Speciation and extinction Notes

What Defines a Species?

1. A species is a group of organisms that can interbreed to produce fertile offspring.
2. Different species cannot interbreed successfully, which keeps their gene pools separate.

How Does Speciation Happen?

1. Speciation requires three conditions working together: isolation, differential selection, and no gene flow.
2. Core mechanism:
   1. Isolation: Populations are separated physically or reproductively.
   2. Different selection pressures: Each population experiences a different environment.
   3. No gene flow: Alleles cannot mix between populations.
3. Over many generations, these differences accumulate until the populations become distinct species.

What Causes Populations to Become Isolated?

1. Geographical isolation (Allopatric speciation)

1. Isolation: A physical barrier (a river, mountain range, desert, rising sea levels) splits the population.
2. Differential selection: Each side faces different environmental pressures (food sources, climate, predators).
3. No gene flow: The barrier prevents interbreeding, so alleles cannot mix.

Example

• Darwin’s finches on Daphne Major evolved different beak shapes because each island offered different food sources.
• Over time, differences in mating songs and beak structures reduced interbreeding, forming new species.

2. Reproductive isolation in the same habitat (Sympatric speciation)

1. Isolation: Populations stop interbreeding due to behavioral, temporal, or chromosomal barriers (not geography).
2. Differential selection: Subgroups experience different microhabitat or resource pressures.
3. No gene flow: Hybrid sterility or assortative mating blocks allele mixing.

What Are Ring Species?

1. In a ring species, populations are distributed around a geographical barrier.
2. Adjacent groups interbreed with their neighbors, creating a continuous gradient of traits and genes.
3. The populations at the far ends of this gradient are the terminal populations.
4. Returning to our 3-part framework of speciation:
   1. Isolation: Populations arranged around a geographical barrier; distant ends are effectively isolated.
   2. Differential selection: Gradients around the ring create changing selection pressures.
   3. No gene flow: Terminal populations cannot interbreed, so alleles at the ends do not mix.

Why Does Speciation Take Time?

1. Speciation depends on accumulating genetic differences through:
   1. Mutation
   2. Different selection pressures
   3. Limited or zero gene flow
2. As these differences build, the populations become incompatible, eventually forming distinct species.

What Causes Species to Go Extinct?

1. A species becomes extinct when all individuals die and no members remain to reproduce.
2. Extinction occurs when environmental change outpaces the species’ ability to adapt.
3. Major causes of extinction include:
   1. Rapid environmental change: climate shifts, habitat loss, altered food availability
   2. Invasive species: new predators or competitors may outcompete or prey on native species
   3. Disease: New or rapidly spreading diseases can eliminate entire populations
   4. Human activity: habitat destruction, pollution, overhunting, climate change
   5. Low reproductive rates: species with slow reproduction (such as polar bears) struggle to recover once numbers decline

How Does Natural Selection Link Speciation and Extinction?

1. Natural selection drives both processes:
   1. If populations adapt to different conditions → speciation
   2. If populations cannot adapt fast enough → extinction
2. Selection pressures determine which path a species follows.

Why Is the Fossil Record Incomplete?

1. Fossils provide evidence of past species, but the record is patchy.
2. Reasons for incomplete fossil records include:
   1. Only hard parts (bones, shells, teeth) fossilize well
   2. Many organisms decay before fossilization
   3. Geological activity can destroy fossils
   4. Some species were rare, leaving few fossils
   5. Fossil formation requires specific conditions