Chapter 19

Changes in Species

SpeciationQ: When are two populations new

species? A: When populations no longer interbreed

they are thought to be separate species. •As natural selection proceeds, populations occupying different environments will diverge into races, subspecies, and finally separate species.

•Barriers to gene flow between populations isolate those populations, ultimately leading to the formation of new and separate species

Reproductive Isolating Mechanisms•Pre-zygotic Isolating Mechanisms- these act before fertilization can happen

- Ecological: - Different habitats

» Example Lions and Tigers

– Geographical: – Mountains, islands, or rivers separate

» Chipmunks in the Grand Canyon– Seasonal/Temporal:

– Different breeding season» Blue whales in northern and southern hemisphere

– Mechanical: – Parts don’t fit

» bushbabies– Behavioral:

– Different mating behavior» Crickets & Birds of Paradise

Post-zygotic Isolating Mechanisms-

Gametic incompatibility

Sperm transfer takes place, but egg is not fertilized.

Zygotic mortality Egg is fertilized, but zygote does not develop.

Hybrid inviability Hybrid embryo forms, but of reduced viability.

Hybrid sterility Hybrid is viable, but resulting adult is sterile.

Hybrid breakdown First generation (F1) hybrids are viable and fertile, but further

hybrid generations (F2 and backcrosses) may be inviable or sterile.

• these happen, even if fertilization happens

Paths of Speciation:

• Allopatric Speciation

– Populations begin to diverge when gene flow between them is restricted.– Geographic isolation is often the first step in allopatric speciation.

•Phylentic Speciation –

a single population is transformed enough to be considered a new species

Parapatric Speciation- • The splitting of a population into 2 species

under conditions where members of each population reside in adjacent areas

Hybridization

• Two species mate and make a hybrid that cannot mate with either parent.

Patterns of Evolution:

• Divergent Evolution- two species evolve from a common ancestor and become different over time.

• Adaptive Radiation- many species evolve from a single ancestral line

• Convergent Evolution- species with different ancestors colonize similar habitats with a superficial resemblance

Comparing them:

Convergent evolution takes place when species of different ancestry begin to share analogous traits because of a shared environment or other selection pressure

Parallel evolution occurs when two species evolve independently of each other, but maintain the same level of similarity. Parallel evolution usually occurs between unrelated species that do not occupy the same or similar niches in a given habitat.

When people hear the word "evolution," they usually think of divergent evolution, the pattern where two species gradually become increasingly different.

ExtinctionExtinction occurs when the population cannot adapt to changing environmental conditions.

The golden toad of Costa Rica’s Monteverde cloud forest became extinct because of climate change.

video

Pace of Evolution• Gradualism-

– Very gradually, over a long time, a population changes. – Over a short period of time it is hard to notice. – Change is slow, constant, and consistent.– The steady uninterrupted process has small adaptive steps– Darwin’s view

• Punctuated Equilibrium-• change comes in spurts. There is a

period of very little change, and then one or a few huge changes occur

• Often due to major changes in the environment.

• Gaps in the fossil record