CHAPTER 24: The Origin of Species
Macroevolution: The origin of new taxonomic groups (new species, genera, families. . .)
Speciation: The origin of new species. Speciation is the central process of macroevolution because the
appearance of new species is the source of biological diversity.
Two Basic Patterns of Speciation (See page 472, Figure 24.2):
1. Anagenesis: (phyletic evolution) the accumulation of changes that gradually transform a given
species into a species with different characteristics.
2. Cladogenesis: (branching evolution) The splitting of a gene pool into two or more separate pools,
which each give rise to one or more new species. **promotes biological diversity by increasing the
number of species**
24-1: The biological species concept emphasizes reproductive isolation
Species (Latin for ‘kind’ or ‘appearance’): is the basic biological classification of life. Morphology,
physiology, biochemistry, DNA, etc are all part of identifying a species, but a true species is hard to
define (more to come).
The Biological Species Concept: A species is a population or group of populations whose members have the
potential to interbreed in nature and produce viable, fertile offspring. . .A reproductively compatible
group (See page 473, Fig. 24.3).
Reproductive Isolation: the existence of biological factors that impede members of two different
species from producing viable, fertile hybrids. There are two types of barriers: Prezygotic and
Postzygotic (See pages 474-475).
Prezygotic Barriers (Before the zygote): Impede mating between species or hinder the
fertilization of the ova if members of different species attempt to mate. Barriers include:
1. Habitat Isolation: Species that occupy different habitats will not interbreed, Ex. Water and land
snakes
2. Temporal Isolation: Species that breed during different times of the day, different seasons, or
different years will not interbreed, Ex. Skunks that mate in winter or summer.
3. Behavioral Isolation: Courtship rituals, pheromones, songs, firefly lights, etc, that attract specific
mates, Ex. The ‘script’ mating ritual of blue-footed boobies.
4. Mechanical Isolation: Morphological differences that prevent successful mating (the parts are
anatomically incompatible), Ex. Shapes of the monkey flower and their pollinators
5. Gametic Isolation: Sperm cannot fertilize the eggs, because of biochemical barriers in the female, Ex.
The gametes of sea urchins.
Postzygotic Barriers (after the zygote): Prevent the hybrid zygote from developing into a viable, fertile adult.
Barriers include:
1. Reduced Hybrid Viability: The genes of different parent species may interact and impair the hybrid’s
development, Ex. Salamander hybrids do not complete development or they are very frail.
2. Reduced Hybrid Fertility: Production of sterile offspring because the parent species differ in number
or structure of chromosomes, Ex. The mule
3. Hybrid Breakdown: First-generation hybrids are fertile, but when they mate, their offspring are feeble
or sterile, Ex. Rice strains.
The biological species concept has a few limitations. There is no way to evaluate the
reproductive isolation of fossils or asexual organisms (like prokaryotes). It is also difficult to use this
model on sexual organisms about which very little is known.
There are many other definitions of a species, including morphological, paleontological, ecological,
phylogenetic, etc (Page 476). Could you create a better definition of a species?
24-2: Speciation can take place with or without geographic separation
There are two Modes of Speciation: Allopatric and Sympatric (See page 476, Fig. 24.5)
Allopatric Speciation (Other Country): A population forms a new species while geographically isolated from
its parent population (Mountains, rivers, sandbars, etc).
 The smallest population that has been split, known as the splinter or peripheral isolate, is a
good candidate for speciation because of mutations, sexual selection, genetic drift, natural
selection, etc.
Sympatric Speciation (Same Country): Speciation takes place in geographically overlapping populations
because of the following factors:
 Polyploidy: extra sets of chromosomes (commonly found in plants)
o Autopolyploid: An individual who has more than two chromosome sets, all derived
from a single species. Figure 24.8, page 478.
o Allopolyploid: A common form of polyploidy in which two different species
interbreed and produce a hybrid. The hybrid may propagate itself asexually (like
plants), and form an allopolyploid. Figure 24.9, page 479.
Adaptive Radiation: the evolution of many diversely adapted species from a common ancestor upon
introduction to various new environmental opportunities and challenges. This typically occurs when a
few organisms make their way to new, often distant areas (See Figure 24.11) or when extinctions
open up new ecological niches for the survivors (Figure 24.12).
Flashback: The Tempo of Speciation: Gradualism vs. Punctuated Equilibrium (Figure 24.13, page 482).
24-3: Macroevolution changes can accumulate through many speciation events
Speciation results from the accumulation of small differences. These changes occur time and time again,
resulting in more pronounced differences.
Evolutionary Novelties: Complex structures can evolve in two ways:
1. Complex novelties can arise in increments from much simpler versions that performed the same basic
function (See Figure 24.14). Remember, that all of the simpler versions have an effective
function!!!!! The biggest is not the best, so to speak.
2. Complex novelties can also arise from structures that originally evolve in one context but can be used
in a different context. These structures are called exaptations. For example, the honeycomb bones of
birds.
Developmental Evolution: Slight genetic divergences can be magnified into major morphological
differences between species. This can occur because of changes in timing and changes in spatial
patterns of development.
Heterochrony (different time): an evolutionary change in the rate or timing of developmental events.
 Allometric Growth: the differential growth of body parts during development (See Figure 24.15, page
484).
 Paedomorphosis: occurs when reproductive development occurs more quickly than somatic
development. It results in a sexually mature stage of a species with body features that are juvenile
structures in an ancestral species (See Figure 24.17)
Changes in spatial patterns can also result from alterations in genes:
 Homeotic genes determine the basic features in an organism, such as where a pair of wings and a pair
of legs will develop on a bird or how a plant’s flowers are arranged. The products of one class of
homeotic genes, called Hox genes, provide positional information in an animal embryo. (See Figure
24.18, page 485).
EVOLUTION IS NOT GOAL ORIENTED
Species Selection: The changes a species undergoes over time. The species that endure and reproduce
the most determine the direction of evolutionary trends, there is no ultimate goal. (See Figure 24.20,
page 487).
CHAPTER 25: Phylogeny and Systematics
Phylogeny: the evolutionary history of a species or group. Phylogenies are based on common ancestries
inferred from fossil, morphological, and molecular evidence.
Systematics: an analytical approach to understanding the diversity and relationships of organisms, both
present-day and extinct.
Molecular systematics: uses comparisons of DNA, RNA, and other molecules to infer evolutionary
relationships between individual genes and entire genomes. (Figure 25.2, page 491)
Systematists must distinguish between structures that do and do not share evolutionary history
Convergent Evolution: Evolution that occurs when similar environmental pressures and natural
selection produce similar adaptations in organisms from different evolutionary lineages (Fig. 25.5,
page 494). These common characteristics are called analogous characters (analogy).
Taxonomy: the ordered division of organisms into categories based on a set of characteristics used to
assess similarities and differences (Carolus Linneaus).
Binomial Nomenclature: The two-word system used to classify every organism. It gives each species a
Latin genus and species name.
Hierarchical Classification (from largest to smallest):
Domain
Kingdom
Phylum
Class
Order
Family
Genus
Species
Phylogenetic Trees: Branching diagrams that depict systematists’ hypotheses about evolutionary
relationships (See Figure 25.9, page 497).
Cladograms: Depict shared characteristics in graphic forms (See Figure 25.11, page 499).
Shared Characters: are characters that show an evolutionary novelty that is unique to a particular clade.