Ch 24 (4)

Darwin facd many unanswered questions when he began to ponder that __, __. He found answers to some of thouse questions when he realized that evolution by __ helped to explain both the diversity of life and the __ of organisms.

We can gather info about how long it takes new species to form from broad patterns in the __ and from studies that use __ (including fossils) or __ to assess the time interval between speciation events in part. groups of organisms.

The __ includes many episodes, in which new species appear suddenly in a __, persist essentially unchanged through several __, and then disappear.
o Paleontologists coined the term __ to describe these periods of apparent stasis punctuated by sudden change. Other species do not show a __; instead they change more gradually over long periods of time.

What do punctuated and gradual patterns tell us about how long it takes new species to form?
- Suppose that a species survived for 5 mil years, but most of the __ changes that caused it to be designated a new species occurred during the first 50,000 years of its existence- just 1% of tis total lifetime. Time periods this short (in geologic terms) often cannot be distinguished in __, in part because the rate of sediment accumulation is too slow to separate lyers this close in time. Thus, based on its fossils, the species would seem to have appeared suddenly and then lingere with little or no change before becoming extinct.

Even though such a species may have originated more slowly than its fossils suggest (in this case taking 50 G years), a __ indicates that __ occurred relatively rapidly. For speices whose fossils change much more gradually, we also cannot tell exactly when a new biologica species forms, since info about reproductive isolation does not fossilize. However it is liely that speciation in such groups occurred relatively slowly, perhaps taking millions of years.

The __ suggets that once the process begins, speciation can be completed relatively rapidly- a suggestion confirmed by a growing number of studies.

What can we learn from such data?
- First, the data suggest than on average, millions of years may pass before a newly formed species will itself give rise to another new species. This result has implications for how long it takes Earth's life to recover from mass extinction events.
Second, the extreme variability in the time it takes new species to form indicates that organisms do not have a "__" ticking inside them, causing them to produce new species at regular time intervals. Instead, __ begins only after __ between populations is interrupted, perhaps by an unpredictable event such as a storn that transports a few individuals to an isolated area. Furthermore, once __ has been interrupted, the populations must diverge genetically to such an extent that they become reproductively isolated- all before another event causes __ to resume, reversing the __ process.

Studies of ongoing __ (as in hybrid zones) can reveal traits that cause __. By identifying genes that control those traits, scientists can explore a fundamental question of evolutionary biology: How many genes change when a new species forms?

In a few cases, the evolution of __ is due to a change in a single gene.

In other organisms, the __ process is influenced by larger numers of genes and gene interactions.

Overall, studies conducted to date suggest that few or many genes can influence the evolution of __ and hence the emergence of a new species- a new addition to the great diversity of life.

__ may begin with differences as seemingly small as the color on a cichlid's back. However, as __ occurs againa nd again, such difference can accumulate and become more pronounced, eventually leading to the formation of new groups of organisms that differ greatly from their ancestors.

Furthermore, as one group of organisms increasees in size by producing many new species, another group of organisms may shrink, losing species to extinction. The cumulative effects of many such __ and __ events have helped to shape the sweeping evolutionary changes that are documented in the __.