Theory of Evolution
descent with modification
natural selection
Genetic Variation is the substrate of natural selection.
mutation/horizontal gene
transfer
sexual recombination
Tracking alleles and genotypes from one generation to the next.
Mendelian genetics
Hardy-Weinberg Equilibrium
Genetic divergence leads to speciation.
species concepts
patterns and rates of
speciation
How do you define a species?
Figure 24.1
Gould, Stephen Jay. 1992. What is a
species? Discover 13(12):40-44.
1941-2002
• paleontologist and evolutionary biologist
• Harvard University Professor
According to Gould, are species real?
Many people suppose that species must be arbitrary divisions
of an evolutionary continuum in the same way that state
boundaries are conventional divisions of unbroken land….I
write this article to argue that such a reading of evolutionary
theory is wrong that that species are almost always objective
entities in nature. --- Stephen Jay Gould
Domain Species are the real and objective
Kingdom items of nature’s morphology. They
are “out there” in the world as
Phylum historically distinct and functionally
Subphylum separate populations with their own
historical role and tendency.
Class
Order
Species are unique in the Linnaean
Family
hierarchy as the only category with
Genus such objectivity. All higher units…are
Species human conventions…
---Stephen Jay Gould
A species concept is a way of defining the concept of a
species and/or of providing an approach to distinguish one
species from another. [Brooker et al. (2023), p. 512]
Species Concept Explanation Year
Proposed
Biological Individuals of the same species can 1942
Species Concept interbreed and produce viable, fertile
offspring
Evolutionary Series of species in a line of descent 1961
Lineage Concept where each new species evolved from the
immediate, previous ancestor
Ecological Each species occupies an ecological niche 1976
Species Concept
General Lineage Each species is a population of an 1998
Concept independently evolving lineage
• Speciation occurs when one
ancestral species evolves
into one or more new
species.
• Some event separates a
population:
• time
• space
• genetics
• Populations then diverge
along their own evolutionary
path
• Either gradually or
rapidly...(Figure 24.16)
In the graduated model, a species acquires
small adaptations to its environment over
millions of years.
© 2017 Pearson Education, Inc. Chapter Table of Contents
In the punctuated equilibrium model, there
are periods of stasis interrupted by occasional
bursts of speciation.
530 million years ago during
a period called the Cambrian
explosion, the rate of
evolution was an order of
magnitude higher than the
normal rate. (It still required
millions of years.)
How does speciation occur?
..in almost all cases of large-scale evolution is a story of
branching, not transformation of a single line - bushes, not
ladders, in my usual formulation.
---Stephen Jay Gould
Allopatric speciation arises by geographic isolation of a few
members of the original population or species.
• genetic drift is more pronounced
• natural selection takes the isolate in a
different direction
Hybrid zones are areas where separated populations
interbreed.
Figure 24.12
Sympatric speciation most often arises by instantaneous
speciation through polyploidy.
• drastic changes in chromosome number
• chromosomal rearrangements
• disruptive selection
How do we declare a population a new, or
different, species?
The Biological
Species Concept
• Proposed by Ernst
Mayr in 1942
• A biological species
is a group of
interbreeding natural
populations that are
reproductively
isolated from other
such groups.
Reproductive barriers maintain species.
Prezygotic isolating mechanisms
Prevent mating attempts:
• Behavioral isolation:
Members of a species often
identify each other through
specific rituals.
• Temporal isolation: Many
species reproduce only at specific
times.
• Habitat isolation:
If species live in slightly
different habitats, they may
never meet.
Reproductive barriers maintain species.
Prezygotic isolation mechanisms
Mating is attempted:
• Mechanical isolation:
Members of different species
often cannot mate because their
anatomies are incompatible.
• Gametic isolation:
The gametes (sperm and egg)
of different species usually
cannot fertilize each other.
Reproductive barriers maintain species.
Postzygotic isolating mechanisms
Fertilization occurs:
• Hybrid inviability: The
fertilized egg fails to develop.
• Hybrid sterility: An
interspecies hybrid survives, but it
is sterile.
• Hybrid breakdown:
The F1 interspecies hybrid is
viable and fertile but succeeding
generations (F2 and so on)
become increasing inviable. Figure 24.5
The Biological Species Concept
applies to:
• sexually reproducing species
• living (not extinct) species
• species with gene flow
(immigration) between
populations.
n n 2n
Figure 24.2 zygote
The Biological Species Concept cannot be
applied to all situations…...
• species that reproduce asexually
• extinct organisms (fossils only)
• limited gene flow between
populations
