The Origin of Species chapter 24. Figure 24.0 A Galápagos Islands tortoise.

The Origin of Species

chapter 24

Figure 24.0 A Galápagos Islands tortoise

the origin of species • The beginning of new forms of life.• Speciation, key process.• Explains, macroevolution, the origin

of new taxonomic groups.• Two patterns: 1) anagenesis- linear evolution in

which the entire population changes to be different from and to replace the ancestral population. (Lamarckian)

2) cladogenesis- branching evolution that creates a greater diversity of sister organisms. Each branch is called a clade.

Figure 24.24 The branched evolution of horses

populations & speciesPopulations are groups of individuals

thata) are the same speciesb) live in the same geographical area at

the same time.A species is the largest unit of populationa) reproductively compatibleb) Gene flow: possible to produce

viable fertile offspring Regardless of geographical barriers

• A species can be divided into subspecies, if they become reproductively isolated.

• Subspecies are different due to pre and/or postzygotic barriers:

a) Prezygotic reproductive barriers:impede mating between species or hinder the fertilization of ova.

b) Postzygotic reproductive barriers: prevent hybrid zygote from developing into a viable,

fertile adult.• If subspecies occur together, but remain

reproductively isolated, these subspecies may eventually become two distinct species.

The biological species concept

is based on infertility rather than physical similarity. (horse + donkey = mule)

species

• species is Latin for “kind” or “appearance”

biological species concept:• Population or group of populations• members have the potential to interbreed with

one another to produce viable, fertile offspring• cannot produce viable, fertile offspring with

members of other species.

Figure 24.5 A summary of reproductive barriers between closely related species

prezygotic barriers: Factors that lead to Reproductive Isolation

A) Ecographic Isolation: (geographic) isolationex. Asian and African elephants

B) Habitat Isolation: two species live in different habitats within the same area.ex. Garter snakes- one aquatic, one terrestrial

C) Seasonal/Temporal Isolation: two species that breed during different times of the day, seasons, or years cannot mix their gametes.ex. Skunks: S. gracilis mates in late summer;

S. putorius mates in late winter.

D) Behavioral Isolation: signals to attract mates, elaborate behaviors, courtship rituals differ between species.

ex. Eastern & Western Meadowlark songs differ

Figure 24.3 Courtship ritual as a behavioral barrier between species

D) Behavioral Isolation: signals to attract mates, elaborate behaviors, courtship rituals differ between species.

ex. Eastern & Western Meadowlark songs differE) Mechanical Isolation: anatomical incompatibility.

ex. Insect copulatory organs don’t fit together floral anatomy specialized to one pollinatorF) Gamete Isolation: incompatibility between sperm/egg.

ex. Sperm of one species may not be able to survive in the environment of the female reproductive tract of another species.

gamete recognition based on complementary molecules found on sperm/egg surfaces.

postzygotic barriers: Examples that expend (waste) energy and lead to reproductive isolation

G) Reduced Hybrid Viability: genetically incompatible hybrid zygotes abort development at some embryonic stage.

ex. frogs in genus Rana

H) Reduced Hybrid Fertility: results in completely or largely sterile hybrids. Chromosomal differences (structure or number) results in malformed gametes during meiosis.

ex. Mule- (sterile) but robust hybrid of a horse and donkey

I) Hybrid Breakdown: first generation hybrids are viable but second generation offspring are feeble or sterile.

ex. cotton

How do new species arise?• 1) By geographic

isolation: • This is the way the flora

and fauna of the Galapagos Islands evolved.

• The barrier prevents gene flow.

• When two different species arise this way, it is called allopatric speciation.

• Greek: allos, other & patria, homeland

Figure 24.8 Has speciation occurred during geographic isolation?

• The factors that lead to divergence:

A) size of population (small)

• The founder effect- genetic drift attributed to colonization by a limited number of individuals from a parent population.

B) ability of organism to move about (isolation)

C) harshness/ differences of new environment.

Allopatric speciation of squirrels in the Grand Canyon

A famous example of divergent evolution/speciation: Adaptive radiation• Adapative radiation is evolution of

many diversely adapted species from a common ancestor.

Example: Darwin’s Finches• The 14 species of Finch evolved from

one species of ancestral finch.• They have adapted to exploit different

food sources with differently shaped beaks and feeding behaviors.

• They exhibit character displacement - evolutionary change driven by competition among species for a limited resource (eg. Food)

• Gause’s Law- competitive exclusion principle.

2) If two different species arise from a population without geographic barriers, it is called sympatric speciation.

• Examples of sympatric speciation: balanced polymorphism, polyploidy, hybridization.

• Polyploidy (having more than the diploid number of chromosomes) and chromosomal change

• This condition is common in plants and less common in animals.

• It can make offspring reproductively isolated from their parental species. (post-zygotic barrier is created in one generation)

• Polyploid population can self-pollinate, mate with other polyploids, or reproduce by asexual propagation.

Figure 24.13 Sympatric speciation by autopolyploidy in plants

Figure 24.15 One mechanism for allopolyploid speciation in plants

Causes of Polyploidy: 1) accidents during meiosis (autopolyploidy)

results in the wrong number of sets of chromosomes in the gametes

2) the contribution of two different species to a polyploid hybrid (allopolyploidy) non-homologous chromosomes can’t align during meiosis.

• The chemical colchicine induces polyploidy.

summary

• In allopatric speciation, a new species forms while geographically isolated from its ancestor.

• Sympatric speciation requires the emergence of some type of reproductive barrier that isolates the gene pool of a subset of a population without geographic separation from the parent population.

PATTERNS OF EVOLUTION1. Divergent Evolution- two or more species originate from a common

ancestor. homologous traits.2. Convergent Evolution- two unrelated species that share similar

traits. Arise not from a common ancestor but because each species has independently adapted to similar ecological conditions or lifestyles. analogous traits. Ex. Shark, porpoises, penguins bodies

Ex. Eyes of squids and vertebrates.3. Parallel Evolution- two related species making similar evolutionary

changes after their divergence. Ex. Marsupial and Placental mammals. analogous traits.

4. Coevolution- tit-for-tat evolution of one species in response to new adaptation that appear in another species.

ex. Pollinators-Flowering Plants

Figure 25.10 Convergent evolution and analogous structures

analagous structuresconvergent evolution

Punctuated Equilibrium (proposed by Stephen J. Gould)• A catastrophic event or major genetic change

occurs, rapid evolution and speciation occurs.• The new population works back toward a long

period of no evolution (few or no transitional forms.)

• The Cambrian Explosion represents a period in time(560 MYA) where we see diversification of animal phyla.

Patterns of macroevolution

• Phyletic gradualism- evolution occurs by the gradual accumulation of small changes. The intermediate stages of evolution not represented by fossils merely testifies to the incompleteness of the fossil record.

• Punctuated Equilibrium- evolutionary history consists of geologically long periods of stasis with little or no evolution, interrupted or “punctuated” by geologically short periods of rapid evolution.