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

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.

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.

Evidence for Evolution

• Fossils• Anatomy

– ________________ Structures: structures that do not have a common evolutionary origin but are similar in function

Analogous Structures

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.

Homologous Structures

Homologous Structures

Homologous Structures

Evidence of Evolution• Homologous Structures

Turtle Alligator Bird Mammal

Ancient, lobe-finned fish

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

Vestigial Structures

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 ____________ __________.

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.

Variation Wrap-Up

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