• Usually involves the removal of a predator or a change in the environment
• Convergent Evolution- biological trait evolves in two unrelated
species as a result of an exposure to a similar environment
• Analogous trait- evolve in response to a similar lifestyle or habitat
• Homologous- evolved from a common ancestor
Principles of Macroevolution
• Gradualistic model- speciation occurs after populations are
isolated, with each group continuing on its own evolutionary path.
• Punctuated Equilibrium model- assembly of species in the fossil
record can be explained by periods of stasis punctuated by periods
of rapid, abrupt speciation
Developmental Genes
• Specific genes can bring about radical changes in body shapes and
organs
• Pax6 gene is required for eye formation in all animals (Ex- flies, human,
squid, mice)
• Tbx5 gene is expressed in both humans and birds in developing limb buds
• Hox genes control the number and appearance of repeated structures along
the main body axes of vertebrates