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Mom, Dad…There’s something you need to know…

I’m a MAMMAL!

Chapter 18

WHAT IS SPECIATION?

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So…what is a species? defined by Ernst Mayr (Evolutionary Biologist) A population whose members can interbreed & produce

viable, fertile offspring Being reproductively compatible is a key component

Western Meadowlark

Sturnella magna

Eastern Meadowlark

Sturnella neglecta

Distinct species:songs & behaviors are different enough to prevent interbreeding

Distinct species:songs & behaviors are different enough to prevent interbreeding

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How and why do new species originate?

New species are formed by a series of evolutionary processes which cause

reproductive isolation. Reproductive Isolating MechanismsReproductive Isolating Mechanisms

Pre-zygotic barriers are various forms of isolation

Post-zygotic barriers are generally related to fertility & viability of offspring

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This means there is an obstacle to mating or to fertilization if mating occurs

PRE-Reproduction (pre-zygotic) Barriers

behavioral isolation

geographic isolation ecological isolation temporal isolation

mechanical isolation gametic isolation

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1. Geographic isolation New species occur in different areas

physical barrier separated ancestor species

Harris’s antelope squirrel inhabits the canyon’s south rim (L). Just a few miles away on the north rim (R) lives the

closely related white-tailed antelope

squirrel

Harris’s antelope squirrel inhabits the canyon’s south rim (L). Just a few miles away on the north rim (R) lives the

closely related white-tailed antelope

squirrel

Ammospermophilus spp

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2. Ecological isolation Species occur in same region, but occupy

different habitats so they rarely encounter each other

2 species of garter snake, Thamnophis, occur in same area, but one lives in water & other is terrestrial

2 species of garter snake, Thamnophis, occur in same area, but one lives in water & other is terrestrial

lions & tigers could hybridize, but they

live in different habitats:

lions in grasslands tigers in rainforest

lions & tigers could hybridize, but they

live in different habitats:

lions in grasslands tigers in rainforest

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3. Temporal “time” isolation Species that breed during different times of

day, different seasons, or different years cannot mix gametes.

Eastern spotted skunk (L) & Western spotted skunk (R)

overlap in range but eastern mates in late winter & western mates in late

summer

Eastern spotted skunk (L) & Western spotted skunk (R)

overlap in range but eastern mates in late winter & western mates in late

summer

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4. Behavioral isolation Unique behavioral patterns & rituals isolate species

identifies members of species attract mates of same species

courtship rituals, mating calls

Blue footed boobies mate only after a courtship display unique to their species

Blue footed boobies mate only after a courtship display unique to their species

That’s hot…

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AP Biologyfirefly courtship displaysfirefly courtship displays

courtship display of Gray-Crowned Cranes, Kenya

courtship display of Gray-Crowned Cranes, Kenya

courtship songs of species of lacewings

(that live in the same area)

courtship songs of species of lacewings

(that live in the same area)

Recognizing your own species

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5. Mechanical isolation Morphological differences can prevent

successful mating

Even in closely related species of plants, the

flowers often have distinct appearances that attract

different pollinators. These 2 species of monkey

flower differ greatly in shape & color, therefore

cross-pollination does not happen.

Even in closely related species of plants, the

flowers often have distinct appearances that attract

different pollinators. These 2 species of monkey

flower differ greatly in shape & color, therefore

cross-pollination does not happen.

Plants

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Mechanical isolation For many insects, male & female sex organs

of closely related species do not fit together, preventing sperm transfer

lack of “fit” between sexual organs: hard to imagine for us… but a big issue for insects with different shaped genitals!

Damsel fly penises

Animals

Get that thing away from me!

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6. Gametic isolation Sperm of one species may not be able to

fertilize eggs of another species biochemical barrier: sperm cannot penetrate egg

receptor recognition: lock & key between egg & sperm

chemical incompatibility sperm cannot survive in female reproductive tract

Sea urchins release sperm & eggs into surrounding waters where they fuse & form zygotes.

Gametes of different species are unable to fuse.

Sea urchins release sperm & eggs into surrounding waters where they fuse & form zygotes.

Gametes of different species are unable to fuse.

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POST-Reproduction (Post-Zygotic) Barriers

Prevent hybrid offspring from developing into a viable, fertile adult

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Post Zygotic: Reduced hybrid viability

Genes of different parent species may interact & impair the hybrid’s development

Species of salamander genus, Ensatina, may interbreed, but most

hybrids do not complete development & those

that do are frail.

Species of salamander genus, Ensatina, may interbreed, but most

hybrids do not complete development & those

that do are frail.

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Mules are vigorous, but sterile

Post Zygotic: Reduced hybrid fertility

Even if hybrids are vigorous they may be sterile chromosomes of parents may differ in number or

structure & meiosis in hybrids may fail to produce normal gametes

Donkeys have 62 chromosomes

(31 pairs)

Horses have 64 chromosomes

(32 pairs) Mules have 63 chromosomes!

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Post Zygotic: Hybrid breakdown Hybrids may be fertile & viable in first

generation, but when they mate offspring are feeble or sterile

In strains of cultivated rice, hybrids are vigorous but

plants in next generation are small & sterile.

In strains of cultivated rice, hybrids are vigorous but

plants in next generation are small & sterile.

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Rate of Speciation Current debate:

Does speciation happen gradually or rapidly? Or both? Gradualism

Punctuated equilibrium

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Gradualism Gradual divergence

over long spans of time assume that big

changes occur as the accumulation of many small ones

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Punctuated Equilibrium Rate of speciation is

not constant rapid bursts of

change long periods of little

or no change species undergo

rapid change when they 1st bud from parent population

Time

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Modes of Speciation: Allopatric Speciation

A physical barrier arises and prevents gene flow between populations

Reproductive isolating mechanisms evolve in the genetically diverging populations

Speciation occurs when the members of the divergent populations no longer interbreed

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Modes of Speciation: Sympatric Speciation

A new species may form within the home range of an existing species (there is no physical barrier)

May be: Temporal Mechanical

Behavioral

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Modes of Speciation: Parapatric Speciation

Daughter species form a small proportion of individuals along a

common border between 2 populations.

Hybrid Zone is formed

BULLOCK’S ORIOLE

BALTIMORE ORIOLE

HYBRID ZONE

Figure 18.10Page 299

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The two types of plants are close enough that tolerant and non-tolerant individuals could potentially fertilize each other—so they seem to meet the first

requirement of parapatric speciation, that of a continuous population. However, the two types of plants have evolved different flowering times. This

change could be the first step in cutting off gene flow entirely between the two groups

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