Chapter 24 Species & Speciation. Slide 2 of 25 Chapter 24 in a nutshell Microevolution v. Macroevolution Speciation Prezygotic Postzygotic Gradualism.

Chapter 24

Species & Speciation

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Chapter 24 in a nutshell

Microevolution v. Macroevolution

Speciation Prezygotic Postzygotic

Gradualism v. Punctuated equilibrium

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Species

Morphological species concept Divisions of organisms into discrete units called species If two organisms look different enough, then they are

considered different species

Biological species concept Population or group of populations whose members have

the potential to interbreed in nature and produce viable, fertile offspring

This population is unable to produce viable, fertile offspring with members of another population

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Micro- vs. Macro- Evolutions

Microevolution Change in the genetic makeup of a population over

generations Changes (adaptations) confined to a particular gene pool

or population

Macroevolution Changes (evolution) above the species level Changes used to define higher taxa

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Speciation

The process that produces a new species

May be the result of microevolution OR macroevolution

How could microevolution lead to speciation?

How could a new species arise from macroevolution?

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Reproductive Isolation

Biological barriers that impede members of 2 species from producing viable, fertile offspring

There are 2 types of barriers that lead to reproductive isolation Prezygotic

Pre-mating Post-mating

Postzygotic

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Prezygotic Barriers

Pre-Mating Habitat isolation Temporal isolation Behavioral isolation

Post-Mating Mechanical isolation Gametic isolation

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Prezygotic Barriers (Page 1)

Pre-Mating Habitat isolation

2 species occupy different habitats in the geographic area Tree-dwelling species, Bush-dwelling species

Temporal isolation Temporal = time Species breed at different times

Different time of day, different seasons, or different years Behavioral isolation

Certain signals or types of behavior are integral to courtship, and these signals/behaviors are unique to the species

Other species do not respond to these signals or behaviors

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Different habitats – water or terrestrial

Different mating season – late winter or late summer

Boobies!! (blue-footed)

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Prezygotic Barriers (Post-Mating)

Mechanical isolation Anatomical incompatibility Insect copulatory organs are elaborated for a particular

species

Gametic isolation Even if gametes meet, they are unable to fuse Sperm may not be able to survive in the reproductive

tract of females from another species Purple + Red sea urchins cannot mate b/c gametes

cannot fuse

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Postzygotic Barriers

Reduced hybrid vitality

Reduced hybrid fertility

Hybrid Breakdown

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Postzygotic Barriers

Reduced Hybrid vitality Although zygote forms, genetic incompatibility causes

developmental cessation In some salamanders, hybrids form but do not complete

development

Reduced Hybrid fertility Sterile hybrids Donkey + Horse = mule (Sterile)

Hybrid breakdown Although first-generation hybrids survive & reproduce,

their offspring are feeble or sterile

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Geographic Barrier No Geographic Barrier

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Allopatric Speciation

Causes Geologic events or processes that fragment the

population Emergence of a mountain range Formation of a land bridge Evaporation of a large lake into multiple smaller lakes

Leads to a significant alteration of a gene pool

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Sympatric Speciation

Sympatric = sym (same) + patr (fatherland) Same country

Geographically overlapping populations

Major mechanism of sympatric speciation is Polyploidy Autopolyploidy Allopolyploidy

Preferential Habitation

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Polyploidy Extra set(s) of chromosomes

Autopolyploidy > 2n Typically, nondisjunction (Meiosis) in Plants All chromosomes from the same species

Example Tetraploid (4n) + diploid (2n) Produce triploid (3n) offspring which are sterile But plant tetraploids can self fertilize or mate with other

tetraploids = viable, fertile polyploid offspring Tetraploids are reproductively isolated from the diploid

population

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Polyploidy (Page 2)

Allopolyploidy Plants 2 different species interbreed = hybrid Hybrids cannot breed with either original species But hybrids can successfully mate with each other Self-pollination successfully occurs as well

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Polyploidy (Page 3)

Can happen in animals but not common

In animals, sympatric speciation is usually due to part of the population switching to a new habitat, food source, or other resource

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Adaptive Radiation

Many new species arise from a common ancestor given new environmental opportunities or challenges

Think Darwin’s finches & Galapagos islands As the finches moved from South America to the

Galapagos, different environments and food sources were encountered

Hawaiian Plants As the plants spread to Hawaiian islands, different

environments on different islands = evolution into several distinct forms

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Hawaiian Plants that had a common ancestor

Arose from Adaptive Radiation

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Gradualism vs. Punctuated Equilibrium

Gradualism – species descended from a common ancestor and gradually diverge in morphology (acquiring unique adaptations) until they are reproductively isolated

Punctuated Equilibrium – long periods of apparent stasis punctuated by brief periods of sudden and rapid change Fossil record confirms the punctuated equilibrium model Also eliminates the necessity of finding “missing links”