1. 7d. Know variation within a species increases the likelihood that at least some members of a species will survive under changed environmental conditions.
15-3 Darwin Presents His Case

2. Evolution by Natural Selection
Because of its similarities to artificial selection, Darwin referred to the survival of the fittest as _____________ ______________.
In natural selection, the traits being selected contribute to an organism's fitness in its environment.

3. Evolution by Natural Selection
Over time, natural selection results in changes in the inherited characteristics of a population. These changes increase a species‘ ___________ in its environment.

4. Evidence for Evolution
Fossils
Anatomy
________________ Structures: structures that do not have a common evolutionary origin but are similar in function

5. Analogous Structures

6. Evidence of Evolution Homologous Body Structures
Structures that have different mature forms but develop from the same embryonic tissues are called __________________ structures.
Similarities and differences in homologous structures help biologists group animals according to how recently they last shared a common ancestor.

7. Homologous Structures

8. Homologous Structures

9. Homologous Structures

10. Evidence of Evolution Homologous Structures Turtle Alligator Bird
Mammal
The limbs of these four modern vertebrates are homologous structures. They provide evidence of a common ancestor whose bones may have resembled those of the ancient fish shown here. Notice that the same colors are used to show related structures. Homologous structures are one type of evidence for the evolution of living things. Homologous structures are one type of evidence for the evolution of living things.
Ancient, lobe-
finned fish

11. And Yet More Evidence for Evolution
_______________ Structures: a body structure that has no function in a present-day organism but was probably useful to an ancestor

12. Vestigial Structures

13. Evidence of Evolution
Not all homologous structures serve important functions.
The organs of many animals are so reduced in size that they are just vestiges, or traces, of homologous organs in other species.
These organs are called ____________ __________.

14. Variation Wrap-Up
As environmental factors change, natural selection of adaptive traits must also be realigned.
_________________ within a species stemming either from mutation or from genetic recombination broadens the opportunity for that species to adapt to change, increasing the probability that at least some members of the species will be suitably adapted to the new conditions.

15. Variation Wrap-Up
_____________ diversity promotes survival of a species should the environment change significantly, and sameness can mean vulnerability that could lead to ______________-.