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Bio 1101 Lecture 14 Chapter 14: How Biological Diversity Evolves

• Last time, we discussed evolution

– How Darwin came up with his theory of evolution by natural selection

– The types of evidence for evolution

– The mechanisms that can lead to evolution

– Possible outcomes of natural selection

Chapter 14: How Biological Diversity Evolves

• Microevolution = the generation-to-generation change in a population’s frequencies of alleles

• Macroevolution = major biological changes, including the formation of new species

• How did the diversity of species on earth arise?

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• Branching evolution is responsible for the multiplication of species

• But what is a species?

• Biological Species Concept: groups of interbreeding natural populations that are reproductively isolated from other such groups

• What keeps closely related species from interbreeding?

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• Pre-zygotic isolating mechanisms

– Prevent the formation of a zygote

• Temporal isolation

• Geographic isolation

• Behavioral isolation

• Mechanical isolation

• Gametic isolation

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• Post-zygotic isolation

– Prevents hybrid offspring from developing or reproducing • Hybrid inviability

• Hybrid sterility – Male lion and female tiger

liger

– Male tiger and female lion tigon

– Male donkey & female horse mule

• Allopatric Speciation – A geographic barrier divides a population,

preventing gene flow

– Example 1: A river flowing through a population eventually carves a canyon in the middle of the population, isolating the individuals on either side. (2 different species of squirrel on either side of Grand Canyon)

– Example 2: During a storm, some individuals get blown off a mainland onto an island and are then isolated from the mainland population

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• Islands offer excellent examples of how new species can arise when isolated from their parent populations – Example 1: Galapagos finches; beak shape adapted

for food sources on different islands

– Example 2: watch the video on Honeycreepers in Hawaiian islands

http://www.untamedscience.com/biology/genetics/

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– Genetic isolation of two populations may or may not result in speciation

• Speciation requires the evolution of reproductive barriers between the isolated population and its parent population

• Sympatric Speciation – The origin of new species without geographic

isolation

– Not common among animals, but fairly common in plants

– Often a result of a genetic mutation that results in the formation of a new species in one generation

– Usually from an accident during cell division resulting in an extra set of chromosomes, called polyploidy; the increased number of chromosomes makes the individual reproductively incompatible with the parent species

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– Examples of polyploid species: the gray treefrog and the Chinese hibiscus

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• What is the tempo of speciation?

• 2 Concepts: Gradualism and Punctuated Equilibrium

• Gradualism: continuous, slow accumulation of changes over a very, very, very long period of time

• Punctuated Equilibrium – Most species-defining characteristics change over

a relatively short period of time in the overall lifetime for that particular species

– Most species will live a few million years… but only a few thousand of those years may be involved in the physical evolution of that species

– Change occurs in relatively short “bursts,” followed by long periods of stasis

– Likely a better fit to the way species actually evolve

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– Species evolve in response to changes in environment

• Organisms are Classified based on their Evolutionary Relationships

– Our taxonomic system was developed by Carolus Linnaeus in the mid-1700s

– “Binomial nomenclature” – a two-part scientific name for each species

• The Genus name

• The Specific Epithet

• Both parts are written in italics; the genus name is capitalized

• Examples: Homo sapiens = modern humans; Rana pipiens = leopard frog; Rana sylvatica = wood frog

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• Specific epithet cannot be used by itself; must be used in combination with the genus name

• Example: Arum maculatum (jack-in-the-pulpit plant) and Ambystoma maculatum (spotted salamander) have the same specific epithet, but they are very different species!

• Taxonomy

• Organisms are organized into a hierarchical classification system – Domain

– Kingdom

– Phylum

– Class

– Order

– Family

– Genus

– Species

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• Organisms are classified into these groups based on their characteristics

– Organisms that share homologous structures (derived from a common ancestor)

– The more homologous characters they share, the more closely related they are

– Watch this Khan Academy video on how to build phylogenetic trees: https://www.youtube.com/watch?v=6_XMKmFQ_w8

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The Geological Time Scale

• A History of the Earth (Table 14.1)

– Earth was formed approximately 4.6 BYA (important date to remember)

– 3.5 BYA – first prokaryotes evolved (important date to remember)

– 2.7 BYA – oxygen began accumulating

– 2.2 BYA – oldest eukaryotes

– 700 MYA – oldest animal fossils

– 600 MYA – invertebrates diversify; algae diversify

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– ~540 MYA – the Cambrian Explosion; most modern animal phyla evolved

– ~500 MYA – origin of true plants

– See Table 14.1 • First vertebrates, the fishes evolved about 430 MYA

• By 300 MYA, there were extensive land forests, amphibians were the dominant land animals, and reptiles originated

• About 65 MYA, dinosaurs went extinct, and flowering plants evolved; this was followed by diversification of mammals, birds, and pollinating insects

• First Homo sapiens were the Neanderthals, about 130,000 years ago

• 5-minute break

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• Plate Tectonics and Biogeography – The continents and seafloors form a thin layer of rock

called the crust, which moves over a mass of hot, viscous material called the mantle

– The crust is not one continuous piece, but is divided into giant, irregularly shaped plates that move

– Where plates touch, there is geologic activity such as volcanos, mountain uplift, and earthquakes when the plates scrape past each other and collide

– About 250 million years ago, plate movements caused all of the previously separated land masses to come together and form a supercontinent called Pangea

– Movement of plates is slow, but over the course of history they have moved thousands of miles

Video: Bill Nye on plate tectonics and Pangea https://www.youtube.com/watch?v=lJiAUvB1vEU

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• Biogeography: the study of past and present distribution of organisms – The formation of Pangea must have had dramatic

impacts on species that had formerly been isolated

– When Pangea broke up during the mid-Mesozoic era, the separation of once-contiguous land masses and populations once again forced species to adapt to new conditions • Example of Madagascar (large island off the southern coast

of Africa) – nearly all animals and plants here are unique, having diversified from ancestral populations when the island broke away from Africa and India – More than 50 species of lemurs, which evolved from a common

ancestor over the past 40 million years

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• Continental drift also separated Australia from other landmasses – Australia and its adjacent islands are home to over 200

species of marsupials (most found nowhere else in world – they are “endemic”)

– Most other parts of the world are dominated by placental mammals; why so few marsupials?

– Distribution of marsupials makes sense in light of continental drift-- marsupials must have originated when the continents were joined • Fossil evidence indicates they originated in what is now Asia, later

dispersing to South America while it was still connected to Antarctica; they made their way to Australia before continental drift broke apart Pangea

• The marsupials in Australia thrived, while placental mammals did better in other landmasses

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Mechanisms of Macroevolution

• Scientists working at the interface of evolutionary biology and developmental biology,

– the research field abbreviated evo-devo,

– are studying how slight genetic changes can become magnified into major structural differences between species.

• Homeotic genes, the master control genes, program development by controlling changes in an organism’s form as it develops from a zygote into an adult, affecting the

– rate,

– timing, and

– spatial pattern of organisms.

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• Changes in rate of developmental events explain changes in the homologous limb bones of vertebrates.

– Increased growth rates produced the extra-long “finger” bones in bat wings.

– Slower growth rates of leg and pelvic bones led to the eventual loss of hind limbs in whales.

– The front limbs of many vertebrates have the same basic bone structure; the different bones (e.g. radius, ulna, etc.) have simply been modified to be shorter or longer, fatter or thinner, to fit different purposes in different environments (see image on next slide)

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• Paedomorphosis

– is the retention in the adult of body structures that were juvenile features in an ancestral species and

– has occurred in the evolution of

• axolotl salamanders (keep their gills, unlike most salamanders)

• Humans (keep a relatively large brain case, relative to chimpanzees)

– In human evolution, genetic changes slowed the growth of the jaw relative to other parts of the skull and produced an adult with head proportions similar to a child

– This resulted in larger brain case and bigger brain (growth of brain switched off later than in chimpanzees), so our brains grow for several more years – a prolonged juvenile process

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Structure/Function: Adaptation of Old Structures for New Functions

• The feathered flight of birds is a perfect marriage of structure and function.

• In flight, the shapes and arrangements of various feathers

– produce lift,

– smooth airflow, and

– help with steering and balance.

• How did such a beautifully intricate structure evolve?

• Reptilian features apparent in fossils of Archaeopteryx, one of the earliest birds, offered clues in Darwin’s time.

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• Thousands of fossils of feathered dinosaurs have been found and classified into more than 30 different species.

• But the feathers seen in these fossils could not have been used for flight, nor would their reptilian anatomy have been suited to flying.

• Their first utility may have been for insulation. • Once flight itself became an advantage, natural

selection would have gradually remodeled feathers and wings to fit their additional function.

• Structures such as feathers that evolve in one context but become co-opted for another function are called exaptations.

From Simple to Complex Structures in Gradual Stages

• Most complex structures have evolved in small steps from simpler versions having the same basic function, a process of refinement rather than the sudden appearance of complexity.

• The evolution of complex eyes can be traced from a simple ancestral patch of photoreceptor cells through a series of incremental modifications that benefited their owners at each stage.

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Figure 14.21

Patch of

pigmented cells Eyecup

Simple

pinhole eye Eye with

primitive lens

Complex camera

lens-type eye

Pigmented cells

(photoreceptors)

Pigmented

cells Fluid-filled cavity

Transparent protective

tissue (cornea)

Lens Lens

Cornea

Nerve

fibers

Eyecup

Nerve

fibers Optic

nerve

Layer of

pigmented

cells

(retina)

Retina Optic

nerve

Optic

nerve

Retina

Limpet Abalone Nautilus Marine snail Squid

• Frequently a difficult concept to grasp – how something as complex as an eye could evolve – but step by step, small genetic changes can result in increasingly complex organs for perceiving light information

• Video on evolution of the eye: https://www.youtube.com/watch?v=dWFteFfg2J0

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• All for today, except…

Random Animal of the Day!

• The Hawaiian Bobtail Squid! – This species forms a mutualism with bioluminescent bacteria called

Vibrio fischeri, which live in a special light organ in its mantle

– The bacteria produce light that is equal to the amount of light hitting the mantle from above; this makes the squid invisible to its prey below!

– In return, the squid feeds the bacteria sugar and amino acids