Biology Lecture 3: How Biological Diversity Evolves

Biological species concept:

– It defines a species as a

group of populations whose members have the potential to interbreed with one another in nature and produce fertile offspring (offspring that can reproduce)

We cannot apply the biological species concept to

all situations. Ex:

– organisms that only reproduce asexually (Prokaryotes)

– fossils

A reproductive barrier is

anything that prevents individuals of closely related

species from interbreeding

Postzygotic Barriers

• Operate if interspecies mating actually occurs and results in hybrid zygotes

• In this context, “hybrid” means that the egg comes from one species and the sperm from another species

Postzygotic Barriers

• In some cases, hybrid offspring die before

reaching reproductive maturity

reduced hybrid viability

A mule, for example, is the hybrid offspring of a female horse and a male donkey. Mules are sterile — they cannot successfully breed with each other. Thus, horses and donkeys remain distinct species

Temporal Isolation, habitat isolation, behavioral isolation, Mechanical isolation, gametic isolation are examples of

Prezygotic barriers

Reduced hybrid viability, reduced hybrid fertility, hybrid breakdown

Postzygotic barriers

When the Reproductive Barriers Between Species is Ignored

• The liger is a hybrid offspring of a male lion and a female tiger

• A tigon or tiglon is a hybrid cross between a male

tiger and a female lion

With its gene pool isolated, the splinter population can follow its own evolutionary course

Allopatric speciation

the initial block to gene flow is a geographic barrier that physically isolates the splinter population

With its gene pool isolated, the splinter population can follow its own evolutionary course

Sympatric speciation

the origin of a new species without geographic isolation. The splinter population becomes reproductively isolated even though it is in the midst of the parent population

Macroevolution

• Evolutionary change above the species level, for example, the origin of a new group of organisms through a series of speciation events

• Also includes the impact of mass extinctions on the diversity of life and its subsequent recovery

The Fossil Record

• Geologic time scale

scale established by geologists that divides Earth’s history into a sequence of geologic periods

The Fossil Record

Geologic time scale Separated into

four broad divisions

• Geologic time scale:

• Separated into four broad divisions:

1. Precambrian (a general term for the time before about 540 million years ago)

2. Paleozoic

3. Mesozoic

4. Cenozoic

• Each of these divisions represents a distinct age in the history of Earth and its life

The boundaries between eras are marked by

mass extinctions, when many forms of life disappeared from the fossil record and were replaced by species that diversified from the survivors

Radiometric dating

– the most common method geologists use to learn the ages of rocks and the fossils they contain

– a method based on the decay of radioactive isotopes

Scientists measure the ratio of

carbon-14 to carbon-12 in a fossil to calculate its age

The crust is divided into giant, irregularly shaped plates that float atop a mass of hot, viscous material called the

mantle

In a process called continental drift

movements in the mantle cause the plates to move

Japan sits atop

four different plates

frequent earthquakes and subsequent tsunamis occur as the plates move and bump against each other

Most plate movement is extremely slow

 The boundaries of some plates are

hotspots of geologic activity

• In some cases, we feel an immediate, violent symptom of this activity, as when an earthquake signals that two plates are scraping past or colliding with each other

About 250 million years ago, near the end of the Paleozoic era, plate movements brought all the previously separated landmasses together into a supercontinent called

Pangaea

The history of continental mergers and separations

explains many patterns of biogeography,

the study of the past and present distribution of organisms

Continental drift also separated Australia from other landmasses. Australia and its neighboring islands are home to more than 200 species of

Marsupials most of which are found nowhere else in the world

Fossil record reveals that

five mass extinctions have occurred over the last 540 million years

• In each of these events, 50% or more of Earth’s species died out

Permian mass extinction

at about the time the merging continents formed Pangaea, claimed about 96% of marine species

mass extinction at the end of the Cretaceous period

when dinosaurs became extinct

Paedomorphosis

The retention in the adult of body structures that were juvenile features in an ancestral species

Natural selection cannot predict the future; it can only

improve an existing structure in the context of its current use

Exaptations

describes a trait that has been co-opted for a use other than the one for which natural selection has built it

Classifying the Diversity of Life

• Systematics

– includes taxonomy

– is a discipline of biology that focuses on classifying organisms and determining their evolutionary relationships

Classifying the Diversity of Life

• Homologous structures

in different species may vary in form and function but exhibit fundamental similarities because they evolved from the same structure in a common ancestor:

– the whale forelimb is adapted for steering in the water, whereas the bat wing is adapted for flight (similarities in the bones supporting these two structures

Convergent Evolution:

species from different evolutionary branches may have certain structures that are superficially similar if natural selection has shaped analogous adaptations

Similarity due to convergence is called

Analogy

the wings of insects and those of birds are *** flight equipment: They evolved independently and are built from entirely different structures