CHAPTER 24THE ORIGIN OF

SPECIES

Question?

What is a species? Evolution theory must also

explain how species originate. Darwin’s “Mystery of mysteries” In the Galápagos Islands Darwin

discovered plants and animals found nowhere else on Earth

Overview: That “Mystery of Mysteries”

Speciation, the origin of new species, is at the focal point of evolutionary theory

Evolutionary theory must explain how new species originate and how populations evolve

Fig. 24-1

How did this flightless bird come to live on the isolated Galápagos Islands?

Microevolution consists of adaptations that evolve within a population, confined to one gene pool

Macroevolution refers to evolutionary change above the species levelExample – the appearance of feathers during the evolution of birds from one group of dinosaurs

Two Basic Patterns of Evolutionary Change”

1. Anagenesis ana = new, genos = raceAccumulation of heritable changes that gradually transforms a species into a species with different characteristics

Two Basic Patterns of Evolutionary Change”

2. Cladogenesis klados = branch, genos = race

Branching evolution in which a new species arises from a population that buds from a parent species

Concept 24.1: The biological species concept emphasizes

reproductive isolation Species is a Latin word meaning

“kind” or “appearance” Biologists compare morphology,

physiology, biochemistry, and DNA sequences when grouping organisms

Fig. 24-2a

Similarity between different species

Fig. 24-2b

(b) Diversity within a speciesDiversity within a species

Question

How many species of African Violets are here?

The Biological Species Concept

• The biological species concept states that a species is a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring; they do not breed successfully with other populations

• Gene flow between populations holds the phenotype of a population together

Fig. 24-3c

Grey-crowned babblers

Does gene flow occur between widely separated populations?

Reproductive Isolation

Reproductive isolation is the existence of biological factors (barriers) that impede two species from producing viable, fertile offspring

Hybrids are the offspring of crosses between different species

Reproductive isolation can be classified by whether factors act before or after fertilization

Prezygotic barriers block fertilization from occurring by:Impeding different species from attempting to mate

Preventing the successful completion of mating

Hindering fertilization if mating is successful

Postzygotic barriers prevent the hybrid zygote from developing into a viable, fertile adult:Reduced hybrid viabilityReduced hybrid fertilityHybrid breakdown

EXAMPLES OF PREZYGOTIC

BARRIERS

Habitat isolation: Two species encounter each other rarely, or not at all, because they occupy different habitats, even though not isolated by physical barriers

Terrestrial Thamnophis

Water-dwelling Thamnophis

Habitat Isolation

Populations live in different habitats or ecological niches.

Ex – mountains vs lowlands.

Temporal isolation: Species that breed at different times of the day, different seasons, or different years cannot mix their gametes

Eastern spotted skunk(Spilogale putorius)

Western spotted skunk(Spilogale gracilis)

Temporal Isolation

Breeding seasons or time of day different.

Ex – flowers open in morning or evening.

Behavioral isolation: Courtship rituals and other behaviors unique to a species are effective barriers

Courtship ritual of blue-footed boobies

Behavioral Isolation

Mating or courtship behaviors different.

Different sexual attractions operating.

Ex – songs and dances in birds.

Mechanical isolation: Morphological differences can prevent successful mating

Bradybaena with shells spiraling in opposite directions

Mechanical Isolation Structural differences that prevent

gamete transfer. Ex – anthers not positioned to put

pollen on a bee, but will put pollen on a bird.

Gametic isolation: Sperm of one species may not be able to fertilize eggs of another species

Seaurchins

Gametic Isolation

Gametes fail to attract each other and fuse.

Ex – chemical markers on egg and sperm fail to match.

EXAMPLES OF POSTZYGOTIC

BARRIERS

Reduced Hybrid Viability

Zygote fails to develop or mature.

Ex – when different species of frogs or salamanders hybridize.

Reduced hybrid viability: Genes of the different parent species may interact and impair the hybrid’s development

Ensatina hybrid

Reduced hybrid fertility: Even if hybrids are vigorous, they may be sterile

Hybrids are viable, but can't reproduce sexually.

Chromosome count often “odd” so meiosis won’t work.

Ex – donkeys and horses produce mules

Fig. 24-4m

(i)

Donkey Donkey

Fig. 24-4n

( j)

Horse

Fig. 24-4o

(k)

Mule (sterile hybrid)

Hybrids

‘Heaven Scent’

an F1 hybrid between 2 species, but sterile.

Hybrid Breakdown

Offspring are fertile, but can't compete as well as the “pure breeds”.

Ex – many plant hybrids

Hybrid breakdown: Some first-generation hybrids are fertile, but when they mate with another species or with either parent species, offspring of the next generation are feeble or sterile

Hybrid cultivated rice plants withstunted offspring (center)

Limitations of the Biological Species Concept

The biological species concept cannot be applied to fossils or asexual organisms (including all prokaryotes)

Other Definitions of Species

Other species concepts emphasize the unity within a species rather than the separateness of different species

The morphological species concept defines a species by structural features It applies to sexual and asexual

species but relies on subjective criteria

Morphospecies

Organisms with very similar morphology or physical form.

Problem

Where does extensive phenotype variation fit?

The ecological species concept views a species in terms of its ecological niche It applies to sexual and asexual

species and emphasizes the role of disruptive selection

The phylogenetic species concept: defines a species as the smallest group of individuals on a phylogenetic tree It applies to sexual and asexual

species, but it can be difficult to determine the degree of difference required for separate species

Question

How many species of African Violets are here?

African Violets 1950-60’s –20 species described 70,000 cultivars or human created hybrids

African Violets

2007 – lumping occurred to 6 species and 10 subspecies

2008 – AVSA now recognizes 9 species and 8 subspecies

point – species definitions can be fluid and change