Download Chapter 24: The Origin of Species 24.1: The biological species

Chapter 24: The Origin of Species
24.1: The biological species concept emphasizes reproductive isolation
That “Mystery of Mysteries”
 Speciation = process by which one species splits into two or more species
o Responsible for diversity of life, yielding new species
o Explains differences and similarities between species
o When one species splits in two the species that result share many
characteristics because they are descended from a common ancestor
 Microevolution = changes over time in allele frequencies
 Macroevolution = broad pattern of evolution above species level
The biological species concept emphasizes reproductive isolation
 Compare morphology (body form), physiology, biochemistry, and DNA
sequences to determine order out of life forms
 Morphologically distinct species are discrete groups, differing in many ways
beside body forms
The Biological Species Concept = definition of species
 Species = group of populations whose members have the potential to
interbreed in nature and produce viable, fertile offspring
o Don’t produce viable, fertile offspring with members of other groups
o Members of species are united by being reproductively compatible
 Absence of gene flow plays role in formation of new species and keeping
different species apart
 Reproductive isolation = existence of biological factors that impede members
of two species from interbreeding and producing viable, fertile offspring
o Limits formation of hybrids = offspring resulting from interspecific
mating
o Prezygotic barriers block fertilization from occurring
 Impede members from different species from trying to mate
 Preventing attempted mating from being successful
 Hindering fertilization of mating is completed successfully
 Habitat isolation = two species that occupy different habitats
within the same area may encounter each other rarely, if at all,
even though they are not isolated by obvious physical barriers
 Temporal Isolation = species that breed during different times
of day, different seasons, or different years cannot mix gametes
 Behavioral Isolation = courtship rituals that attract mates and
other behaviors unique to a species are effective reproductive
barriers because they enable mate recognition
 Mechanical Isolation = mating is attempted but morphological
differences prevent successful completion
 Gametic Isolation = sperm of one species may not be able to
fertilize eggs of another species- sperm cant survive in
reproductive tract or biochemical mechanisms prevent sperm
from penetrating egg’s membrane
o Postzygotic barriers = when prezygotic barriers are overcome,
contributes to reproductive isolation after hybrid zygote is formed
 Developmental errors reducing survival of embryos
 Problems after birth cause infertile hybrids
 Reduced Hybrid Viability = genes of different parent species
interact in ways that impair hybrid’s development/survival
 Reduced hybrid fertility = hybrids are sterile, chromosomes of
two parent species differ in number or structure  meiosis
fails to produce normal gametes
 Hybrid breakdown = first generation hybrids are viable and
fertile but next generation are sterile

Limitations of the Biological Species Concept
o Limit on number of species to which concept can be applied
o Can’t evaluate reproductive isolation of fossils
o Doesn’t apply to organisms that reproduce asexually
o Species are designated by absence of gene flow, but there are species
that are morphologically and ecologically distinct but gene flow
occurs between them (Ex: polar bear and grizzly bear)
Other Definitions of Species emphasize unity within a species
 Morphological species concept characterizes species structural features
o Can be applied to asexual and sexual organisms and used without info
on gene flow
o Disadvantage: relies on subjective criteria
 Ecological species concept views species in terms of ecological niche
o Sum of how members of the species interact with nonliving and living
parts of environment
o Accommodates asexual and sexual species
o Emphasizes role of disruptive natural selection
 Phylogenetic species concept defines species as smallest group of individuals
that share a common ancestor (one branch on tree of life)
o Trace phylogenetic history of species by
comparing its characteristics
(morphology/molecular sequences) with
other organisms
o Distinguishes groups of individuals that are
sufficiently different to be considered
separate species
o Difficult to determine degree of difference
required to be separate
24.2: Speciation can take place with or without
geographic separation
Allopatric speciation = when gene flow is interrupted
when a population is divided into geographically isolated
subpopulations
 Can occur through geologic remodeling or when
descendants become geographically isolated from
parent population
 Process of Allopatric Speciation
o How formidable a geographic barrier must
be depends on ability of organisms to move
about
o Once geographic separation occurs,
separated gene polls may diverge
o Different mutations arise, natural selection and genetic drift may alter
allele frequencies in different ways
o Reproductive isolation may arise as a by-product of selection or drift
o Ex: mosquitofish on Andros Island became isolated in ponds, ponds have
similar environment but some have predatory fishes so the fish that
have body shape that lets them swim faster in ponds with predatory
fishes have different shape than fish in ponds without the predators
 Fish like to mate with fish with similar body type
 Evidence of Allopatric Speciation
o Regions that are isolated or highly subdivided by barriers have more
species than similar regions without such features
o Lab/field tests provide evidence that reproductive isolation between
two populations increases as distance between them increases
o Reproductive isolation also occurs when populations are isolated
experimentally and subjected to different environments
o 30 species of snapping shrimp that live on different sides of Isthmus of
Panama, land bridge that connects South and North America
 15 on one side, 15 on the other = sibling species that arose due to
geographic separation
Sympatric speciation = when reproductive barriers and
speciation occurs in populations that live in the same
geographic area
 Less common than allopatric speciation
 Factors/mechanisms can also promote allopatric
speciation
 Polyploidy = accident during cell division that
results in extra sets of chromosomes
o More common in plants (~80% of plant
species today are descended from ancestors
from polyploidy speciation)
o Autopolyploid = individual with more than
two chromosome sets that are all derived
from a single species (2n, 3n, 4n, etc.)
 Can produce fertile offspring by selfpollinating or by mating with other
mutants with the same chromosome
number
 Reproductively isolated from
organisms of different chromosome
number because the mutated
offspring have reduced fertility
 Can generate reproductive isolation in
just one generation without
geographic separation
o Allopolyploid = fertile polyploidy
Occurs when two different species interbreeds and produces
hybrid offspring
 Most hybrid are sterile because set of chromosomes can’t pair,
but infertile hybrid can propagate asexually
 After subsequent generations a sterile hybrid becomes fertile,
but cannot interbreed with either parent species = new species
 Ex: species of weeds and wheat for bread arose through
allopolyploidy
 Habitat Differentiation occurs when genetic factors enable a subpopulation
to exploit a habitat/resource not used by parent population
o Ex: maggot fly originally fed on hawthorn tree, then some fed on apple
trees  natural selection favors apple feeding flies which now have
reproductive isolation from hawthorn feeding flies
 Alleles that benefit one type of fly and harm the other =
postzygotic barrier further limiting gene flow
 Sexual Selection drives sympatric speciation
o Ex: Africa’s Lake Victoria had 600+ species of cichlids which originated
within last 100,000 years from small number of species from other
rivers/lakes, number of species may have increased due to females
selecting males based on appearance
24.3: Hybrid zones reveal factors that cause reproductive isolation
Patterns Within Hybrid Zones
 Hybrid zone = a region where members of different species meet and mate
producing offspring of mixed ancestry
o Occurs when species with incomplete reproductive barriers come in
contact with each other
 Some form as narrow bands of geographic area
o Ex: two species of toads, hybrid zone where high altitude of one toad
meets low altitude of other toad altering pattern of allele frequencies
 Obstacle to gene flow causes pattern of allele frequencies across hybrid zone
o Ex: hybrid toads have increased rates of embryonic mortality and
morphological abnormalities = poor survival and reproduction
 Other hybrid zones have more complicated spatial patterns
o Ex: plant species only occur in locations that have a particular set of
environmental conditions- favorable patches with such conditions can
be scattered randomly across landscape but isolated = hybrid zone

Hybrid Zones over Time
 Hybrids may become reproductively isolated from their parents and form a
new species
 If not, other outcomes:
o Fusion of species: reproductive barriers are weakened
o Stability: hybrids continue to be produced
 Reinforcement = Strengthening Reproductive Barriers
o When hybrids are less fit than parent species prezygotic barriers are
strengthened, reducing formation of unfit hybrids
o Barriers to reproduction should be stronger for sympatric than
allopatric populations
o Ex: allopatric populations of flycatcher birds resemble each other while
in sympatric populations they look very different so females make
mistakes in choosing in allopatric populations but not in sympatric
populations  reproductive barriers stronger in sympatric populations
 Fusion = Weakening of Reproductive Barriers
o So much gene flow occurs that reproductive barriers are weakened
and the gene pools of the two species become increasingly alike
o Causes two hybridizing species to fuse into a single species
o Ex: Lake Victoria cichlids: 200 of former 600 species vanished in some
causes through species fusion because while females prefer to mate with
fish of their own species of a specific color, murky water caused by
pollution may have made females unable to distinguish
o Ex: polar bears evolved from North American grizzly bears, recently
global warming has reduced ice packs where polar bears hunt so they
go to land where they encounter grizzly bears = hybrid zones
 Stability = Continued Formation of Hybrid Individuals
o Occurs because hybrids survive or reproduce better than members of
either parent species
o Also occurs in cases where hybrids are selected against
o Ex: Bombina hybrid zone where hybrid toads are more unfit but
reproductive reinforcement doesn’t occur because the hybrid zone is so
narrow that parents migrate into the zone = lots of gene flow, continued
production of hybrids overwhelming selection for reproductive isolation
24.4: Speciation can occur rapidly or slowly and can result from changes in
few or many genes
The Time Course of Speciation
 Gather info about how long it takes new species to form from broad patterns
in fossil record and from studies that use morphological data like fossils or
molecular data
 Punctuated equilibria = periods of stasis punctuated by sudden change
o Fossil record shows episodes where new species appear suddenly in
stratum, persist unchanged through strata, and then disappear
o Once process of speciation begins it can be completed rapidly


o Ex: rapid speciation of wild
sunflower shown through genetic
evidence indicating that species
originated by hybridization of two
other sunflower species because
natural selection produced
extensive genetic changes in hybrid
populations over short periods of
time
o Other ex’s: apple maggot fly, Lake
Victoria fish, fruit flies
Other species show more gradual change
(through fossils) = gradualism
Length of time between speciation events
o Interval consists of time that
elapses before populations of a
newly formed species start to
diverge from one another + time it
takes for speciation to be complete
once divergence begins
o Total time between speciation
events varies
o Millions of years may pass before
newly formed species will itself
give rise to another new species
o Speciation begins after gene flow
between populations is interrupted
by changing environmental
conditions, unpredictable events,
etc.
 Populations must also
diverge genetically to become reproductively isolated before
other events resume gene flow
Studying the Genetics of Speciation
 Studies of ongoing speciation like hybrid zones can reveal traits that cause
reproductive isolation
 Identifying genes that control those traits can help explore how many genes
change when new species form
 Can be influenced by small amount of genes
o Ex: one gene in a snail can create a mechanical barrier to reproduction
o Ex: small number of genes controls prezygotic and postzygotic barriers
to different species of monkey flower which are visited by different
pollinators preferring different colors  hybridization can impact
pollinator preference
 Can be influenced by large amount of genes
o Ex: hybrid sterility of two types of fruit fly due to at least four loci
o Ex: sunflower postzygotic isolation influenced by 26 segments
From Speciation to Macroevolution
 Speciation can begin with small differences which accumulate and become
more pronounced leading to formation of new groups of organisms
 One group of organisms may increase in size by producing new species while
other groups may shrink, loosing species
 Cumulative effects of speciation and extinction help shape evolutionary
changes in fossil record