How Biological Diversity Evolves
When Darwin visited the Galápagos Islands, he realized that he was visiting a place
of origins
Though the volcanic islands were geologically young, they were already
home to many plants and animals known nowhere else in the world
Darwin thought that not all of these species could have been among
the original colonists
He realized that some must have evolved later on, modified by natural
selection from those original ancestors
The Origin of Species
In the 150 years since the publication of Darwin’s book On the Origin of
Species by Means of Natural Selection, new discoveries and technological
advances have given scientists a wealth of new information about the
evolution of life
The incredible diversity of life evolved through speciation, the process in which one
species branches into additional species
The most accepted definition of a species comes from the biological species concept:
“a group of populations whose members have the potential to interbreed with one
another and produce fertile offspring (offspring that can reproduce).”
That gives us a better criterion than simply morphological similarities and
differences
There are difficulties with our concept of species
Asexual species
Extinct species
Hybrid species
What Makes a Species?
Reproductive Barriers Between Species
Since the biological concept of species is dependent upon reproduction,
processes that lead to reproductively isolated populations can produce new
species
Prezygotic barriers prevent mating or fertilization between species
Postzygotic barriers operate if
interspecies mating occurs and
hybrid zygotes form
How do these reproductive isolating mechanism evolve? How does speciation
occur?
Species can form via
allopatric speciation, in which the initial block to gene flow is a geographic
barrier that physically isolates the splinter population, or
sympatric speciation, without geographic isolation
Allopatric Speciation
Geologic processes can fragment a population into two or more isolated
populations and contribute to allopatric speciation
Sympatric Speciation
A species may originate from an accident during cell division that results in
an extra set of chromosomes, a condition called polyploidy
Polyploid speciation
has been found in some animal species, especially fish and amphibians, but
is most common in plants - an estimated 80% of present-day plant species
are descended from ancestors that arose by polyploid speciation
1. Polyploidy can arise from a single parent species. For example, a
failure of cell division might double the chromosome number from the
original diploid number (2n) to tetraploid (4n).
2. Two different species might interbreed and produce hybrid
offspring.
Polyploid speciation has given us oats, potatoes, bananas, strawberries,
peanuts, apples, sugarcane, and wheat
We can observe a likely case of sympatric speciation occurring in flies even today
Rhagoletis pomonella - in North America, one species of fruit fly initially fed on the
hawthorn fruit
In the period of 1800-1850, a race of flies spontaneously emerged that
preferred apples
This coincided with the introduction of apples to North America by
Europeans
The apple feeding race does not normally feed on the hawthorn; the
hawthorn feeding race does not normally feed on apples
Flies usually choose their mates around their preferred fruits
Island Showcases of Speciation
Why can we find such incredible diversity on islands?
Diverse ecological opportunities
Volcanic islands, such as the Galápagos and Hawaiian island chains, are
initially devoid of life
Over time, colonists arrive via ocean currents or winds
In their new environment, these populations may diverge significantly
from their distant parent populations
Macroevolution
is evolutionary change above the species level and
includes the impact of mass extinctions on the diversity of life and its
subsequent recovery
The Fossil Record
The geologic time scale divides Earth’s history into a consistent sequence of
geologic periods
Plate Tectonics and Biogeography
According to the theory of plate tectonics, the continents and seafloors form
a thin outer layer of solid rock, called the crust, divided into giant, irregularly
shaped plates that float atop the mantle, a mass of hot, viscous material
In the process of continental drift, movements in the mantle cause the plates
to move
The boundaries of some plates are hotspots of geologic activity
Earthquakes occur when two plates are scraping past or colliding with
each other
In the history of the planet, there has never been a bigger geological event
than the formation of Pangea … and its breaking apart
Mass Extinctions and Explosive Diversifications of Life
The fossil record reveals that five mass extinctions have occurred over the
last 540 million years
The Permian mass extinction (250 million years ago)
occurred at about the time Pangaea formed,
claimed about 96% of marine species, and
took a tremendous toll on terrestrial life
The Cretaceous extinction (about 65 million years ago)
occurred at the end of the Cretaceous period,
included the extinction of all the dinosaurs except birds, and
permitted an explosive increase in the diversity of mammals
Each mass extinction has been followed by a period of rapid diversification
as new ecological niches became available - adaptive radiations
Large Effects from Small Genetic Changes
Scientists working at the interface of evolutionary biology and
developmental biology (evo-devo), are studying how slight genetic changes
can become magnified into major structural differences between species
Changes in the rate of developmental events explains 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
Paedomorphosis is the retention in the adult of body structures that were
juvenile features in an ancestral species
Examples are seen in axolotl salamanders and humans
Adaptation of Old Features for New Functions
How do biological innovations come about?
We can find possible answers in transitional forms - organisms that show
features of two groups
Thousands of feathered dinosaur fossils have been found and classified into
>30 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, or escape, or attracting
a mate
Once flight itself became an advantage, natural selection would have
gradually selected for individuals with feathers and wings that best fit
their additional function
Structures such as feathers that evolve in one context but become co-opted
for another function are called exaptations
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
Homologous structures have been very useful to biologists trying to figure out
evolutionary relationships
But they must not be confused with analogous structures which result from
convergent evolution
Homologous structures result from common ancestry; analogous structures do not