How Biological Diversity Evolves

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