Chapter 24 ~Macroevolution

Origin of Species

What is a species?

A population whose members have the potential to interbreed and produce viable, fertile offspring

+Horse

ZebraZorse = not-fertile

Patterns of Evolution

Divergent Evolution• 2 or more species

originate from a common ancestor

Patterns of Evolution

Convergent Evolution• 2 unrelated species

share similar traits due to adaptation to similar environments

• Unrelated species will never evolve into a single species

Patterns of Evolution

Parallel Evolution• 2 or more related

species make similar evolutionary changes

Patterns of Evolution

Co-Evolution• Two interacting

species evolve together

Macroevolution

• Large-scale evolution: – The descent of

different species from a common ancestor over many generations.

For macroevolution to occur, two populations must be reproductively isolated (or separated) from one another

Over time, the two populations of beetles are eventually considered different species, since they can no longer produce viable, fertile offspring.

Pre-zygotic Isolating Mechanisms

Isolating mechanisms that prevent fertilization

Ova/egg

Pre-zygotic Isolating Mechanisms

1) Habitat Isolation

2 populations live in different habitats and rarely meet

Ex. Terrestrial vs. Aquatic Garter Snakes

Pre-zygotic Isolating Mechanisms

2) Behavioral Isolation

2 populations can not recognize differences in courtship rituals

2 subspecies of tropical finches have been separated enough to have variations in song that they do not recognize

Pre-zygotic Isolating Mechanisms

3) Temporal Isolation

2 populations that breed during different times of day or seasons

Day vs. Night Blooming Lily

Pre-zygotic Isolating Mechanisms

4) Mechanical Isolation

incompatible sex-organs/genitalia

5) Gametic Isolation

Sperm can not survive in the reproductive tract of females or gametes fail to recognize each other

Post-zygotic Isolating Mechanisms

Fertilization is successful, but zygote is prevented from developing into a viable adult

Ova/Egg

Post-zygotic Isolating Mechanisms

1) Hybrid Inviability – genetic incompatibility causes early miscarriage

2) Hybrid Sterility - Hybrid is viable, but sterile

3) Hybrid Breakdown – Hybrid is viable and fertile, but its offspring is sterile

Modes of Speciation(How new species are formed)

1) Allopatric Speciation

2) Adaptive Radiation

3) Sympatric Speciation

Allopatric Speciation

~ “other”

~ “homeland”

AllopatricWhen a speciation occurs due to geographic isolation between two populations

Back to Modes of Speciation

Adaptive Radiation• Type of allopatric speciation

• Rapid evolution of many species from a single ancestor due to colonization of an area w/diverse conditions (ex. Darwin’s finches)

Back to Modes of Speciation

Sympatric Speciation

• When new species are formed w/o a geographic barrier (more common in plants)

• This can an occur due to the following reasons:– Polyploidy– Allopolyploidy– Balanced Polymorphism

Sympatric Speciation (due to polyploidy)

• Mutant condition occurs when a non-disjunction error in meiosis causes gametes that are diploid.

• Mutant zygotes are triploid (3N) or tetraploid (4N) as a result and can propagate a new species

Sympatric Speciation (due to allopolyploidy)

• Type of hybridization• When two different species interbreed and

combine their chromosomes• Usually sterile, but can breed new species

asexually

Sympatric Speciation (due to balanced polymorphism)

• When there are different phenotypes in a population, members can self-isolate and “selectively breed” with those of their own.

• Over time, two populations of differing phenotypes (ex. colors) can evolve into two different species

Does macroevolution occur gradually or suddenly?

Two models of evolutionary thought:

1) Gradualism

2) Punctuated Equilibrium

Gradualism

Evolution occurs due to

-gradual accumulation of small changes over time

** not supported by fossil evidence, since existence of “intermediate” fossils are few or inadequate to support the theory

[;’p

;

Punctuated Equilibrium

Evolution occurs due to

-abrupt changes in the environment (ex. Catastrophes) that drives rapid speciation

-New species appear quickly, then changes very little over time

** supported by fossil evidence