Our Expert Voices conversation on why sex exists.

Why bother with sex when self-cloning would seem more efficient? The advantage: Sexual reproduction in a species, cumulated over all its members, stirs the species' gene pool, rebalancing the genes into different combinations. A species needs a continually rebalanced genetic portfolio to survive over the long term in nature's ever-changing environment. At the cellular level, there are two gamete sizes — big (egg) and tiny (sperm). But why not one intermediate gamete size instead? It's to find one another. Imagine getting separated from your friend at a shopping mall. It's easier if one party stays put while the other does the searching. Likewise, a stationary egg and and mobile sperm can meet up more readily than two gametes of the same size. Hence the origin of the distinction between male and female. The bottom line: The purpose of sex is to mix genes. Species whose members self-clone do not last very long evolutionarily. They are twigs on the tree of life whereas sexually reproducing species form its branches. Other voices in the conversation: Sarah Otto, theoretical biologist, University of British Columbia: Sex is evolution's answer to an ever-changing world Curtis Lively, evolutionary biologist, Indiana University Bloomington: Sex protects populations from parasites

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Amy Harder, author of Generate
1 hour ago - Energy & Environment

Climate change goes mainstream in presidential debate

Photo illustration: Aïda Amer/Axios. Photo: Olivier Douliery-Pool/Getty

The most notable part of Thursday’s presidential debate on climate change was the fact it was included as a topic and assumed as a fact.

The big picture: This is the first time in U.S. presidential history that climate change was a featured issue at a debate. It signals how the problem has become part of the fabric of our society. More extreme weather, like the wildfires ravaging Colorado, is pushing the topic to the front-burner.

Finally, a real debate

Photo: Morry Gash/AP

A more disciplined President Trump held back from the rowdy interruptions at tonight's debate in Nashville, while making some assertions so outlandish that Joe Biden chuckled and even closed his eyes. A Trump campaign adviser told Axios: "He finally listened." 

The result: A real debate.

Biden to Trump: "I have not taken a penny from any foreign source ever in my life"

Former VP Joe Biden pushed back Thursday against allegations from President Trump, saying he had never profited from foreign sources. "Nothing was unethical," Biden told debate moderator Kristen Welker about his son Hunter's work in Ukraine while he was vice president.

Why it matters: Earlier on Thursday, Hunter Biden's former business partner, Tony Bobulinski, released a statement saying Joe Biden's claims that he never discussed overseas business dealings with his son were "false."